libdrm/intel/Makefile.am


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# Copyright © 2008 Intel Corporation
#
# Permission is hereby granted, free of charge, to any person obtaining a
# copy of this software and associated documentation files (the "Software"),
# to deal in the Software without restriction, including without limitation
# the rights to use, copy, modify, merge, publish, distribute, sublicense,
# and/or sell copies of the Software, and to permit persons to whom the
# Software is furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice (including the next
# paragraph) shall be included in all copies or substantial portions of the
# Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
# THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
# FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
# IN THE SOFTWARE.
#
# Authors:
#    Eric Anholt <eric@anholt.net>

AM_CFLAGS = \
	$(WARN_CFLAGS) \
	-I$(top_srcdir)/libdrm \
	-I$(top_srcdir)/shared-core

noinst_LTLIBRARIES = libdrm_intel.la

libdrm_intel_la_SOURCES = \
	intel_bufmgr_fake.c \
	intel_bufmgr_gem.c \
	mm.c \
	mm.h

libdrm_intelincludedir = ${includedir}
libdrm_intelinclude_HEADERS = intel_bufmgr.h
1. INTRODUCTION
===============

This document describes the specifications of the “virtio” family of
devices. These are devices are found in virtual environments, yet by
design they are not all that different from physical devices, and this
document treats them as such. This allows the guest to use standard
drivers and discovery mechanisms.

The purpose of virtio and this specification is that virtual
environments and guests should have a straightforward, efficient,
standard and extensible mechanism for virtual devices, rather
than boutique per-environment or per-OS mechanisms.

  Straightforward: Virtio devices use normal bus mechanisms of
  interrupts and DMA which should be familiar to any device driver
  author. There is no exotic page-flipping or COW mechanism: it's just
  a normal device.[1]

  Efficient: Virtio devices consist of rings of descriptors
  for input and output, which are neatly separated to avoid cache
  effects from both guest and device writing to the same cache
  lines.

  Standard: Virtio makes no assumptions about the environment in which
  it operates, beyond supporting the bus attaching the device.  Virtio
  devices are implemented over PCI and other buses, and earlier drafts
  been implemented on other buses not included in this spec.[2]

  Extensible: Virtio PCI devices contain feature bits which are
  acknowledged by the guest operating system during device setup.
  This allows forwards and backwards compatibility: the device
  offers all the features it knows about, and the driver
  acknowledges those it understands and wishes to use.

1.1.1.  Key words
-----------------

The key words must, must not, required, shall, shall not, should,
should not, recommended, may, and optional are to be interpreted as
described in [RFC 2119].  Note that for reasons of style, these words
are not capitalized in this document.

1.1.2.  Definitions
-------------------

term
    Definition

1.1.3.  Key concepts
--------------------

Guest
    Definition...

Host
    Definition

Device
    Definition

Driver
    Definition

1.2. Normative References
=========================

[RFC 2119] S. Bradner, Key words for use in RFCs to Indicate Requirement Levels, http://www.ietf.org/rfc/rfc2119.txt IETF (Internet Engineering Task Force) RFC 2119, March 1997.

1.3. Non-Normative References
=========================


2. The Virtio Standard
=========================

2.1. Basic Facilities of a Virtio Device
=======================================

A virtio device is discovered and identified by a bus-specific
method (see the bus specific sections: "2.3.1. Virtio Over PCI
Bus" and "2.3.2. Virtio Over MMIO").  Each device consists of the
following parts:

o Device Status field
o Feature bits
o Configuration space
o One or more virtqueues

2.1.1. Device Status Field
-------------------------

The Device Status field is updated by the guest to indicate its
progress. This provides a simple low-level diagnostic: it's most
useful to imagine them hooked up to traffic lights on the console
indicating the status of each device.

This field is 0 upon reset, otherwise at least one bit should be set:

  ACKNOWLEDGE (1) Indicates that the guest OS has found the
  device and recognized it as a valid virtio device.

  DRIVER (2) Indicates that the guest OS knows how to drive the
  device. Under Linux, drivers can be loadable modules so there
  may be a significant (or infinite) delay before setting this
  bit.

  DRIVER_OK (4) Indicates that the driver is set up and ready to
  drive the device.

  FAILED (128) Indicates that something went wrong in the guest,
  and it has given up on the device. This could be an internal
  error, or the driver didn't like the device for some reason, or
  even a fatal error during device operation. The device must be
  reset before attempting to re-initialize.

2.1.2. Feature Bits
------------------

Each virtio device lists all the features it understands.  During
device initialization, the guest reads this and tells the device the
subset that it understands.  The only way to renegotiate is to reset
the device.

This allows for forwards and backwards compatibility: if the device is
enhanced with a new feature bit, older guests will not write that
feature bit back to the device and it can go into backwards
compatibility mode. Similarly, if a guest is enhanced with a feature
that the device doesn't support, it see the new feature is not offered
and can go into backwards compatibility mode (or, for poor
implementations, set the FAILED Device Status bit).

Feature bits are allocated as follows:

  0 to 23: Feature bits for the specific device type

  24 to 32: Feature bits reserved for extensions to the queue and
  feature negotiation mechanisms

  33 and above: Feature bits resered for future extensions.

For example, feature bit 0 for a network device (i.e. Subsystem
Device ID 1) indicates that the device supports checksumming of
packets.

In particular, new fields in the device configuration space are
indicated by offering a feature bit, so the guest can check
before accessing that part of the configuration space.

2.1.2.1 Legacy Interface: A Note on transitions from earlier drafts
--------------------------------------

Earlier drafts of this specification (up to 0.9.X) defined a similar, but
different interface between the hypervisor and the guest.
Since these are widely deployed, this specification
accomodates optional features to simplify transition
from these earlier draft interfaces. Specifically:

Legacy Interface
	is an interface specified by an earlier draft of this specification
        (up to 0.9.X)
Legacy Device
	is a device implemented before this specification was released,
        and implementing a legacy interface on the host side
Legacy Driver
	is a driver implemented before this specification was released,
        and implementing a legacy interface on the guest side

Legacy devices and legacy drivers are not compliant with this
specification.

To simplify transition from these earlier draft interfaces,
it is possible to implement:

Transitional Device
	a device supporting both drivers conforming to this
        specification, and allowing legacy drivers.

Transitional Driver
	a driver supporting both devices conforming to this
	specification, and legacy devices.

Transitional devices and transitional drivers can be compliant with
this specification (ie. when not operating in legacy mode).

Devices or drivers with no legacy compatibility are referred to as
non-transitional devices and drivers, respectively.

Transitional Drivers can detect Legacy Devices by detecting that
the feature bit VIRTIO_F_VERSION_1 is not offered.
Transitional devices can detect Legacy drivers by detecting that
VIRTIO_F_VERSION_1 has not been acknowledged by the driver.
In this case device is used through the legacy interface.

To make them easier to locate, specification sections documenting
these transitional features are explicitly marked with 'Legacy
Interface' in the section title.

2.1.3. Configuration Space
-------------------------

Configuration space is generally used for rarely-changing or
initialization-time parameters.

Note that this space is generally the guest's native endian,
rather than PCI's little-endian.

2.1.4. Virtqueues
----------------

The mechanism for bulk data transport on virtio devices is
pretentiously called a virtqueue. Each device can have zero or more
virtqueues: for example, the simplest network device has one for
transmit and one for receive.  Each queue has a 16-bit queue size
parameter, which sets the number of entries and implies the total size
of the queue.

Each virtqueue occupies two or more physically-contiguous pages
(usually defined as 4096 bytes, but depending on the transport)
and consists of three parts:

+-------------------+-----------------------------------+-----------+
| Descriptor Table  |   Available Ring     (padding)    | Used Ring |
+-------------------+-----------------------------------+-----------+

The bus-specific Queue Size field controls the total number of bytes
required for the virtqueue according to the following formula:

	#define ALIGN(x) (((x) + PAGE_SIZE) & ~PAGE_SIZE)
	static inline unsigned vring_size(unsigned int qsz)
	{
	     return ALIGN(sizeof(struct vring_desc)*qsz + sizeof(u16)*(3 + qsz))
	          + ALIGN(sizeof(u16)*3 + sizeof(struct vring_used_elem)*qsz);
	}

This currently wastes some space with padding, but also allows future
extensions such as the VIRTIO_RING_F_EVENT_IDX extension.  The
virtqueue layout structure looks like this:

	struct vring {
		// The actual descriptors (16 bytes each)
		struct vring_desc desc[ Queue Size ];

		// A ring of available descriptor heads with free-running index.
		struct vring_avail avail;

		// Padding to the next PAGE_SIZE boundary.
		char pad[ Padding ];

		// A ring of used descriptor heads with free-running index.
		struct vring_used used;
	};

When the driver wants to send a buffer to the device, it fills in
a slot in the descriptor table (or chains several together), and
writes the descriptor index into the available ring.  It then
notifies the device. When the device has finished a buffer, it
writes the descriptor into the used ring, and sends an interrupt.

2.1.4.1. A Note on Virtqueue Endianness
--------------------------------------

Note that the endian of fields and in the virtqueue is the native
endian of the guest, not little-endian as PCI normally is. This makes
for simpler guest code, and it is assumed that the host already has to
be deeply aware of the guest endian so such an “endian-aware” device
is not a significant issue.

2.1.4.2. Message Framing
-----------------------
The message framing (the particular layout of descriptors) is
independent of the contents of the buffers. For example, a network
transmit buffer consists of a 12 byte header followed by the network
packet. This could be most simply placed in the descriptor table as a
12 byte output descriptor followed by a 1514 byte output descriptor,
but it could also consist of a single 1526 byte output descriptor in
the case where the header and packet are adjacent, or even three or
more descriptors (possibly with loss of efficiency in that case).

Note that, some implementations may have large-but-reasonable
restrictions on total descriptor size (such as based on IOV_MAX in the
host OS). This has not been a problem in practice: little sympathy
will be given to drivers which create unreasonably-sized descriptors
such as by dividing a network packet into 1500 single-byte
descriptors!

2.1.4.2.1. Legacy Interface: Message Framing
-----------------------

Regrettably, initial driver implementations used simple layouts, and
devices came to rely on it, despite this specification wording.  In
addition, the specification for virtio_blk SCSI commands required
intuiting field lengths from frame boundaries (see "2.4.2.5.1 Legacy
Interface: Device Operation")

It is thus recommended that when using legacy interfaces, transitional
drivers be conservative in their assumptions, unless the
VIRTIO_F_ANY_LAYOUT feature is accepted.

2.1.4.3. The Virtqueue Descriptor Table
--------------------------------------

The descriptor table refers to the buffers the guest is using for
the device. The addresses are physical addresses, and the buffers
can be chained via the next field. Each descriptor describes a
buffer which is read-only or write-only, but a chain of
descriptors can contain both read-only and write-only buffers.

No descriptor chain may be more than 2^32 bytes long in total.

	struct vring_desc {
		/* Address (guest-physical). */
		u64 addr;
		/* Length. */
		u32 len;

	/* This marks a buffer as continuing via the next field. */
	#define VRING_DESC_F_NEXT   1
	/* This marks a buffer as write-only (otherwise read-only). */
	#define VRING_DESC_F_WRITE     2
	/* This means the buffer contains a list of buffer descriptors. */
	#define VRING_DESC_F_INDIRECT   4
		/* The flags as indicated above. */
		u16 flags;
		/* Next field if flags & NEXT */
		u16 next;
	};

The number of descriptors in the table is defined by the queue size
for this virtqueue.

2.1.4.3.1. Indirect Descriptors
------------------------------

Some devices benefit by concurrently dispatching a large number
of large requests. The VIRTIO_RING_F_INDIRECT_DESC feature can be
used to allow this (see "2.6. Reserved Feature Bits"). To increase
ring capacity it is possible to store a table of indirect
descriptors anywhere in memory, and insert a descriptor in main
virtqueue (with flags&VRING_DESC_F_INDIRECT on) that refers to memory buffer
containing this indirect descriptor table; fields addr and len
refer to the indirect table address and length in bytes,
respectively. The indirect table layout structure looks like this
(len is the length of the descriptor that refers to this table,
which is a variable, so this code won't compile):

	struct indirect_descriptor_table {
		/* The actual descriptors (16 bytes each) */
		struct vring_desc desc[len / 16];
	};

The first indirect descriptor is located at start of the indirect
descriptor table (index 0), additional indirect descriptors are
chained by next field. An indirect descriptor without next field
(with flags&VRING_DESC_F_NEXT off) signals the end of the indirect descriptor
table, and transfers control back to the main virtqueue. An
indirect descriptor can not refer to another indirect descriptor
table (flags&VRING_DESC_F_INDIRECT must be off). A single indirect descriptor
table can include both read-only and write-only descriptors;
write-only flag (flags&VRING_DESC_F_WRITE) in the descriptor that refers to it
is ignored.

