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Virtio PCI Card Specification
v0.9.1 DRAFT
Rusty Russell <>IBM Corporation (Editor)
2011 August 1.
Purpose and Description
This document describes the specifications of the “virtio” family
of PCI[LaTeX Command: nomenclature] devices. These are devices
are found in virtual environments[LaTeX Command: nomenclature],
yet by design they are not all that different from physical PCI
devices, and this document treats them as such. This allows the
guest to use standard PCI 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 PCI devices use normal PCI 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 PCI device.[footnote:
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.
Efficient: Virtio PCI 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
Standard: Virtio PCI makes no assumptions about the environment
in which it operates, beyond supporting PCI. In fact the virtio
devices specified in the appendices do not require PCI at all:
they have been implemented on non-PCI buses.[footnote:
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).
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.
The mechanism for bulk data transport on virtio PCI devices is
pretentiously called a virtqueue. Each device can have zero or
more virtqueues: for example, the network device has one for
transmit and one for receive.
Each virtqueue occupies two or more physically-contiguous pages
(defined, for the purposes of this specification, as 4096 bytes),
and consists of three parts:
| Descriptor Table | Available Ring (padding) | Used Ring |
When the driver wants to send buffers to the device, it puts them
in one or more slots in the descriptor table, and writes the
descriptor indices into the available ring. It then notifies the
device. When the device has finished with the buffers, it writes
the descriptors into the used ring, and sends an interrupt.
PCI Discovery
Any PCI device with Vendor ID 0x1AF4, and Device ID 0x1000
through 0x103F inclusive is a virtio device[footnote:
The actual value within this range is ignored
]. The device must also have a Revision ID of 0 to match this
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).
| Subsystem Device ID | Virtio Device | Specification |
| 1 | network card | Appendix C |
| 2 | block device | Appendix D |
| 3 | console | Appendix E |
| 4 | entropy source | Appendix F |
| 5 | memory ballooning | Appendix G |
| 6 | ioMemory | - |
| 9 | 9P transport | - |
Device Configuration
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
Device Initialization Sequence
We start with an overview of device initialization, then expand
on the details of the device and how each step is preformed.
Reset the device. This is not required on initial start up.
The ACKNOWLEDGE status bit is set: we have noticed the device.
The DRIVER status bit is set: we know how to drive the device.
Device-specific setup, including reading the Device Feature
Bits, discovery of virtqueues for the device, optional MSI-X
setup, and reading and possibly writing the virtio
configuration space.
The subset of Device Feature Bits understood by the driver is
written to the device.
The DRIVER_OK status bit is set.
The device can now be used (ie. buffers added to the
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.
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).
We now cover the fields required for general setup in detail.
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:
| Bits || 16 | 16 |
| Read/Write || R+W | R+W |
| Purpose || Configuration | Queue |
| (MSI-X) || Vector | Vector |
Finally, if feature bits (VIRTIO_F_FEATURES_HI) this is
immediately followed by two additional fields:
| Bits || 32 | 32
| Read/Write || R | R+W
| Purpose || Device | Guest
| || Features bits 32:63 | Features bits 32:63
Immediately following these general headers, there may be
device-specific headers:
| Bits || Device Specific |
| Read/Write || Device Specific |
| Purpose || Device Specific... |
| || |
Device Status
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.
The device can be reset by writing a 0 to this field, 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
DRIVER_OK (3) Indicates that the driver is set up and ready to
drive the device.
FAILED (8) 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.
Feature Bits
The least significant 31 bits of the first configuration field
indicates the features that the device supports (the high bit is
reserved, and will be used to indicate the presence of future
feature bits elsewhere). If more than 31 feature bits are
supported, the device indicates so by setting feature bit 31 (see
[cha:Reserved-Feature-Bits]). The bits are allocated as follows:
0 to 23 Feature bits for the specific device type
24 to 40 Feature bits reserved for extensions to the queue and
feature negotiation mechanisms
41 to 63 Feature bits reserved 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
The feature bits are negotiated: the device lists all the
features it understands in the Device Features field, and the
guest writes the subset that it understands into the Guest
Features field. The only way to renegotiate is to reset the
In particular, new fields in the device configuration header are
indicated by offering a feature bit, so the guest can check
before accessing that part of the configuration space.
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 Guest Features field and it
can go into backwards compatibility mode. Similarly, if a guest
is enhanced with a feature that the device doesn't support, it
will not see that feature bit in the Device Features field and
can go into backwards compatibility mode (or, for poor
implementations, set the FAILED Device Status bit).