2.1.4.4. The Virtqueue Available Ring
------------------------------------

The available ring refers to what descriptor chains we are offering the
device: each entry refers to the head of a descriptor chain. The “flags” field
is currently 0 or 1: 1 indicating that we do not need an interrupt
when the device consumes a descriptor chain from the available
ring. Alternatively, the guest can ask the device to delay interrupts
until an entry with an index specified by the “used_event” field is
written in the used ring (equivalently, until the idx field in the
used ring will reach the value used_event + 1). The method employed by
the device is controlled by the VIRTIO_RING_F_EVENT_IDX feature bit
(see "2.6. Reserved Feature Bits"). This interrupt suppression is
merely an optimization; it may not suppress interrupts entirely.

The “idx” field indicates where we would put the next descriptor
entry (modulo the queue size). This starts at 0, and increases.

	struct vring_avail {
	#define VRING_AVAIL_F_NO_INTERRUPT      1
		u16 flags;
		u16 idx;
		u16 ring[ /* Queue Size */ ];
		u16 used_event;	/* Only if VIRTIO_RING_F_EVENT_IDX */
	};

2.1.4.5. The Virtqueue Used Ring
-------------------------------

The used ring is where the device returns buffers once it is done
with them. The flags field can be used by the device to hint that
no notification is necessary when the guest adds to the available
ring. Alternatively, the “avail_event” field can be used by the
device to hint that no notification is necessary until an entry
with an index specified by the “avail_event” is written in the
available ring (equivalently, until the idx field in the
available ring will reach the value avail_event + 1). The method
employed by the device is controlled by the guest through the
VIRTIO_RING_F_EVENT_IDX feature bit (see "2.6. Reserved
Feature Bits").[7]

Each entry in the ring is a pair: the head entry of the
descriptor chain describing the buffer (this matches an entry
placed in the available ring by the guest earlier), and the total
of bytes written into the buffer. The latter is extremely useful
for guests using untrusted buffers: if you do not know exactly
how much has been written by the device, you usually have to zero
the buffer to ensure no data leakage occurs.

	/* u32 is used here for ids for padding reasons. */
	struct vring_used_elem {
		/* Index of start of used descriptor chain. */
		u32 id;
		/* Total length of the descriptor chain which was used (written to) */
		u32 len;
	};

	struct vring_used {
	#define VRING_USED_F_NO_NOTIFY  1
		u16 flags;
		u16 idx;
		struct vring_used_elem ring[ /* Queue Size */];
		u16 avail_event; /* Only if VIRTIO_RING_F_EVENT_IDX */
	};

2.1.4.6. Helpers for Operating Virtqueues
----------------------------------------

The Linux Kernel Source code contains the definitions above and
helper routines in a more usable form, in
include/linux/virtio_ring.h. This was explicitly licensed by IBM
and Red Hat under the (3-clause) BSD license so that it can be
freely used by all other projects, and is reproduced (with slight
variation to remove Linux assumptions) in "2.6. virtio_ring.h".

2.2. General Initialization And Device Operation
===============================================

We start with an overview of device initialization, then expand on the
details of the device and how each step is preformed.  This section
should be read along with the bus-specific section which describes
how to communicate with the specific device.

2.2.1. Device Initialization
---------------------------

1. Reset the device. This is not required on initial start up.

2. The ACKNOWLEDGE status bit is set: we have noticed the device.

3. The DRIVER status bit is set: we know how to drive the device.

4. Device-specific setup, including reading the device feature
  bits, discovery of virtqueues for the device, optional per-bus
  setup, and reading and possibly writing the device's virtio
  configuration space.

5. The subset of device feature bits understood by the driver is
   written to the device.

6. The DRIVER_OK status bit is set.

7. The device can now be used (ie. buffers added to the
   virtqueues)[4]

If any of these steps go irrecoverably wrong, the guest should
set the FAILED status bit to indicate that it has given up on the
device (it can reset the device later to restart if desired).

2.2.2. Device Operation
----------------------

There are two parts to device operation: supplying new buffers to
the device, and processing used buffers from the device. As an
example, the simplest virtio network device has two virtqueues: the
transmit virtqueue and the receive virtqueue. The driver adds
outgoing (read-only) packets to the transmit virtqueue, and then
frees them after they are used. Similarly, incoming (write-only)
buffers are added to the receive virtqueue, and processed after
they are used.

2.2.2.1. Supplying Buffers to The Device
---------------------------------------

Actual transfer of buffers from the guest OS to the device
operates as follows:

1. Place the buffer(s) into free descriptor(s).

  (a) If there are no free descriptors, the guest may choose to
    notify the device even if notifications are suppressed (to
    reduce latency).[8]

2. Place the id of the buffer in the next ring entry of the
  available ring.

3. The steps (1) and (2) may be performed repeatedly if batching
  is possible.

4. A memory barrier should be executed to ensure the device sees
  the updated descriptor table and available ring before the next
  step.

5. The available “idx” field should be increased by the number of
  entries added to the available ring.

6. A memory barrier should be executed to ensure that we update
  the idx field before checking for notification suppression.

7. If notifications are not suppressed, the device should be
  notified of the new buffers.

Note that the above code does not take precautions against the
available ring buffer wrapping around: this is not possible since
the ring buffer is the same size as the descriptor table, so step
(1) will prevent such a condition.

In addition, the maximum queue size is 32768 (it must be a power
of 2 which fits in 16 bits), so the 16-bit “idx” value can always
distinguish between a full and empty buffer.

Here is a description of each stage in more detail.

2.2.2.1.1. Placing Buffers Into The Descriptor Table
---------------------------------------------------

A buffer consists of zero or more read-only physically-contiguous
elements followed by zero or more physically-contiguous
write-only elements (it must have at least one element). This
algorithm maps it into the descriptor table:

for each buffer element, b:

  (a) Get the next free descriptor table entry, d

  (b) Set d.addr to the physical address of the start of b

  (c) Set d.len to the length of b.

  (d) If b is write-only, set d.flags to VRING_DESC_F_WRITE,
    otherwise 0.

  (e) If there is a buffer element after this:

    i. Set d.next to the index of the next free descriptor
      element.

    ii. Set the VRING_DESC_F_NEXT bit in d.flags.

In practice, the d.next fields are usually used to chain free
descriptors, and a separate count kept to check there are enough
free descriptors before beginning the mappings.

2.2.2.1.2. Updating The Available Ring
-------------------------------------

The head of the buffer we mapped is the first d in the algorithm
above. A naive implementation would do the following:

	avail->ring[avail->idx % qsz] = head;

However, in general we can add many descriptor chains before we update
the “idx” field (at which point they become visible to the
device), so we keep a counter of how many we've added:

	avail->ring[(avail->idx + added++) % qsz] = head;

2.2.2.1.3. Updating The Index Field
----------------------------------

Once the index field of the virtqueue is updated, the device will
be able to access the descriptor chains we've created and the
memory they refer to. This is why a memory barrier is generally
used before the index update, to ensure it sees the most up-to-date
copy.

The index field always increments, and we let it wrap naturally at
65536:

	avail->idx += added;

2.2.2.1.4. Notifying The Device
------------------------------

The actual method of device notification is bus-specific, but generally
it can be expensive.  So the device can suppress such notifications if it
doesn't need them.  We have to be careful to expose the new index
value before checking if notifications are suppressed: it's OK to notify
gratuitously, but not to omit a required notification. So again,
we use a memory barrier here before reading the flags or the
avail_event field.

If the VIRTIO_F_RING_EVENT_IDX feature is not negotiated, and if the
VRING_USED_F_NOTIFY flag is not set, we go ahead and notify the
device.

If the VIRTIO_F_RING_EVENT_IDX feature is negotiated, we read the
avail_event field in the available ring structure. If the
available index crossed_the avail_event field value since the
last notification, we go ahead and write to the PCI configuration
space.  The avail_event field wraps naturally at 65536 as well,
iving the following algorithm for calculating whether a device needs
notification:

	(u16)(new_idx - avail_event - 1) < (u16)(new_idx - old_idx)

2.2.2.2. Receiving Used Buffers From The Device
----------------------------------------------

Once the device has used a buffer (read from or written to it, or
parts of both, depending on the nature of the virtqueue and the
device), it sends an interrupt, following an algorithm very
similar to the algorithm used for the driver to send the device a
buffer:

1. Write the head descriptor number to the next field in the used
  ring.

2. Update the used ring index.

3. Deliver an interrupt if necessary:

  (a) If the VIRTIO_F_RING_EVENT_IDX feature is not negotiated:
    check if the VRING_AVAIL_F_NO_INTERRUPT flag is not set in
    avail->flags.

  (b) If the VIRTIO_F_RING_EVENT_IDX feature is negotiated: check
    whether the used index crossed the used_event field value
    since the last update. The used_event field wraps naturally
    at 65536 as well:
	(u16)(new_idx - used_event - 1) < (u16)(new_idx - old_idx)

For each ring, guest should then disable interrupts by writing
VRING_AVAIL_F_NO_INTERRUPT flag in avail structure, if required.
It can then process used ring entries finally enabling interrupts
by clearing the VRING_AVAIL_F_NO_INTERRUPT flag or updating the
EVENT_IDX field in the available structure.  The guest should then
execute a memory barrier, and then recheck the ring empty
condition. This is necessary to handle the case where after the
last check and before enabling interrupts, an interrupt has been
suppressed by the device:

	vring_disable_interrupts(vq);

	for (;;) {
		if (vq->last_seen_used != vring->used.idx) {
			vring_enable_interrupts(vq);
			mb();

			if (vq->last_seen_used != vring->used.idx)
				break;
		}

		struct vring_used_elem *e = vring.used->ring[vq->last_seen_used%vsz];
		process_buffer(e);
		vq->last_seen_used++;
	}

2.2.2.3. Notification of Device Configuration Changes
----------------------------------------------------

For devices where the configuration information can be changed, an
interrupt is delivered when a configuration change occurs.


2.3. Virtio Transport Options
============================

Virtio can use various different busses, thus the standard is split
into virtio general and bus-specific sections.

2.3.1. Virtio Over PCI Bus
-------------------------

Virtio devices are commonly implemented as PCI devices.

2.3.1.1. PCI Device Discovery
----------------------------

Any PCI device with Vendor ID 0x1AF4, and Device ID 0x1000 through
0x103F inclusive is a virtio device[3]. The device must also have a
Revision ID of 0 to match this specification.

The Subsystem Device ID indicates which virtio device is
supported by the device. The Subsystem Vendor ID should reflect
the PCI Vendor ID of the environment (it's currently only used
for informational purposes by the guest).

2.3.1.2. PCI Device Layout
-------------------------

To configure the device, we use the first I/O region of the PCI
device. This contains a virtio header followed by a
device-specific region.

There may be different widths of accesses to the I/O region; the
“natural” access method for each field in the virtio header must be
used (i.e. 32-bit accesses for 32-bit fields, etc), but the
device-specific region can be accessed using any width accesses, and
should obtain the same results.

Note that this is possible because while the virtio header is PCI
(i.e. little) endian, the device-specific region is encoded in
the native endian of the guest (where such distinction is
applicable).

2.3.1.2.1. PCI Device Virtio Header
----------------------------------

The virtio header looks as follows:

+------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+
| Bits       || 32                  | 32                  | 32       | 16     | 16      | 16      | 8       | 8      |
+------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+
| Read/Write || R                   | R+W                 | R+W      | R      | R+W     | R+W     | R+W     | R      |
+------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+
| Purpose    || Device              | Guest               | Queue    | Queue  | Queue   | Queue   | Device  | ISR    |
|            || Features bits 0:31  | Features bits 0:31  | Address  | Size   | Select  | Notify  | Status  | Status |
+------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+


If MSI-X is enabled for the device, two additional fields
immediately follow this header:[5]


+------------++----------------+--------+
| Bits       || 16             | 16     |
              +----------------+--------+
+------------++----------------+--------+
| Read/Write || R+W            | R+W    |
+------------++----------------+--------+
| Purpose    || Configuration  | Queue  |
| (MSI-X)    || Vector         | Vector |
+------------++----------------+--------+

Immediately following these general headers, there may be
device-specific headers:

+------------++--------------------+
| Bits       || Device Specific    |
              +--------------------+
+------------++--------------------+
| Read/Write || Device Specific    |
+------------++--------------------+
| Purpose    || Device Specific... |
|            ||                    |
+------------++--------------------+

2.3.1.3. PCI-specific Initialization And Device Operation
--------------------------------------------------------

The page size for a virtqueue on a PCI virtio device is defined as
4096 bytes.