Access to feature bits 32 to 63 is enabled by Guest by setting
feature bit 31. If this bit is unset, Device must assume that all
feature bits > 31 are unset.
Configuration/Queue Vectors
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
/* 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.
Virtqueue Configuration
As a device can have zero or more virtqueues for bulk data
transport (for example, the network driver has two), the driver
needs to configure them as part of the device-specific
This is done as follows, for each virtqueue a device has:
Write the virtqueue index (first queue is 0) to the Queue
Select field.
Read the virtqueue size from the Queue Size field, which is
always a power of 2. This controls how big the virtqueue is
(see below). If this field is 0, the virtqueue does not exist.
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.[footnote:
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.
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.
The Queue Size field controls the total number of bytes required
for the virtqueue according to the following formula:
#define ALIGN(x) (((x) + 4095) & ~4095)
static inline unsigned vring_size(unsigned int qsz)
return ALIGN(sizeof(struct vring_desc)*qsz + sizeof(u16)*(2
+ qsz))
+ ALIGN(sizeof(struct vring_used_elem)*qsz);
This currently wastes some space with padding, but also allows
future extensions. The virtqueue layout structure looks like this
(qsz is the Queue Size field, which is a variable, so this code
won't compile):
struct vring {
/* The actual descriptors (16 bytes each) */
struct vring_desc desc[qsz];
/* A ring of available descriptor heads with free-running
index. */
struct vring_avail avail;
// Padding to the next 4096 boundary.
char pad[];
// A ring of used descriptor heads with free-running index.
struct vring_used used;
A Note on Virtqueue Endianness
Note that the endian of these fields and everything else 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
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. */
/* This marks a buffer as write-only (otherwise read-only). */
/* This means the buffer contains a list of buffer descriptors.
/* The flags as indicated above. */
u16 flags;
/* Next field if flags & NEXT */
u16 next;
The number of descriptors in the table is specified by the Queue
Size field for this virtqueue.
<sub:Indirect-Descriptors>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 [cha: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&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&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&INDIRECT must be off). A single indirect descriptor
table can include both read-only and write-only descriptors;
write-only flag (flags&WRITE) in the descriptor that refers to it
is ignored.
Available Ring
The available ring refers to what descriptors we are offering the
device: it 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 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 [cha: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 ring size). This starts at 0, and increases.
struct vring_avail {
u16 flags;
u16 idx;
u16 ring[qsz]; /* qsz is the Queue Size field read from device
u16 used_event;
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 [cha:Reserved-Feature-Bits]
). [footnote:
These fields are kept here because this is the only part of the
virtqueue written by the device
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 {
u16 flags;
u16 idx;
struct vring_used_elem ring[qsz];
u16 avail_event;
Helpers for Managing 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 Appendix A.
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 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.
Supplying Buffers to The Device
Actual transfer of buffers from the guest OS to the device
operates as follows:
Place the buffer(s) into free descriptor(s).
If there are no free descriptors, the guest may choose to
notify the device even if notifications are suppressed (to
reduce latency).[footnote:
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.
Place the id of the buffer in the next ring entry of the
available ring.
The steps (1) and (2) may be performed repeatedly if batching
is possible.
A memory barrier should be executed to ensure the device sees
the updated descriptor table and available ring before the next
The available “idx” field should be increased by the number of
entries added to the available ring.
A memory barrier should be executed to ensure that we update
the idx field before checking for notification suppression.
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.
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:
Get the next free descriptor table entry, d
Set d.addr to the physical address of the start of b
Set d.len to the length of b.
If b is write-only, set d.flags to VRING_DESC_F_WRITE,
otherwise 0.
If there is a buffer element after this:
Set to the index of the next free descriptor element.
Set the VRING_DESC_F_NEXT bit in d.flags.
In practice, the fields are usually used to chain free
descriptors, and a separate count kept to check there are enough
free descriptors before beginning the mappings.
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 descriptors 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;
Updating The Index Field
Once the idx field of the virtqueue is updated, the device will
be able to access the descriptor entries we've created and the
memory they refer to. This is why a memory barrier is generally
used before the idx update, to ensure it sees the most up-to-date
The idx field always increments, and we let it wrap naturally at
avail->idx += added;
<sub:Notifying-The-Device>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. This can be expensive,
however, so the device can suppress such notifications if it
doesn't need them. We have to be careful to expose the new idx
value before checking the suppression flag: 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 write to
the PCI configuration space.
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:
(u16)(new_idx - avail_event - 1) < (u16)(new_idx - old_idx)
<sub:Receiving-Used-Buffers>Receiving Used Buffers From The
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
Write the head descriptor number to the next field in the used
Update the used ring idx.
Determine whether an interrupt is necessary:
If the VIRTIO_F_RING_EVENT_IDX feature is not negotiated: check
if f the VRING_AVAIL_F_NO_INTERRUPT flag is not set in avail-
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)
If an interrupt is necessary:
If MSI-X capability is disabled:
Set the lower bit of the ISR Status field for the device.
Send the appropriate PCI interrupt for the device.
If MSI-X capability is enabled:
Request the appropriate MSI-X interrupt message for the
device, Queue Vector field sets the MSI-X Table entry
If Queue Vector field value is NO_VECTOR, no interrupt
message is requested for this event.
The guest interrupt handler should:
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.
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.
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, 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:
for (;;) {
if (vq->last_seen_used != vring->used.idx) {
if (vq->last_seen_used != vring->used.idx)
struct vring_used_elem *e =
Dealing With 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:
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.
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.
Creating New Device Types
Various considerations are necessary when creating a new device
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.
What Configuration Space Layout?
Configuration space is generally used for rarely-changing or
initialization-time parameters. But it is a limited resource, so
it might be 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
Note that this space is generally the guest's native endian,
rather than PCI's little-endian.
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[footnote:
] will be sufficient to secure a unique one.
Meanwhile for experimental drivers, use 65535 and work backwards.
How many MSI-X vectors?
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
Message Framing
The descriptors used for a buffer should not effect the semantics
of the message, except for the total length of the buffer. For
example, a network buffer consists of a 10 byte header followed
by the network packet. Whether this is presented in the ring
descriptor chain as (say) a 10 byte buffer and a 1514 byte
buffer, or a single 1524 byte buffer, or even three buffers,
should have no effect.
In particular, no implementation should use the descriptor
boundaries to determine the size of any header in a request.[footnote:
The current qemu device implementations mistakenly insist that
the first descriptor cover the header in these cases exactly, so
a cautious driver should arrange it so.
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[footnote:
Even if it does mean documenting design or implementation
] 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.
[LaTeX Command: printnomenclature]
Appendix A: 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
* modification, are permitted provided that the following
* are met:
* 1. Redistributions of source code must retain the above
* notice, this list of conditions and the following
* 2. Redistributions in binary form must reproduce the above
* notice, this list of conditions and the following
disclaimer in the
* documentation and/or other materials provided with the
* 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 marks a buffer as continuing via the next field. */
/* This marks a buffer as write-only (otherwise read-only). */
/* 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.
/* The Guest uses this in avail->flags to advise the Host: don't
* interrupt me when you consume a buffer. It's unreliable, so
* simply an optimization. */
/* 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[];
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[];
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
* __u16 avail_flags;
* __u16 avail_idx;
* __u16 available[num];
* // Padding to the next align boundary.
* char pad[];
* // A ring of used descriptor heads with free-running
* __u16 used_flags;
* __u16 EVENT_IDX;
* struct vring_used_elem used[num];
* };
* 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]
+ align-1)
& ~(align - 1));
static inline unsigned vring_size(unsigned int num, unsigned long
return ((sizeof(struct vring_desc)*num +
+ align - 1) & ~(align - 1))
+ sizeof(uint16_t)*3 + sizeof(struct
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);
#endif /* VIRTIO_RING_H */
<cha:Reserved-Feature-Bits>Appendix B: 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 [sub: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_BAD_FEATURE(30) This feature should never be
negotiated by the guest; doing so is an indication that the
guest is faulty[footnote:
An experimental virtio PCI driver contained in Linux version
2.6.25 had this problem, and this feature bit can be used to
detect it.
VIRTIO_F_FEATURES_HIGH(31) This feature indicates that the
device supports feature bits 32:63. If unset, feature bits
32:63 are unset.
Appendix C: 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
Subsystem Device ID 1
Virtqueues 0:receiveq. 1:transmitq. 2:controlq[footnote:
Feature bits
VIRTIO_NET_F_CSUM (0) Device handles packets with partial
VIRTIO_NET_F_GUEST_CSUM (1) Guest handles packets with partial
VIRTIO_NET_F_MAC (5) Device has given MAC address.
VIRTIO_NET_F_GSO (6) (Deprecated) device handles packets with
any GSO type.[footnote:
It was supposed to indicate segmentation offload support, but
upon further investigation it became clear that multiple bits
were required.