2.3.1.3.1. Device Initialization
-------------------------------

2.3.1.3.1.1. Queue Vector Configuration
--------------------------------------

When MSI-X capability is present and enabled in the device
(through standard PCI configuration space) 4 bytes at byte offset
20 are used to map configuration change and queue interrupts to
MSI-X vectors. In this case, the ISR Status field is unused, and
device specific configuration starts at byte offset 24 in virtio
header structure. When MSI-X capability is not enabled, device
specific configuration starts at byte offset 20 in virtio header.

Writing a valid MSI-X Table entry number, 0 to 0x7FF, to one of
Configuration/Queue Vector registers, maps interrupts triggered
by the configuration change/selected queue events respectively to
the corresponding MSI-X vector. To disable interrupts for a
specific event type, unmap it by writing a special NO_VECTOR
value:

	/* Vector value used to disable MSI for queue */
	#define VIRTIO_MSI_NO_VECTOR            0xffff

Reading these registers returns vector mapped to a given event,
or NO_VECTOR if unmapped. All queue and configuration change
events are unmapped by default.

Note that mapping an event to vector might require allocating
internal device resources, and might fail. Devices report such
failures by returning the NO_VECTOR value when the relevant
Vector field is read. After mapping an event to vector, the
driver must verify success by reading the Vector field value: on
success, the previously written value is returned, and on
failure, NO_VECTOR is returned. If a mapping failure is detected,
the driver can retry mapping with fewervectors, or disable MSI-X.

2.3.1.3.1.2. Virtqueue Configuration
-----------------------------------

As a device can have zero or more virtqueues for bulk data
transport (for example, the simplest network device has two), the driver
needs to configure them as part of the device-specific
configuration.

This is done as follows, for each virtqueue a device has:

1. Write the virtqueue index (first queue is 0) to the Queue
  Select field.

2. Read the virtqueue size from the Queue Size field, which is
  always a power of 2. This controls how big the virtqueue is
  (see "2.1.4. Virtqueues"). If this field is 0, the virtqueue does not exist.

3. Allocate and zero virtqueue in contiguous physical memory, on
  a 4096 byte alignment. Write the physical address, divided by
  4096 to the Queue Address field.[6]

4. Optionally, if MSI-X capability is present and enabled on the
  device, select a vector to use to request interrupts triggered
  by virtqueue events. Write the MSI-X Table entry number
  corresponding to this vector in Queue Vector field. Read the
  Queue Vector field: on success, previously written value is
  returned; on failure, NO_VECTOR value is returned.

2.3.1.3.2. Notifying The Device
------------------------------

Device notification occurs by writing the 16-bit virtqueue index
of this virtqueue to the Queue Notify field of the virtio header
in the first I/O region of the PCI device.

2.3.1.3.3. Virtqueue Interrupts From The Device
----------------------------------------------

If an interrupt is necessary:

  (a) If MSI-X capability is disabled:

    i. Set the lower bit of the ISR Status field for the device.

    ii. Send the appropriate PCI interrupt for the device.

  (b) If MSI-X capability is enabled:

    i. Request the appropriate MSI-X interrupt message for the
      device, Queue Vector field sets the MSI-X Table entry
      number.

    ii. If Queue Vector field value is NO_VECTOR, no interrupt
      message is requested for this event.

The guest interrupt handler should:

1. If MSI-X capability is disabled: read the ISR Status field,
  which will reset it to zero. If the lower bit is zero, the
  interrupt was not for this device. Otherwise, the guest driver
  should look through the used rings of each virtqueue for the
  device, to see if any progress has been made by the device
  which requires servicing.

2. If MSI-X capability is enabled: look through the used rings of
  each virtqueue mapped to the specific MSI-X vector for the
  device, to see if any progress has been made by the device
  which requires servicing.

2.3.1.3.4. Notification of Device Configuration Changes
------------------------------------------------------

Some virtio PCI devices can change the device configuration
state, as reflected in the virtio header in the PCI configuration
space. In this case:

1. If MSI-X capability is disabled: an interrupt is delivered and
  the second highest bit is set in the ISR Status field to
  indicate that the driver should re-examine the configuration
  space.  Note that a single interrupt can indicate both that one
  or more virtqueue has been used and that the configuration
  space has changed: even if the config bit is set, virtqueues
  must be scanned.

2. If MSI-X capability is enabled: an interrupt message is
  requested. The Configuration Vector field sets the MSI-X Table
  entry number to use. If Configuration Vector field value is
  NO_VECTOR, no interrupt message is requested for this event.

2.3.2. Virtio Over MMIO
----------------------

Virtual environments without PCI support (a common situation in
embedded devices models) might use simple memory mapped device (“
virtio-mmio”) instead of the PCI device.

The memory mapped virtio device behaviour is based on the PCI
device specification. Therefore most of operations like device
initialization, queues configuration and buffer transfers are
nearly identical. Existing differences are described in the
following sections.

2.3.2.1. MMIO Device Discovery
-----------------------------

Unlike PCI, MMIO provides no generic device discovery.  For systems using
a device-tree such as Linux's dtc or Open Firmware, the suggested format is:

	virtio_block@1e000 {
		compatible = "virtio,mmio";
		reg = <0x1e000 0x100>;
		interrupts = <42>;
	}

2.3.2.2. MMIO Device Layout
--------------------------

MMIO virtio devices provides a set of memory mapped control
registers, all 32 bits wide, followed by device-specific
configuration space. The following list presents their layout:

• Offset from the device base address | Direction | Name
 Description

• 0x000 | R | MagicValue
 “virt” string.

• 0x004 | R | Version
 Device version number. Currently must be 1.

• 0x008 | R | DeviceID
 Virtio Subsystem Device ID (ie. 1 for network card).

• 0x00c | R | VendorID
 Virtio Subsystem Vendor ID.

• 0x010 | R | HostFeatures
 Flags representing features the device supports.
 Reading from this register returns 32 consecutive flag bits,
  first bit depending on the last value written to
  HostFeaturesSel register. Access to this register returns bits HostFeaturesSel*32

   to (HostFeaturesSel*32)+31, eg. feature bits 0 to 31 if
  HostFeaturesSel is set to 0 and features bits 32 to 63 if
  HostFeaturesSel is set to 1. Also see "2.1.2. Feature Bits".

• 0x014 | W | HostFeaturesSel
 Device (Host) features word selection.
 Writing to this register selects a set of 32 device feature bits
  accessible by reading from HostFeatures register. Device driver
  must write a value to the HostFeaturesSel register before
  reading from the HostFeatures register.

• 0x020 | W | GuestFeatures
 Flags representing device features understood and activated by
  the driver.
 Writing to this register sets 32 consecutive flag bits, first
  bit depending on the last value written to GuestFeaturesSel
  register. Access to this register sets bits GuestFeaturesSel*32
  to (GuestFeaturesSel*32)+31, eg. feature bits 0 to 31 if
  GuestFeaturesSel is set to 0 and features bits 32 to 63 if
  GuestFeaturesSel is set to 1. Also see "2.1.2. Feature Bits".

• 0x024 | W | GuestFeaturesSel
 Activated (Guest) features word selection.
 Writing to this register selects a set of 32 activated feature
  bits accessible by writing to the GuestFeatures register.
  Device driver must write a value to the GuestFeaturesSel
  register before writing to the GuestFeatures register.

• 0x028 | W | GuestPageSize
 Guest page size.
 Device driver must write the guest page size in bytes to the
  register during initialization, before any queues are used.
  This value must be a power of 2 and is used by the Host to
  calculate Guest address of the first queue page (see QueuePFN).

• 0x030 | W | QueueSel
 Virtual queue index (first queue is 0).
 Writing to this register selects the virtual queue that the
  following operations on QueueNum, QueueAlign and QueuePFN apply
  to.

• 0x034 | R | QueueNumMax
 Maximum virtual queue size.
 Reading from the register returns the maximum size of the queue
  the Host is ready to process or zero (0x0) if the queue is not
  available. This applies to the queue selected by writing to
  QueueSel and is allowed only when QueuePFN is set to zero
  (0x0), so when the queue is not actively used.

• 0x038 | W | QueueNum
 Virtual queue size.
 Queue size is the number of elements in the queue, therefore size
  of the descriptor table and both available and used rings.
 Writing to this register notifies the Host what size of the
  queue the Guest will use. This applies to the queue selected by
  writing to QueueSel.

• 0x03c | W | QueueAlign
 Used Ring alignment in the virtual queue.
 Writing to this register notifies the Host about alignment
  boundary of the Used Ring in bytes. This value must be a power
  of 2 and applies to the queue selected by writing to QueueSel.

• 0x040 | RW | QueuePFN
 Guest physical page number of the virtual queue.
 Writing to this register notifies the host about location of the
  virtual queue in the Guest's physical address space. This value
  is the index number of a page starting with the queue
  Descriptor Table. Value zero (0x0) means physical address zero
  (0x00000000) and is illegal. When the Guest stops using the
  queue it must write zero (0x0) to this register.
 Reading from this register returns the currently used page
  number of the queue, therefore a value other than zero (0x0)
  means that the queue is in use.
 Both read and write accesses apply to the queue selected by
  writing to QueueSel.

• 0x050 | W | QueueNotify
 Queue notifier.
 Writing a queue index to this register notifies the Host that
  there are new buffers to process in the queue.

• 0x60 | R | InterruptStatus
Interrupt status.
Reading from this register returns a bit mask of interrupts
  asserted by the device. An interrupt is asserted if the
  corresponding bit is set, ie. equals one (1).

  – Bit 0 | Used Ring Update
	This interrupt is asserted when the Host has updated the Used
    Ring in at least one of the active virtual queues.

  – Bit 1 | Configuration change
	This interrupt is asserted when configuration of the device has
    changed.

• 0x064 | W | InterruptACK
 Interrupt acknowledge.
 Writing to this register notifies the Host that the Guest
  finished handling interrupts. Set bits in the value clear the
  corresponding bits of the InterruptStatus register.

• 0x070 | RW | Status
 Device status.
 Reading from this register returns the current device status
  flags.
 Writing non-zero values to this register sets the status flags,
  indicating the Guest progress. Writing zero (0x0) to this
  register triggers a device reset.
 Also see "2.2.1. Device Initialization".

• 0x100+ | RW | Config
 Device-specific configuration space starts at an offset 0x100
  and is accessed with byte alignment. Its meaning and size
  depends on the device and the driver.

Virtual queue size is the number of elements in the queue,
therefore size of the descriptor table and both available and
used rings.

All register values are organized as Little Endian.

Writing to registers described as “R” and reading from
registers described as “W” is not permitted and can cause
undefined behavior.

2.3.2.3. MMIO-specific Initialization And Device Operation
---------------------------------------------------------

2.3.2.3.1. Device Initialization
-------------------------------

Unlike the fixed page size for PCI, the virtqueue page size is defined
by the GuestPageSize field, as written by the guest.  This must be
done before the virtqueues are configured.

2.3.2.3.1.1. Virtqueue Configuration
-----------------------------------

1. Select the queue writing its index (first queue is 0) to the
  QueueSel register.

2. Check if the queue is not already in use: read QueuePFN
  register, returned value should be zero (0x0).

3. Read maximum queue size (number of elements) from the
  QueueNumMax register. If the returned value is zero (0x0) the
  queue is not available.

4. Allocate and zero the queue pages in contiguous virtual
  memory, aligning the Used Ring to an optimal boundary (usually
  page size). Size of the allocated queue may be smaller than or
  equal to the maximum size returned by the Host.

5. Notify the Host about the queue size by writing the size to
  QueueNum register.

6. Notify the Host about the used alignment by writing its value
  in bytes to QueueAlign register.

7. Write the physical number of the first page of the queue to
  the QueuePFN register.

2.3.2.3.2. Notifying The Device
------------------------------

The device is notified about new buffers available in a queue by
writing the queue index to register QueueNum.

2.3.2.3.3. Receiving Used Buffers From The Device
------------------------------------------------

The memory mapped virtio device is using single, dedicated
interrupt signal, which is raised when at least one of the
interrupts described in the InterruptStatus register
description is asserted. After receiving an interrupt, the
driver must read the InterruptStatus register to check what
caused the interrupt (see the register description). After the
interrupt is handled, the driver must acknowledge it by writing
a bit mask corresponding to the serviced interrupt to the
InterruptACK register.

2.3.2.4.4. Notification of Device Configuration Changes
------------------------------------------------------

This is indicated by bit 1 in the InterruptStatus register, as
documented in the register description.

2.4. Device Types
================

On top of the queues, config space and feature negotiation facilities
built into virtio, several specific devices are defined.

The following device IDs are used to identify different types of virtio
devices.  Some device IDs are reserved for devices which are not currently
defined in this standard.

Discovering what devices are available and their type is bus-dependent.

+------------+--------------------+
| Device ID  |   Virtio Device    |
+------------+--------------------+
+------------+--------------------+
| 0          | reserved (invalid) |
+------------+--------------------+
| 1          |   network card     |
+------------+--------------------+
| 2          |   block device     |
+------------+--------------------+
| 3          |      console       |
+------------+--------------------+
| 4          |  entropy source    |
+------------+--------------------+
| 5          | memory ballooning  |
+------------+--------------------+
| 6          |     ioMemory       |
+------------+--------------------+
| 7          |       rpmsg        |
+------------+--------------------+
| 8          |     SCSI host      |
+------------+--------------------+
| 9          |   9P transport     |
+------------+--------------------+
| 10         |   mac80211 wlan    |
+------------+--------------------+
| 11         |   rproc serial     |
+------------+--------------------+
| 12         |   virtio CAIF      |
+------------+--------------------+

2.4.1. Network Device
====================

The virtio network device is a virtual ethernet card, and is the
most complex of the devices supported so far by virtio. It has
enhanced rapidly and demonstrates clearly how support for new
features should be added to an existing device. Empty buffers are
placed in one virtqueue for receiving packets, and outgoing
packets are enqueued into another for transmission in that order.
A third command queue is used to control advanced filtering
features.

2.4.1.1. Device ID
-----------------

 1

2.4.1.2. Virtqueues
------------------

 0:receiveq. 1:transmitq. 2:controlq

 Virtqueue 2 only exists if VIRTIO_NET_F_CTRL_VQ set.

2.4.1.3. Feature bits
--------------------

  VIRTIO_NET_F_CSUM (0) Device handles packets with partial checksum

  VIRTIO_NET_F_GUEST_CSUM (1) Guest handles packets with partial checksum

  VIRTIO_NET_F_CTRL_GUEST_OFFLOADS (2) Control channel offloads
	reconfiguration support.

  VIRTIO_NET_F_MAC (5) Device has given MAC address.

  VIRTIO_NET_F_GSO (6) (Deprecated) device handles packets with
    any GSO type.[12]

  VIRTIO_NET_F_GUEST_TSO4 (7) Guest can receive TSOv4.

  VIRTIO_NET_F_GUEST_TSO6 (8) Guest can receive TSOv6.

  VIRTIO_NET_F_GUEST_ECN (9) Guest can receive TSO with ECN.

  VIRTIO_NET_F_GUEST_UFO (10) Guest can receive UFO.

  VIRTIO_NET_F_HOST_TSO4 (11) Device can receive TSOv4.

  VIRTIO_NET_F_HOST_TSO6 (12) Device can receive TSOv6.

  VIRTIO_NET_F_HOST_ECN (13) Device can receive TSO with ECN.

  VIRTIO_NET_F_HOST_UFO (14) Device can receive UFO.

  VIRTIO_NET_F_MRG_RXBUF (15) Guest can merge receive buffers.

  VIRTIO_NET_F_STATUS (16) Configuration status field is
    available.

  VIRTIO_NET_F_CTRL_VQ (17) Control channel is available.

  VIRTIO_NET_F_CTRL_RX (18) Control channel RX mode support.

  VIRTIO_NET_F_CTRL_VLAN (19) Control channel VLAN filtering.

  VIRTIO_NET_F_GUEST_ANNOUNCE(21) Guest can send gratuitous
    packets.

  Device configuration layout Two configuration fields are
  currently defined. The mac address field always exists (though
  is only valid if VIRTIO_NET_F_MAC is set), and the status field
  only exists if VIRTIO_NET_F_STATUS is set. Two read-only bits
  are currently defined for the status field:
  VIRTIO_NET_S_LINK_UP and VIRTIO_NET_S_ANNOUNCE.

	#define VIRTIO_NET_S_LINK_UP	1
	#define VIRTIO_NET_S_ANNOUNCE	2

	struct virtio_net_config {
		u8 mac[6];
		u16 status;
	};

2.4.1.4. Device Initialization
-----------------------------

1. The initialization routine should identify the receive and
  transmission virtqueues.

2. If the VIRTIO_NET_F_MAC feature bit is set, the configuration
  space “mac” entry indicates the “physical” address of the the
  network card, otherwise a private MAC address should be
  assigned. All guests are expected to negotiate this feature if
  it is set.

3. If the VIRTIO_NET_F_CTRL_VQ feature bit is negotiated,
  identify the control virtqueue.

4. If the VIRTIO_NET_F_STATUS feature bit is negotiated, the link
  status can be read from the bottom bit of the “status” config
  field. Otherwise, the link should be assumed active.

5. The receive virtqueue should be filled with receive buffers.
  This is described in detail below in “Setting Up Receive
  Buffers”.

6. A driver can indicate that it will generate checksumless
  packets by negotating the VIRTIO_NET_F_CSUM feature. This “
  checksum offload” is a common feature on modern network cards.

7. If that feature is negotiated[13], a driver can use TCP or UDP
  segmentation offload by negotiating the VIRTIO_NET_F_HOST_TSO4 (IPv4
  TCP), VIRTIO_NET_F_HOST_TSO6 (IPv6 TCP) and VIRTIO_NET_F_HOST_UFO
  (UDP fragmentation) features. It should not send TCP packets
  requiring segmentation offload which have the Explicit Congestion
  Notification bit set, unless the VIRTIO_NET_F_HOST_ECN feature is
  negotiated.[14]

8. The converse features are also available: a driver can save
  the virtual device some work by negotiating these features.[15]
   The VIRTIO_NET_F_GUEST_CSUM feature indicates that partially
  checksummed packets can be received, and if it can do that then
  the VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6,
  VIRTIO_NET_F_GUEST_UFO and VIRTIO_NET_F_GUEST_ECN are the input
  equivalents of the features described above. See “Receiving
  Packets” below.

2.4.1.5. Device Operation
------------------------

Packets are transmitted by placing them in the transmitq, and
buffers for incoming packets are placed in the receiveq. In each
case, the packet itself is preceeded by a header:

	struct virtio_net_hdr {
	#define VIRTIO_NET_HDR_F_NEEDS_CSUM    1
		u8 flags;
	#define VIRTIO_NET_HDR_GSO_NONE        0
	#define VIRTIO_NET_HDR_GSO_TCPV4       1
	#define VIRTIO_NET_HDR_GSO_UDP		 3
	#define VIRTIO_NET_HDR_GSO_TCPV6       4
	#define VIRTIO_NET_HDR_GSO_ECN      0x80
		u8 gso_type;
		u16 hdr_len;
		u16 gso_size;
		u16 csum_start;
		u16 csum_offset;
	/* Only if VIRTIO_NET_F_MRG_RXBUF: */
		u16 num_buffers;
	};

The controlq is used to control device features such as
filtering.

2.4.1.5.1. Packet Transmission
-----------------------------

Transmitting a single packet is simple, but varies depending on
the different features the driver negotiated.

1. If the driver negotiated VIRTIO_NET_F_CSUM, and the packet has
  not been fully checksummed, then the virtio_net_hdr's fields
  are set as follows. Otherwise, the packet must be fully
  checksummed, and flags is zero.

  • flags has the VIRTIO_NET_HDR_F_NEEDS_CSUM set,

  • csum_start is set to the offset within the packet to begin checksumming,
    and

  • csum_offset indicates how many bytes after the csum_start the
    new (16 bit ones' complement) checksum should be placed.[16]

2. If the driver negotiated
  VIRTIO_NET_F_HOST_TSO4, TSO6 or UFO, and the packet requires
  TCP segmentation or UDP fragmentation, then the “gso_type”
  field is set to VIRTIO_NET_HDR_GSO_TCPV4, TCPV6 or UDP.
  (Otherwise, it is set to VIRTIO_NET_HDR_GSO_NONE). In this
  case, packets larger than 1514 bytes can be transmitted: the
  metadata indicates how to replicate the packet header to cut it
  into smaller packets. The other gso fields are set:

  • hdr_len is a hint to the device as to how much of the header
    needs to be kept to copy into each packet, usually set to the
    length of the headers, including the transport header.[17]

  • gso_size is the maximum size of each packet beyond that
    header (ie. MSS).

  • If the driver negotiated the VIRTIO_NET_F_HOST_ECN feature,
    the VIRTIO_NET_HDR_GSO_ECN bit may be set in “gso_type” as
    well, indicating that the TCP packet has the ECN bit set.[18]

3. If the driver negotiated the VIRTIO_NET_F_MRG_RXBUF feature,
  the num_buffers field is set to zero.

4. The header and packet are added as one output buffer to the
  transmitq, and the device is notified of the new entry (see "2.4.1.4.
  Notifying The Device").[19]

2.4.1.5.1.1. Packet Transmission Interrupt
-----------------------------------------

Often a driver will suppress transmission interrupts using the
VRING_AVAIL_F_NO_INTERRUPT flag (see "2.4.2. Receiving Used Buffers From
The Device") and check for used packets in the transmit path of following
packets. However, it will still receive interrupts if the
VIRTIO_F_NOTIFY_ON_EMPTY feature is negotiated, indicating that
the transmission queue is completely emptied.

The normal behavior in this interrupt handler is to retrieve and
new descriptors from the used ring and free the corresponding
headers and packets.

2.4.1.5.2. Setting Up Receive Buffers

It is generally a good idea to keep the receive virtqueue as
fully populated as possible: if it runs out, network performance
will suffer.

If the VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6 or
VIRTIO_NET_F_GUEST_UFO features are used, the Guest will need to
accept packets of up to 65550 bytes long (the maximum size of a
TCP or UDP packet, plus the 14 byte ethernet header), otherwise
1514. bytes. So unless VIRTIO_NET_F_MRG_RXBUF is negotiated, every
buffer in the receive queue needs to be at least this length [20]

If VIRTIO_NET_F_MRG_RXBUF is negotiated, each buffer must be at
least the size of the struct virtio_net_hdr.

2.4.1.5.2.1. Packet Receive Interrupt
------------------------------------

When a packet is copied into a buffer in the receiveq, the
optimal path is to disable further interrupts for the receiveq
(see 2.2.2.2. Receiving Used Buffers From The Device) and process
packets until no more are found, then re-enable them.

Processing packet involves:

1. If the driver negotiated the VIRTIO_NET_F_MRG_RXBUF feature,
  then the “num_buffers” field indicates how many descriptors
  this packet is spread over (including this one). This allows
  receipt of large packets without having to allocate large
  buffers. In this case, there will be at least “num_buffers” in
  the used ring, and they should be chained together to form a
  single packet. The other buffers will not begin with a struct
  virtio_net_hdr.

2. If the VIRTIO_NET_F_MRG_RXBUF feature was not negotiated, or
  the “num_buffers” field is one, then the entire packet will be
  contained within this buffer, immediately following the struct
  virtio_net_hdr.

3. If the VIRTIO_NET_F_GUEST_CSUM feature was negotiated, the
  VIRTIO_NET_HDR_F_NEEDS_CSUM bit in the “flags” field may be
  set: if so, the checksum on the packet is incomplete and the “
  csum_start” and “csum_offset” fields indicate how to calculate
  it (see Packet Transmission point 1).

4. If the VIRTIO_NET_F_GUEST_TSO4, TSO6 or UFO options were
  negotiated, then the “gso_type” may be something other than
  VIRTIO_NET_HDR_GSO_NONE, and the “gso_size” field indicates the
  desired MSS (see Packet Transmission point 2).

2.4.1.5.3. Control Virtqueue
---------------------------

The driver uses the control virtqueue (if VIRTIO_NET_F_VTRL_VQ is
negotiated) to send commands to manipulate various features of
the device which would not easily map into the configuration
space.

All commands are of the following form:

	struct virtio_net_ctrl {
		u8 class;
		u8 command;
		u8 command-specific-data[];
		u8 ack;
	};

	/* ack values */
	#define VIRTIO_NET_OK     0
	#define VIRTIO_NET_ERR    1

The class, command and command-specific-data are set by the
driver, and the device sets the ack byte. There is little it can
do except issue a diagnostic if the ack byte is not
VIRTIO_NET_OK.

2.4.1.5.2.1. Packet Receive Filtering
------------------------------------

If the VIRTIO_NET_F_CTRL_RX feature is negotiated, the driver can
send control commands for promiscuous mode, multicast receiving,
and filtering of MAC addresses.

Note that in general, these commands are best-effort: unwanted
packets may still arrive.

Setting Promiscuous Mode

	#define VIRTIO_NET_CTRL_RX    0
	 #define VIRTIO_NET_CTRL_RX_PROMISC      0
	 #define VIRTIO_NET_CTRL_RX_ALLMULTI     1

The class VIRTIO_NET_CTRL_RX has two commands:
VIRTIO_NET_CTRL_RX_PROMISC turns promiscuous mode on and off, and
VIRTIO_NET_CTRL_RX_ALLMULTI turns all-multicast receive on and
off. The command-specific-data is one byte containing 0 (off) or
1 (on).

2.4.1.5.2.2. Setting MAC Address Filtering
-----------------------------------------

	struct virtio_net_ctrl_mac {
		u32 entries;
		u8 macs[entries][ETH_ALEN];
	};

	#define VIRTIO_NET_CTRL_MAC    1
	 #define VIRTIO_NET_CTRL_MAC_TABLE_SET        0

The device can filter incoming packets by any number of destination
MAC addresses.[21] This table is set using the class
VIRTIO_NET_CTRL_MAC and the command VIRTIO_NET_CTRL_MAC_TABLE_SET. The
command-specific-data is two variable length tables of 6-byte MAC
addresses. The first table contains unicast addresses, and the second
contains multicast addresses.

2.4.1.5.3.3. VLAN Filtering
--------------------------

If the driver negotiates the VIRTION_NET_F_CTRL_VLAN feature, it
can control a VLAN filter table in the device.

	#define VIRTIO_NET_CTRL_VLAN       2
	 #define VIRTIO_NET_CTRL_VLAN_ADD             0
	 #define VIRTIO_NET_CTRL_VLAN_DEL             1

Both the VIRTIO_NET_CTRL_VLAN_ADD and VIRTIO_NET_CTRL_VLAN_DEL
command take a 16-bit VLAN id as the command-specific-data.

2.4.1.5.3.4. Gratuitous Packet Sending
-------------------------------------

If the driver negotiates the VIRTIO_NET_F_GUEST_ANNOUNCE (depends
on VIRTIO_NET_F_CTRL_VQ), it can ask the guest to send gratuitous
packets; this is usually done after the guest has been physically
migrated, and needs to announce its presence on the new network
links. (As hypervisor does not have the knowledge of guest
network configuration (eg. tagged vlan) it is simplest to prod
the guest in this way).

	#define VIRTIO_NET_CTRL_ANNOUNCE       3
	 #define VIRTIO_NET_CTRL_ANNOUNCE_ACK             0

The Guest needs to check VIRTIO_NET_S_ANNOUNCE bit in status
field when it notices the changes of device configuration. The
command VIRTIO_NET_CTRL_ANNOUNCE_ACK is used to indicate that
driver has recevied the notification and device would clear the
VIRTIO_NET_S_ANNOUNCE bit in the status filed after it received
this command.

Processing this notification involves:

1. Sending the gratuitous packets or marking there are pending
  gratuitous packets to be sent and letting deferred routine to
  send them.

2. Sending VIRTIO_NET_CTRL_ANNOUNCE_ACK command through control
  vq.

2.4.1.5.3.5. Offloads State Configuration
-------------------------------------

If the VIRTIO_NET_F_CTRL_GUEST_OFFLOADS feature is negotiated, the driver can
send control commands for dynamic offloads state configuration.

2.4.1.5.4.3.1. Setting Offloads State
-------------------------------------

	u64 offloads;

	#define VIRTIO_NET_F_GUEST_CSUM       1
	#define VIRTIO_NET_F_GUEST_TSO4       7
	#define VIRTIO_NET_F_GUEST_TSO6       8
	#define VIRTIO_NET_F_GUEST_ECN        9
	#define VIRTIO_NET_F_GUEST_UFO        10

	#define VIRTIO_NET_CTRL_GUEST_OFFLOADS       5
	 #define VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET   0

The class VIRTIO_NET_CTRL_GUEST_OFFLOADS has one command:
VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET applies the new offloads configuration.

u64 value passed as command data is a bitmask, bits set define
offloads to be enabled, bits cleared - offloads to be disabled.

There is a corresponding device feature for each offload. Upon feature
negotiation corresponding offload gets enabled to preserve backward
compartibility.

Corresponding feature must be negotiated at startup in order to allow dynamic
change of specific offload state.


2.4.2. Block Device
==================

The virtio block device is a simple virtual block device (ie.
disk). Read and write requests (and other exotic requests) are
placed in the queue, and serviced (probably out of order) by the
device except where noted.

2.4.2.1. Device ID
-----------------
  2

2.4.2.2. Virtqueues
------------------
  0:requestq

2.4.2.3. Feature bits
--------------------

  VIRTIO_BLK_F_BARRIER (0) Host supports request barriers.

  VIRTIO_BLK_F_SIZE_MAX (1) Maximum size of any single segment is
    in “size_max”.

  VIRTIO_BLK_F_SEG_MAX (2) Maximum number of segments in a
    request is in “seg_max”.

  VIRTIO_BLK_F_GEOMETRY (4) Disk-style geometry specified in “
    geometry”.

  VIRTIO_BLK_F_RO (5) Device is read-only.

  VIRTIO_BLK_F_BLK_SIZE (6) Block size of disk is in “blk_size”.

  VIRTIO_BLK_F_SCSI (7) Device supports scsi packet commands.

  VIRTIO_BLK_F_FLUSH (9) Cache flush command support.

  Device configuration layout The capacity of the device
  (expressed in 512-byte sectors) is always present. The
  availability of the others all depend on various feature bits
  as indicated above.

	struct virtio_blk_config {
		u64 capacity;
		u32 size_max;
		u32 seg_max;
		struct virtio_blk_geometry {
			u16 cylinders;
			u8 heads;
			u8 sectors;
		} geometry;
		u32 blk_size;
	};

2.4.2.3. Device Initialization
-----------------------------

1. The device size should be read from the “capacity”
  configuration field. No requests should be submitted which goes
  beyond this limit.

2. If the VIRTIO_BLK_F_BLK_SIZE feature is negotiated, the
  blk_size field can be read to determine the optimal sector size
  for the driver to use. This does not effect the units used in
  the protocol (always 512 bytes), but awareness of the correct
  value can effect performance.

3. If the VIRTIO_BLK_F_RO feature is set by the device, any write
  requests will fail.

2.4.2.5. Device Operation
------------------------

The driver queues requests to the virtqueue, and they are used by
the device (not necessarily in order). Each request is of form:

	struct virtio_blk_req {
		u32 type;
		u32 ioprio;
		u64 sector;
		char data[][512];
		u8 status;
	};

If the device has VIRTIO_BLK_F_SCSI feature, it can also support
scsi packet command requests, each of these requests is of form:

	struct virtio_scsi_pc_req {
		u32 type;
		u32 ioprio;
		u64 sector;
		char cmd[];
		char data[][512];
#define SCSI_SENSE_BUFFERSIZE   96
		u8 sense[SCSI_SENSE_BUFFERSIZE];
		u32 errors;
		u32 data_len;
		u32 sense_len;
		u32 residual;
		u8 status;
	};

The type of the request is either a read (VIRTIO_BLK_T_IN), a write
(VIRTIO_BLK_T_OUT), a scsi packet command (VIRTIO_BLK_T_SCSI_CMD or
VIRTIO_BLK_T_SCSI_CMD_OUT[22]) or a flush (VIRTIO_BLK_T_FLUSH or
VIRTIO_BLK_T_FLUSH_OUT[23]). If the device has VIRTIO_BLK_F_BARRIER
feature the high bit (VIRTIO_BLK_T_BARRIER) indicates that this
request acts as a barrier and that all preceeding requests must be
complete before this one, and all following requests must not be
started until this is complete. Note that a barrier does not flush
caches in the underlying backend device in host, and thus does not
serve as data consistency guarantee. Driver must use FLUSH request to
flush the host cache.

	#define VIRTIO_BLK_T_IN           0
	#define VIRTIO_BLK_T_OUT          1
	#define VIRTIO_BLK_T_SCSI_CMD     2
	#define VIRTIO_BLK_T_SCSI_CMD_OUT 3
	#define VIRTIO_BLK_T_FLUSH        4
	#define VIRTIO_BLK_T_FLUSH_OUT    5
	#define VIRTIO_BLK_T_BARRIER	 0x80000000

The ioprio field is a hint about the relative priorities of
requests to the device: higher numbers indicate more important
requests.

The sector number indicates the offset (multiplied by 512) where
the read or write is to occur. This field is unused and set to 0
for scsi packet commands and for flush commands.

The cmd field is only present for scsi packet command requests,
and indicates the command to perform. This field must reside in a
single, separate read-only buffer; command length can be derived
from the length of this buffer.

Note that these first three (four for scsi packet commands)
fields are always read-only: the data field is either read-only
or write-only, depending on the request. The size of the read or
write can be derived from the total size of the request buffers.

The sense field is only present for scsi packet command requests,
and indicates the buffer for scsi sense data.

The data_len field is only present for scsi packet command
requests, this field is deprecated, and should be ignored by the
driver. Historically, devices copied data length there.

The sense_len field is only present for scsi packet command
requests and indicates the number of bytes actually written to
the sense buffer.

The residual field is only present for scsi packet command
requests and indicates the residual size, calculated as data
length - number of bytes actually transferred.

The final status byte is written by the device: either
VIRTIO_BLK_S_OK for success, VIRTIO_BLK_S_IOERR for host or guest
error or VIRTIO_BLK_S_UNSUPP for a request unsupported by host:

	#define VIRTIO_BLK_S_OK        0
	#define VIRTIO_BLK_S_IOERR     1
	#define VIRTIO_BLK_S_UNSUPP    2

2.4.2.5.1 Legacy Interface: Device Operation
------------------------

Historically, devices assumed that the fields type, ioprio and
sector reside in a single, separate read-only buffer; the fields
errors, data_len, sense_len and residual reside in a single,
separate write-only buffer; the sense field in a separate
write-only buffer of size 96 bytes, by itself; the fields errors,
data_len, sense_len and residual in a single write-only buffer;
and the status field is a separate read-only buffer of size 1
byte, by itself.


2.4.3. Console Device
====================

The virtio console device is a simple device for data input and
output. A device may have one or more ports. Each port has a pair
of input and output virtqueues. Moreover, a device has a pair of
control IO virtqueues. The control virtqueues are used to
communicate information between the device and the driver about
ports being opened and closed on either side of the connection,
indication from the host about whether a particular port is a
console port, adding new ports, port hot-plug/unplug, etc., and
indication from the guest about whether a port or a device was
successfully added, port open/close, etc.. For data IO, one or
more empty buffers are placed in the receive queue for incoming
data and outgoing characters are placed in the transmit queue.

2.4.3.1. Device ID
-----------------

  3

2.4.3.2. Virtqueues
------------------

   0:receiveq(port0). 1:transmitq(port0), 2:control receiveq, 3:control transmitq, 4:receiveq(port1), 5:transmitq(port1),
  ...

  Ports 2 onwards only exist if VIRTIO_CONSOLE_F_MULTIPORT is set.

2.4.3.3. Feature bits
--------------------

  VIRTIO_CONSOLE_F_SIZE (0) Configuration cols and rows fields
    are valid.

  VIRTIO_CONSOLE_F_MULTIPORT(1) Device has support for multiple
    ports; configuration fields nr_ports and max_nr_ports are
    valid and control virtqueues will be used.

2.4.3.4. Device configuration layout
-----------------------------------

  The size of the console is supplied
  in the configuration space if the VIRTIO_CONSOLE_F_SIZE feature
  is set. Furthermore, if the VIRTIO_CONSOLE_F_MULTIPORT feature
  is set, the maximum number of ports supported by the device can
  be fetched.

	struct virtio_console_config {
		u16 cols;
		u16 rows;
		u32 max_nr_ports;
	};

2.4.3.5. Device Initialization
-----------------------------

1. If the VIRTIO_CONSOLE_F_SIZE feature is negotiated, the driver
  can read the console dimensions from the configuration fields.

2. If the VIRTIO_CONSOLE_F_MULTIPORT feature is negotiated, the
  driver can spawn multiple ports, not all of which may be
  attached to a console. Some could be generic ports. In this
  case, the control virtqueues are enabled and according to the
  max_nr_ports configuration-space value, the appropriate number
  of virtqueues are created. A control message indicating the
  driver is ready is sent to the host. The host can then send
  control messages for adding new ports to the device. After
  creating and initializing each port, a
  VIRTIO_CONSOLE_PORT_READY control message is sent to the host
  for that port so the host can let us know of any additional
  configuration options set for that port.

3. The receiveq for each port is populated with one or more
  receive buffers.

2.4.3.6. Device Operation
------------------------

1. For output, a buffer containing the characters is placed in
  the port's transmitq.[24]

2. When a buffer is used in the receiveq (signalled by an
  interrupt), the contents is the input to the port associated
  with the virtqueue for which the notification was received.

3. If the driver negotiated the VIRTIO_CONSOLE_F_SIZE feature, a
  configuration change interrupt may occur. The updated size can
  be read from the configuration fields.

4. If the driver negotiated the VIRTIO_CONSOLE_F_MULTIPORT
  feature, active ports are announced by the host using the
  VIRTIO_CONSOLE_PORT_ADD control message. The same message is
  used for port hot-plug as well.

5. If the host specified a port `name', a sysfs attribute is
  created with the name filled in, so that udev rules can be
  written that can create a symlink from the port's name to the
  char device for port discovery by applications in the guest.

6. Changes to ports' state are effected by control messages.
  Appropriate action is taken on the port indicated in the
  control message. The layout of the structure of the control
  buffer and the events associated are:

	struct virtio_console_control {
		uint32_t id;    /* Port number */
		uint16_t event; /* The kind of control event */
		uint16_t value; /* Extra information for the event */
	};

	/* Some events for the internal messages (control packets) */
	#define VIRTIO_CONSOLE_DEVICE_READY     0
	#define VIRTIO_CONSOLE_PORT_ADD         1
	#define VIRTIO_CONSOLE_PORT_REMOVE      2
	#define VIRTIO_CONSOLE_PORT_READY       3
	#define VIRTIO_CONSOLE_CONSOLE_PORT     4
	#define VIRTIO_CONSOLE_RESIZE           5
	#define VIRTIO_CONSOLE_PORT_OPEN        6
	#define VIRTIO_CONSOLE_PORT_NAME        7

2.4.4. Entropy Device
====================

The virtio entropy device supplies high-quality randomness for
guest use.

2.4.4.1. Device ID
-----------------
  4

2.4.4.2. Virtqueues
------------------
  0:requestq.

2.4.4.3. Feature bits
--------------------
  None currently defined

2.4.4.4. Device configuration layout
-----------------------------------
  None currently defined.

2.4.4.5. Device Initialization
-----------------------------

1. The virtqueue is initialized

2.4.4.6. Device Operation
------------------------

When the driver requires random bytes, it places the descriptor
of one or more buffers in the queue. It will be completely filled
by random data by the device.

2.4.5. Memory Balloon Device
===========================

The virtio memory balloon device is a primitive device for
managing guest memory: the device asks for a certain amount of
memory, and the guest supplies it (or withdraws it, if the device
has more than it asks for). This allows the guest to adapt to
changes in allowance of underlying physical memory. If the
feature is negotiated, the device can also be used to communicate
guest memory statistics to the host.

2.4.5.1. Device ID
-----------------
  5

2.4.5.2. Virtqueues
------------------
  0:inflateq. 1:deflateq. 2:statsq.

  Virtqueue 2 only exists if VIRTIO_BALLON_F_STATS_VQ set.

2.4.5.3. Feature bits
--------------------
  VIRTIO_BALLOON_F_MUST_TELL_HOST (0) Host must be told before
    pages from the balloon are used.

  VIRTIO_BALLOON_F_STATS_VQ (1) A virtqueue for reporting guest
    memory statistics is present.

2.4.5.4. Device configuration layout
-----------------------------------
  Both fields of this configuration
  are always available. Note that they are little endian, despite
  convention that device fields are guest endian:

	struct virtio_balloon_config {
		u32 num_pages;
		u32 actual;
	};

2.4.5.5. Device Initialization
-----------------------------

1. The inflate and deflate virtqueues are identified.

2. If the VIRTIO_BALLOON_F_STATS_VQ feature bit is negotiated:

  (a) Identify the stats virtqueue.

  (b) Add one empty buffer to the stats virtqueue and notify the
    host.

Device operation begins immediately.

2.4.5.6. Device Operation
------------------------

Memory Ballooning The device is driven by the receipt of a
configuration change interrupt.

1. The “num_pages” configuration field is examined. If this is
  greater than the “actual” number of pages, memory must be given
  to the balloon. If it is less than the “actual” number of
  pages, memory may be taken back from the balloon for general
  use.

2. To supply memory to the balloon (aka. inflate):

  (a) The driver constructs an array of addresses of unused memory
    pages. These addresses are divided by 4096[25] and the descriptor
    describing the resulting 32-bit array is added to the inflateq.

3. To remove memory from the balloon (aka. deflate):

  (a) The driver constructs an array of addresses of memory pages
    it has previously given to the balloon, as described above.
    This descriptor is added to the deflateq.

  (b) If the VIRTIO_BALLOON_F_MUST_TELL_HOST feature is negotiated, the
    guest may not use these requested pages until that descriptor
    in the deflateq has been used by the device.

  (c) Otherwise, the guest may begin to re-use pages previously
    given to the balloon before the device has acknowledged their
    withdrawl. [26]

4. In either case, once the device has completed the inflation or
  deflation, the “actual” field of the configuration should be
  updated to reflect the new number of pages in the balloon.[27]

2.4.5.6.1. Memory Statistics
---------------------------

The stats virtqueue is atypical because communication is driven
by the device (not the driver). The channel becomes active at
driver initialization time when the driver adds an empty buffer
and notifies the device. A request for memory statistics proceeds
as follows:

1. The device pushes the buffer onto the used ring and sends an
  interrupt.

2. The driver pops the used buffer and discards it.

3. The driver collects memory statistics and writes them into a
  new buffer.

4. The driver adds the buffer to the virtqueue and notifies the
  device.

5. The device pops the buffer (retaining it to initiate a
  subsequent request) and consumes the statistics.

  Memory Statistics Format Each statistic consists of a 16 bit
  tag and a 64 bit value. Both quantities are represented in the
  native endian of the guest. All statistics are optional and the
  driver may choose which ones to supply. To guarantee backwards
  compatibility, unsupported statistics should be omitted.

	struct virtio_balloon_stat {
	#define VIRTIO_BALLOON_S_SWAP_IN  0
	#define VIRTIO_BALLOON_S_SWAP_OUT 1
	#define VIRTIO_BALLOON_S_MAJFLT   2
	#define VIRTIO_BALLOON_S_MINFLT   3
	#define VIRTIO_BALLOON_S_MEMFREE  4
	#define VIRTIO_BALLOON_S_MEMTOT   5
		u16 tag;
		u64 val;
	} __attribute__((packed));

2.4.5.6.2. Memory Statistics Tags
--------------------------------

  VIRTIO_BALLOON_S_SWAP_IN The amount of memory that has been
  swapped in (in bytes).

  VIRTIO_BALLOON_S_SWAP_OUT The amount of memory that has been
  swapped out to disk (in bytes).

  VIRTIO_BALLOON_S_MAJFLT The number of major page faults that
  have occurred.

  VIRTIO_BALLOON_S_MINFLT The number of minor page faults that
  have occurred.

  VIRTIO_BALLOON_S_MEMFREE The amount of memory not being used
  for any purpose (in bytes).

  VIRTIO_BALLOON_S_MEMTOT The total amount of memory available
  (in bytes).


2.4.6. SCSI Host Device
======================

The virtio SCSI host device groups together one or more virtual
logical units (such as disks), and allows communicating to them
using the SCSI protocol. An instance of the device represents a
SCSI host to which many targets and LUNs are attached.

The virtio SCSI device services two kinds of requests:

• command requests for a logical unit;

• task management functions related to a logical unit, target or
  command.

The device is also able to send out notifications about added and
removed logical units. Together, these capabilities provide a
SCSI transport protocol that uses virtqueues as the transfer
medium. In the transport protocol, the virtio driver acts as the
initiator, while the virtio SCSI host provides one or more
targets that receive and process the requests.

2.4.6.1. Device ID
-----------------
  8

2.4.6.2. Virtqueues
------------------
   0:controlq; 1:eventq; 2..n:request queues.

2.4.6.3. Feature bits
--------------------

  VIRTIO_SCSI_F_INOUT (0) A single request can include both
    read-only and write-only data buffers.

  VIRTIO_SCSI_F_HOTPLUG (1) The host should enable
    hot-plug/hot-unplug of new LUNs and targets on the SCSI bus.

  VIRTIO_SCSI_F_CHANGE (2) The host will report changes to LUN
    parameters via a VIRTIO_SCSI_T_PARAM_CHANGE event.

2.4.6.4. Device configuration layout
-----------------------------------

  All fields of this configuration are always available. sense_size
  and cdb_size are writable by the guest.

	struct virtio_scsi_config {
		u32 num_queues;
		u32 seg_max;
		u32 max_sectors;
		u32 cmd_per_lun;
		u32 event_info_size;
		u32 sense_size;
		u32 cdb_size;
		u16 max_channel;
		u16 max_target;
		u32 max_lun;
	};

  num_queues is the total number of request virtqueues exposed by
    the device. The driver is free to use only one request queue,
    or it can use more to achieve better performance.

  seg_max is the maximum number of segments that can be in a
    command. A bidirectional command can include seg_max input
    segments and seg_max output segments.

  max_sectors is a hint to the guest about the maximum transfer
    size it should use.

  cmd_per_lun is a hint to the guest about the maximum number of
    linked commands it should send to one LUN. The actual value
    to be used is the minimum of cmd_per_lun and the virtqueue
    size.

  event_info_size is the maximum size that the device will fill
    for buffers that the driver places in the eventq. The driver
    should always put buffers at least of this size. It is
    written by the device depending on the set of negotated
    features.

  sense_size is the maximum size of the sense data that the
    device will write. The default value is written by the device
    and will always be 96, but the driver can modify it. It is
    restored to the default when the device is reset.

  cdb_size is the maximum size of the CDB that the driver will
    write. The default value is written by the device and will
    always be 32, but the driver can likewise modify it. It is
    restored to the default when the device is reset.

  max_channel, max_target and max_lun can be used by the driver
    as hints to constrain scanning the logical units on the
    host.h

2.4.6.5. Device Initialization
-----------------------------

The initialization routine should first of all discover the
device's virtqueues.

If the driver uses the eventq, it should then place at least a
buffer in the eventq.

The driver can immediately issue requests (for example, INQUIRY
or REPORT LUNS) or task management functions (for example, I_T
RESET).

2.4.6.6. Device Operation
------------------------

Device operation consists of operating request queues, the control
queue and the event queue.

2.4.6.6.1. Device Operation: Request Queues
------------------------------------------

The driver queues requests to an arbitrary request queue, and
they are used by the device on that same queue. It is the
responsibility of the driver to ensure strict request ordering
for commands placed on different queues, because they will be
consumed with no order constraints.

Requests have the following format:

	struct virtio_scsi_req_cmd {
		// Read-only
		u8 lun[8];
		u64 id;
		u8 task_attr;
		u8 prio;
		u8 crn;
		char cdb[cdb_size];
		char dataout[];
		// Write-only part
		u32 sense_len;
		u32 residual;
		u16 status_qualifier;
		u8 status;
		u8 response;
		u8 sense[sense_size];
		char datain[];
	};


	/* command-specific response values */
	#define VIRTIO_SCSI_S_OK                0
	#define VIRTIO_SCSI_S_OVERRUN           1
	#define VIRTIO_SCSI_S_ABORTED           2
	#define VIRTIO_SCSI_S_BAD_TARGET        3
	#define VIRTIO_SCSI_S_RESET             4
	#define VIRTIO_SCSI_S_BUSY              5
	#define VIRTIO_SCSI_S_TRANSPORT_FAILURE 6
	#define VIRTIO_SCSI_S_TARGET_FAILURE    7
	#define VIRTIO_SCSI_S_NEXUS_FAILURE     8
	#define VIRTIO_SCSI_S_FAILURE           9

	/* task_attr */
	#define VIRTIO_SCSI_S_SIMPLE            0
	#define VIRTIO_SCSI_S_ORDERED           1
	#define VIRTIO_SCSI_S_HEAD              2
	#define VIRTIO_SCSI_S_ACA               3

The lun field addresses a target and logical unit in the
virtio-scsi device's SCSI domain. The only supported format for
the LUN field is: first byte set to 1, second byte set to target,
third and fourth byte representing a single level LUN structure,
followed by four zero bytes. With this representation, a
virtio-scsi device can serve up to 256 targets and 16384 LUNs per
target.

The id field is the command identifier (“tag”).

task_attr, prio and crn should be left to zero. task_attr defines
the task attribute as in the table above, but all task attributes
may be mapped to SIMPLE by the device; crn may also be provided
by clients, but is generally expected to be 0. The maximum CRN
value defined by the protocol is 255, since CRN is stored in an
8-bit integer.

All of these fields are defined in SAM. They are always
read-only, as are the cdb and dataout field. The cdb_size is
taken from the configuration space.

sense and subsequent fields are always write-only. The sense_len
field indicates the number of bytes actually written to the sense
buffer. The residual field indicates the residual size,
calculated as “data_length - number_of_transferred_bytes”, for
read or write operations. For bidirectional commands, the
number_of_transferred_bytes includes both read and written bytes.
A residual field that is less than the size of datain means that
the dataout field was processed entirely. A residual field that
exceeds the size of datain means that the dataout field was
processed partially and the datain field was not processed at
all.

The status byte is written by the device to be the status code as
defined in SAM.

The response byte is written by the device to be one of the
following:

  VIRTIO_SCSI_S_OK when the request was completed and the status
  byte is filled with a SCSI status code (not necessarily
  "GOOD").

  VIRTIO_SCSI_S_OVERRUN if the content of the CDB requires
  transferring more data than is available in the data buffers.

  VIRTIO_SCSI_S_ABORTED if the request was cancelled due to an
  ABORT TASK or ABORT TASK SET task management function.

  VIRTIO_SCSI_S_BAD_TARGET if the request was never processed
  because the target indicated by the lun field does not exist.

  VIRTIO_SCSI_S_RESET if the request was cancelled due to a bus
  or device reset (including a task management function).

  VIRTIO_SCSI_S_TRANSPORT_FAILURE if the request failed due to a
  problem in the connection between the host and the target
  (severed link).

  VIRTIO_SCSI_S_TARGET_FAILURE if the target is suffering a
  failure and the guest should not retry on other paths.

  VIRTIO_SCSI_S_NEXUS_FAILURE if the nexus is suffering a failure
  but retrying on other paths might yield a different result.

  VIRTIO_SCSI_S_BUSY if the request failed but retrying on the
  same path should work.

  VIRTIO_SCSI_S_FAILURE for other host or guest error. In
  particular, if neither dataout nor datain is empty, and the
  VIRTIO_SCSI_F_INOUT feature has not been negotiated, the
  request will be immediately returned with a response equal to
  VIRTIO_SCSI_S_FAILURE.

2.4.6.6.2. Device Operation: controlq
------------------------------------

The controlq is used for other SCSI transport operations.
Requests have the following format:

	struct virtio_scsi_ctrl {
		u32 type;
	...
		u8 response;
	};

	/* response values valid for all commands */
	#define VIRTIO_SCSI_S_OK                       0
	#define VIRTIO_SCSI_S_BAD_TARGET               3
	#define VIRTIO_SCSI_S_BUSY                     5
	#define VIRTIO_SCSI_S_TRANSPORT_FAILURE        6
	#define VIRTIO_SCSI_S_TARGET_FAILURE           7
	#define VIRTIO_SCSI_S_NEXUS_FAILURE            8
	#define VIRTIO_SCSI_S_FAILURE                  9
	#define VIRTIO_SCSI_S_INCORRECT_LUN            12

The type identifies the remaining fields.

The following commands are defined:

  Task management function
	#define VIRTIO_SCSI_T_TMF                      0

	#define VIRTIO_SCSI_T_TMF_ABORT_TASK           0
	#define VIRTIO_SCSI_T_TMF_ABORT_TASK_SET       1
	#define VIRTIO_SCSI_T_TMF_CLEAR_ACA            2
	#define VIRTIO_SCSI_T_TMF_CLEAR_TASK_SET       3
	#define VIRTIO_SCSI_T_TMF_I_T_NEXUS_RESET      4
	#define VIRTIO_SCSI_T_TMF_LOGICAL_UNIT_RESET   5
	#define VIRTIO_SCSI_T_TMF_QUERY_TASK           6
	#define VIRTIO_SCSI_T_TMF_QUERY_TASK_SET       7

	struct virtio_scsi_ctrl_tmf
	{
		// Read-only part
		u32 type;
		u32 subtype;
		u8 lun[8];
		u64 id;
		// Write-only part
		u8 response;
	}

	/* command-specific response values */
	#define VIRTIO_SCSI_S_FUNCTION_COMPLETE        0
	#define VIRTIO_SCSI_S_FUNCTION_SUCCEEDED       10
	#define VIRTIO_SCSI_S_FUNCTION_REJECTED        11

  The type is VIRTIO_SCSI_T_TMF; the subtype field defines. All
  fields except response are filled by the driver. The subtype
  field must always be specified and identifies the requested
  task management function.

  Other fields may be irrelevant for the requested TMF; if so,
  they are ignored but they should still be present. The lun
  field is in the same format specified for request queues; the
  single level LUN is ignored when the task management function
  addresses a whole I_T nexus. When relevant, the value of the id
  field is matched against the id values passed on the requestq.

  The outcome of the task management function is written by the
  device in the response field. The command-specific response
  values map 1-to-1 with those defined in SAM.

  Asynchronous notification query

	#define VIRTIO_SCSI_T_AN_QUERY                    1

	struct virtio_scsi_ctrl_an {
	    // Read-only part
	    u32 type;
	    u8  lun[8];
	    u32 event_requested;
	    // Write-only part
	    u32 event_actual;
	    u8  response;
	}

	#define VIRTIO_SCSI_EVT_ASYNC_OPERATIONAL_CHANGE  2
	#define VIRTIO_SCSI_EVT_ASYNC_POWER_MGMT          4
	#define VIRTIO_SCSI_EVT_ASYNC_EXTERNAL_REQUEST    8
	#define VIRTIO_SCSI_EVT_ASYNC_MEDIA_CHANGE        16
	#define VIRTIO_SCSI_EVT_ASYNC_MULTI_HOST          32
	#define VIRTIO_SCSI_EVT_ASYNC_DEVICE_BUSY         64

  By sending this command, the driver asks the device which
  events the given LUN can report, as described in paragraphs 6.6
  and A.6 of the SCSI MMC specification. The driver writes the
  events it is interested in into the event_requested; the device
  responds by writing the events that it supports into
  event_actual.

  The type is VIRTIO_SCSI_T_AN_QUERY. The lun and event_requested
  fields are written by the driver. The event_actual and response
  fields are written by the device.

  No command-specific values are defined for the response byte.

  Asynchronous notification subscription
	#define VIRTIO_SCSI_T_AN_SUBSCRIBE                2

	struct virtio_scsi_ctrl_an {
		// Read-only part
		u32 type;
		u8  lun[8];
		u32 event_requested;
		// Write-only part
		u32 event_actual;
		u8  response;
	}

  By sending this command, the driver asks the specified LUN to
  report events for its physical interface, again as described in
  the SCSI MMC specification. The driver writes the events it is
  interested in into the event_requested; the device responds by
  writing the events that it supports into event_actual.

  Event types are the same as for the asynchronous notification
  query message.

  The type is VIRTIO_SCSI_T_AN_SUBSCRIBE. The lun and
  event_requested fields are written by the driver. The
  event_actual and response fields are written by the device.

  No command-specific values are defined for the response byte.

2.4.6.6.3. Device Operation: eventq
----------------------------------

The eventq is used by the device to report information on logical
units that are attached to it. The driver should always leave a
few buffers ready in the eventq. In general, the device will not
queue events to cope with an empty eventq, and will end up
dropping events if it finds no buffer ready. However, when
reporting events for many LUNs (e.g. when a whole target
disappears), the device can throttle events to avoid dropping
them. For this reason, placing 10-15 buffers on the event queue
should be enough.

Buffers are placed in the eventq and filled by the device when
interesting events occur. The buffers should be strictly
write-only (device-filled) and the size of the buffers should be
at least the value given in the device's configuration
information.

Buffers returned by the device on the eventq will be referred to
as "events" in the rest of this section. Events have the
following format:

	#define VIRTIO_SCSI_T_EVENTS_MISSED   0x80000000

	struct virtio_scsi_event {
		// Write-only part
		u32 event;
		u8  lun[8];
		i32 reason;
	}

If bit 31 is set in the event field, the device failed to report
an event due to missing buffers. In this case, the driver should
poll the logical units for unit attention conditions, and/or do
whatever form of bus scan is appropriate for the guest operating
system.

The meaning of the reason field depends on the
contents of the event field. The following events are defined:

  No event
	#define VIRTIO_SCSI_T_NO_EVENT         0

  This event is fired in the following cases:

  • When the device detects in the eventq a buffer that is
    shorter than what is indicated in the configuration field, it
    might use it immediately and put this dummy value in the
    event field. A well-written driver will never observe this
    situation.

  • When events are dropped, the device may signal this event as
    soon as the drivers makes a buffer available, in order to
    request action from the driver. In this case, of course, this
    event will be reported with the VIRTIO_SCSI_T_EVENTS_MISSED
    flag.

  Transport reset
	#define VIRTIO_SCSI_T_TRANSPORT_RESET  1

	#define VIRTIO_SCSI_EVT_RESET_HARD         0
	#define VIRTIO_SCSI_EVT_RESET_RESCAN       1
	#define VIRTIO_SCSI_EVT_RESET_REMOVED      2

  By sending this event, the device signals that a logical unit
  on a target has been reset, including the case of a new device
  appearing or disappearing on the bus.The device fills in all
  fields. The event field is set to
  VIRTIO_SCSI_T_TRANSPORT_RESET. The lun field addresses a
  logical unit in the SCSI host.

  The reason value is one of the three #define values appearing
  above:

  • VIRTIO_SCSI_EVT_RESET_REMOVED (“LUN/target removed”) is used
    if the target or logical unit is no longer able to receive
    commands.

  • VIRTIO_SCSI_EVT_RESET_HARD (“LUN hard reset”) is used if the
    logical unit has been reset, but is still present.

  • VIRTIO_SCSI_EVT_RESET_RESCAN (“rescan LUN/target”) is used if
    a target or logical unit has just appeared on the device.

  The “removed” and “rescan” events, when sent for LUN 0, may
  apply to the entire target. After receiving them the driver
  should ask the initiator to rescan the target, in order to
  detect the case when an entire target has appeared or
  disappeared. These two events will never be reported unless the
  VIRTIO_SCSI_F_HOTPLUG feature was negotiated between the host
  and the guest.

  Events will also be reported via sense codes (this obviously
  does not apply to newly appeared buses or targets, since the
  application has never discovered them):

  • “LUN/target removed” maps to sense key ILLEGAL REQUEST, asc
    0x25, ascq 0x00 (LOGICAL UNIT NOT SUPPORTED)

  • “LUN hard reset” maps to sense key UNIT ATTENTION, asc 0x29
    (POWER ON, RESET OR BUS DEVICE RESET OCCURRED)

  • “rescan LUN/target” maps to sense key UNIT ATTENTION, asc
    0x3f, ascq 0x0e (REPORTED LUNS DATA HAS CHANGED)

  The preferred way to detect transport reset is always to use
  events, because sense codes are only seen by the driver when it
  sends a SCSI command to the logical unit or target. However, in
  case events are dropped, the initiator will still be able to
  synchronize with the actual state of the controller if the
  driver asks the initiator to rescan of the SCSI bus. During the
  rescan, the initiator will be able to observe the above sense
  codes, and it will process them as if it the driver had
  received the equivalent event.

  Asynchronous notification
	#define VIRTIO_SCSI_T_ASYNC_NOTIFY     2

  By sending this event, the device signals that an asynchronous
  event was fired from a physical interface.

  All fields are written by the device. The event field is set to
  VIRTIO_SCSI_T_ASYNC_NOTIFY. The lun field addresses a logical
  unit in the SCSI host. The reason field is a subset of the
  events that the driver has subscribed to via the "Asynchronous
  notification subscription" command.

  When dropped events are reported, the driver should poll for
  asynchronous events manually using SCSI commands.

  LUN parameter change
	#define VIRTIO_SCSI_T_PARAM_CHANGE  3

  By sending this event, the device signals that the configuration parameters
  (for example the capacity) of a logical unit have changed.
  The event field is set to VIRTIO_SCSI_T_PARAM_CHANGE.
  The lun field addresses a logical unit in the SCSI host.

  The same event is also reported as a unit attention condition.
  The reason field contains the additional sense code and additional sense code qualifier,
  respectively in bits 0..7 and 8..15.
  For example, a change in capacity will be reported as asc 0x2a, ascq 0x09
  (CAPACITY DATA HAS CHANGED).

  For MMC devices (inquiry type 5) there would be some overlap between this
  event and the asynchronous notification event.
  For simplicity, as of this version of the specification the host must
  never report this event for MMC devices.

2.5. Reserved Feature Bits
=========================

Currently there are five device-independent feature bits defined:

  VIRTIO_F_NOTIFY_ON_EMPTY (24) Negotiating this feature
  indicates that the driver wants an interrupt if the device runs
  out of available descriptors on a virtqueue, even though
  interrupts are suppressed using the VRING_AVAIL_F_NO_INTERRUPT
  flag or the used_event field. An example of this is the
  networking driver: it doesn't need to know every time a packet
  is transmitted, but it does need to free the transmitted
  packets a finite time after they are transmitted. It can avoid
  using a timer if the device interrupts it when all the packets
  are transmitted.

  VIRTIO_F_RING_INDIRECT_DESC (28) Negotiating this feature indicates
  that the driver can use descriptors with the VRING_DESC_F_INDIRECT
  flag set, as described in "2.1.4.3.1. Indirect Descriptors".

  VIRTIO_F_RING_EVENT_IDX(29) This feature enables the used_event
  and the avail_event fields. If set, it indicates that the
  device should ignore the flags field in the available ring
  structure. Instead, the used_event field in this structure is
  used by guest to suppress device interrupts. Further, the
  driver should ignore the flags field in the used ring
  structure. Instead, the avail_event field in this structure is
  used by the device to suppress notifications. If unset, the
  driver should ignore the used_event field; the device should
  ignore the avail_event field; the flags field is used

  VIRTIO_F_VERSION_1(32) This feature must be offered by any device
  compliant with this specification, and acknowledged by all device
  drivers.

In addition, bit 30 is used by qemu's implementation to check for experimental
early versions of virtio which did not perform correct feature negotiation,
and should not be used.

2.5.1 Legacy Interface: Reserved Feature Bits
--------------------------------------------

Legacy or transitional devices may offer the following:

VIRTIO_F_ANY_LAYOUT (27) This feature indicates that the device
  accepts arbitrary descriptor layouts, as described in Section
  "2.1.4.2.1. Legacy Interface: Message Framing".


2.6. virtio_ring.h
=================

#ifndef VIRTIO_RING_H
#define VIRTIO_RING_H
/* An interface for efficient virtio implementation.
 *
 * This header is BSD licensed so anyone can use the definitions
 * to implement compatible drivers/servers.
 *
 * Copyright 2007, 2009, IBM Corporation
 * Copyright 2011, Red Hat, Inc
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of IBM nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL IBM OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */
#include <stdint.h>

/* This marks a buffer as continuing via the next field. */
#define VRING_DESC_F_NEXT       1
/* This marks a buffer as write-only (otherwise read-only). */
#define VRING_DESC_F_WRITE      2
/* This means the buffer contains a list of buffer descriptors. */
#define VRING_DESC_F_INDIRECT   4

/* The Host uses this in used->flags to advise the Guest: don't kick me
 * when you add a buffer.  It's unreliable, so it's simply an
 * optimization.  Guest will still kick if it's out of buffers. */
#define VRING_USED_F_NO_NOTIFY  1
/* The Guest uses this in avail->flags to advise the Host: don't
 * interrupt me when you consume a buffer.  It's unreliable, so it's
 * simply an optimization.  */
#define VRING_AVAIL_F_NO_INTERRUPT      1

/* Support for indirect descriptors */
#define VIRTIO_RING_F_INDIRECT_DESC    28

/* Support for avail_idx and used_idx fields */
#define VIRTIO_RING_F_EVENT_IDX        29

/* Arbitrary descriptor layouts. */
#define VIRTIO_F_ANY_LAYOUT            27

/* Virtio ring descriptors: 16 bytes.
 * These can chain together via "next". */
struct vring_desc {
        /* Address (guest-physical). */
        uint64_t addr;
        /* Length. */
        uint32_t len;
        /* The flags as indicated above. */
        uint16_t flags;
        /* We chain unused descriptors via this, too */
        uint16_t next;
};

struct vring_avail {
        uint16_t flags;
        uint16_t idx;
        uint16_t ring[];
        /* Only if VIRTIO_RING_F_EVENT_IDX: uint16_t used_event; */
};

/* u32 is used here for ids for padding reasons. */
struct vring_used_elem {
        /* Index of start of used descriptor chain. */
        uint32_t id;
        /* Total length of the descriptor chain which was written to. */
        uint32_t len;
};

struct vring_used {
        uint16_t flags;
        uint16_t idx;
        struct vring_used_elem ring[];
        /* Only if VIRTIO_RING_F_EVENT_IDX: uint16_t avail_event; */
};

struct vring {
        unsigned int num;

        struct vring_desc *desc;
        struct vring_avail *avail;
        struct vring_used *used;
};

/* The standard layout for the ring is a continuous chunk of memory which
 * looks like this.  We assume num is a power of 2.
 *
 * struct vring {
 *      // The actual descriptors (16 bytes each)
 *      struct vring_desc desc[num];
 *
 *      // A ring of available descriptor heads with free-running index.
 *      __u16 avail_flags;
 *      __u16 avail_idx;
 *      __u16 available[num];
 *      __u16 used_event_idx; // Only if VIRTIO_RING_F_EVENT_IDX
 *
 *      // Padding to the next align boundary.
 *      char pad[];
 *
 *      // A ring of used descriptor heads with free-running index.
 *      __u16 used_flags;
 *      __u16 used_idx;
 *      struct vring_used_elem used[num];
 *      __u16 avail_event_idx; // Only if VIRTIO_RING_F_EVENT_IDX
 * };
 * Note: for virtio PCI, align is 4096.
 */
static inline void vring_init(struct vring *vr, unsigned int num, void *p,
                              unsigned long align)
{
        vr->num = num;
        vr->desc = p;
        vr->avail = p + num*sizeof(struct vring_desc);
        vr->used = (void *)(((unsigned long)&vr->avail->ring[num] + sizeof(uint16_t)
                              + align-1)
                            & ~(align - 1));
}

static inline unsigned vring_size(unsigned int num, unsigned long align)
{
        return ((sizeof(struct vring_desc)*num + sizeof(uint16_t)*(3+num)
                 + align - 1) & ~(align - 1))
                + sizeof(uint16_t)*3 + sizeof(struct vring_used_elem)*num;
}

static inline int vring_need_event(uint16_t event_idx, uint16_t new_idx, uint16_t old_idx)
{
         return (uint16_t)(new_idx - event_idx - 1) < (uint16_t)(new_idx - old_idx);
}

/* Get location of event indices (only with VIRTIO_RING_F_EVENT_IDX) */
static inline uint16_t *vring_used_event(struct vring *vr)
{
        /* For backwards compat, used event index is at *end* of avail ring. */
        return &vr->avail->ring[vr->num];
}

static inline uint16_t *vring_avail_event(struct vring *vr)
{
        /* For backwards compat, avail event index is at *end* of used ring. */
        return (uint16_t *)&vr->used->ring[vr->num];
}
#endif /* VIRTIO_RING_H */


2.7. Creating New Device Types
==============================

Various considerations are necessary when creating a new device
type.

2.7.1. How Many Virtqueues?
---------------------------

It is possible that a very simple device will operate entirely
through its configuration space, but most will need at least one
virtqueue in which it will place requests. A device with both
input and output (eg. console and network devices described here)
need two queues: one which the driver fills with buffers to
receive input, and one which the driver places buffers to
transmit output.

2.7.2. What Configuration Space Layout?
---------------------------------------

Configuration space should only be used for initialization-time
parameters.  It is a limited resource with no synchronization, so for
most uses it is better to use a virtqueue to update configuration
information (the network device does this for filtering,
otherwise the table in the config space could potentially be very
large).

2.7.3. What Device Number?
--------------------------

Currently device numbers are assigned quite freely: a simple
request mail to the author of this document or the Linux
virtualization mailing list[9] will be sufficient to secure a unique one.

Meanwhile for experimental drivers, use 65535 and work backwards.

2.7.4. How many MSI-X vectors?  (for PCI)
-----------------------------------------

Using the optional MSI-X capability devices can speed up
interrupt processing by removing the need to read ISR Status
register by guest driver (which might be an expensive operation),
reducing interrupt sharing between devices and queues within the
device, and handling interrupts from multiple CPUs. However, some
systems impose a limit (which might be as low as 256) on the
total number of MSI-X vectors that can be allocated to all
devices. Devices and/or device drivers should take this into
account, limiting the number of vectors used unless the device is
expected to cause a high volume of interrupts. Devices can
control the number of vectors used by limiting the MSI-X Table
Size or not presenting MSI-X capability in PCI configuration
space. Drivers can control this by mapping events to as small
number of vectors as possible, or disabling MSI-X capability
altogether.

2.7.5. Device Improvements
--------------------------

Any change to configuration space, or new virtqueues, or
behavioural changes, should be indicated by negotiation of a new
feature bit. This establishes clarity[11] and avoids future expansion problems.

Clusters of functionality which are always implemented together
can use a single bit, but if one feature makes sense without the
others they should not be gratuitously grouped together to
conserve feature bits. We can always extend the spec when the
first person needs more than 24 feature bits for their device.


FOOTNOTES:
==========

[1] This lack of page-sharing implies that the implementation of the
device (e.g. the hypervisor or host) needs full access to the
guest memory. Communication with untrusted parties (i.e.
inter-guest communication) requires copying.

[2] The Linux implementation further separates the PCI virtio code
from the specific virtio drivers: these drivers are shared with
the non-PCI implementations (currently lguest and S/390).

[3] The actual value within this range is ignored

[4] Historically, drivers have used the device before steps 5 and 6.
This is only allowed if the driver does not use any features
which would alter this early use of the device.

[5] ie. once you enable MSI-X on the device, the other fields move.
If you turn it off again, they move back!

[6] The 4096 is based on the x86 page size, but it's also large
enough to ensure that the separate parts of the virtqueue are on
separate cache lines.

[7] These fields are kept here because this is the only part of the
virtqueue written by the device

[8] The Linux drivers do this only for read-only buffers: for
write-only buffers, it is assumed that the driver is merely
trying to keep the receive buffer ring full, and no notification
of this expected condition is necessary.

[9] https://lists.linux-foundation.org/mailman/listinfo/virtualization

[11] Even if it does mean documenting design or implementation
mistakes!


[12] It was supposed to indicate segmentation offload support, but
upon further investigation it became clear that multiple bits
were required.

[13] ie. VIRTIO_NET_F_HOST_TSO* and VIRTIO_NET_F_HOST_UFO are
dependent on VIRTIO_NET_F_CSUM; a dvice which offers the offload
features must offer the checksum feature, and a driver which
accepts the offload features must accept the checksum feature.
Similar logic applies to the VIRTIO_NET_F_GUEST_TSO4 features
depending on VIRTIO_NET_F_GUEST_CSUM.

[14] This is a common restriction in real, older network cards.

[15] For example, a network packet transported between two guests on
the same system may not require checksumming at all, nor segmentation,
if both guests are amenable.

[16] For example, consider a partially checksummed TCP (IPv4) packet.
It will have a 14 byte ethernet header and 20 byte IP header
followed by the TCP header (with the TCP checksum field 16 bytes
into that header). csum_start will be 14+20 = 34 (the TCP
checksum includes the header), and csum_offset will be 16. The
value in the TCP checksum field should be initialized to the sum
of the TCP pseudo header, so that replacing it by the ones'
complement checksum of the TCP header and body will give the
correct result.

[17] Due to various bugs in implementations, this field is not useful
as a guarantee of the transport header size.

[18] This case is not handled by some older hardware, so is called out
specifically in the protocol.

[19] Note that the header will be two bytes longer for the
VIRTIO_NET_F_MRG_RXBUF case.

[20] Obviously each one can be split across multiple descriptor
elements.

[21] Since there are no guarentees, it can use a hash filter or
silently switch to allmulti or promiscuous mode if it is given too
many addresses.

[22] The SCSI_CMD and SCSI_CMD_OUT types are equivalent, the device
does not distinguish between them.

[23] The FLUSH and FLUSH_OUT types are equivalent, the device does not
distinguish between them

[24] Because this is high importance and low bandwidth, the current
Linux implementation polls for the buffer to be used, rather than
waiting for an interrupt, simplifying the implementation
significantly. However, for generic serial ports with the
O_NONBLOCK flag set, the polling limitation is relaxed and the
consumed buffers are freed upon the next write or poll call or
when a port is closed or hot-unplugged.

[25] This is historical, and independent of the guest page size

[26] In this case, deflation advice is merely a courtesy

[27] As updates to configuration space are not atomic, this field
isn't particularly reliable, but can be used to diagnose buggy guests.