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
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.
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. Only one bit is
currently defined for the status field: VIRTIO_NET_S_LINK_UP. #define VIRTIO_NET_S_LINK_UP 1
struct virtio_net_config {
u8 mac[6];
u16 status;
Device Initialization
The initialization routine should identify the receive and
transmission virtqueues.
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.
If the VIRTIO_NET_F_CTRL_VQ feature bit is negotiated, identify
the control virtqueue.
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.
The receive virtqueue should be filled with receive buffers.
This is described in detail below in “Setting Up Receive
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.
If that feature is negotiated, a driver can use TCP or UDP
segmentation offload by negotiating the VIRTIO_NET_F_HOST_TSO4
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.[footnote:
This is a common restriction in real, older network cards.
The converse features are also available: a driver can save the
virtual device some work by negotiating these features.[footnote:
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.
] The VIRTIO_NET_F_GUEST_CSUM feature indicates that partially
checksummed packets can be received, and if it can do that then
equivalents of the features described above. See “Receiving
Packets” below.
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 {
u8 flags;
u8 gso_type;
u16 hdr_len;
u16 gso_size;
u16 csum_start;
u16 csum_offset;
u16 num_buffers
The controlq is used to control device features such as
Packet Transmission
Transmitting a single packet is simple, but varies depending on
the different features the driver negotiated.
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,
<ite:csum_start-is-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.[footnote:
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 will be 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.
<enu:If-the-driver>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.[footnote:
Due to various bugs in implementations, this field is not useful
as a guarantee of the transport header size.
gso_size is the size of the packet beyond that header (ie.
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.[footnote:
This case is not handled by some older hardware, so is called out
specifically in the protocol.
If the driver negotiated the VIRTIO_NET_F_MRG_RXBUF feature,
the num_buffers field is set to zero.
The header and packet are added as one output buffer to the
transmitq, and the device is notified of the new entry (see [sub:Notifying-The-Device]
Note that the header will be two bytes longer for the
Packet Transmission Interrupt
Often a driver will suppress transmission interrupts using the
VRING_AVAIL_F_NO_INTERRUPT flag (see [sub:Receiving-Used-Buffers]
) 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.
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.
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 [footnote:
Obviously each one can be split across multiple descriptor
If VIRTIO_NET_F_MRG_RXBUF is negotiated, each buffer must be at
least the size of the struct virtio_net_hdr.
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 [sub:Receiving-Used-Buffers]) and process packets until no
more are found, then re-enable them.
Processing packet involves:
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
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
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 [ite:csum_start-is-set]).
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 [enu:If-the-driver]).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
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
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
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).
Setting MAC Address Filtering
struct virtio_net_ctrl_mac {
u32 entries;
u8 macs[entries][ETH_ALEN];
The device can filter incoming packets by any number of
destination MAC addresses.[footnote:
Since there are no guarentees, it can use a hash filter
orsilently switch to allmulti or promiscuous mode if it is given
too many addresses.
] 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.
VLAN Filtering
If the driver negotiates the VIRTION_NET_F_CTRL_VLAN feature, it
can control a VLAN filter table in the device.
command take a 16-bit VLAN id as the command-specific-data.
Appendix D: 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.
Subsystem Device ID 2
Virtqueues 0:requestq.
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 “
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;
Device Initialization
The device size should be read from the “capacity”
configuration field. No requests should be submitted which goes
beyond this limit.
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.
If the VIRTIO_BLK_F_RO feature is set by the device, any write
requests will fail.
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];
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
the SCSI_CMD and SCSI_CMD_OUT types are equivalent, the device
does not distinguish between them
]) or a flush (VIRTIO_BLK_T_FLUSH or VIRTIO_BLK_T_FLUSH_OUT[footnote:
the FLUSH and FLUSH_OUT types are equivalent, the device does not
distinguish between them
]). 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_BARRIER 0x80000000
The ioprio field is a hint about the relative priorities of
requests to the device: higher numbers indicate more important
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
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.
Appendix E: 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.
Subsystem Device ID 3
Virtqueues 0:receiveq(port0). 1:transmitq(port0), 2:control
Ports 2 onwards only if VIRTIO_CONSOLE_F_MULTIPORT is set
], 3:control transmitq, 4:receiveq(port1), 5:transmitq(port1),
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.
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;
Device Initialization
If the VIRTIO_CONSOLE_F_SIZE feature is negotiated, the driver
can read the console dimensions from the configuration fields.
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.
The receiveq for each port is populated with one or more
receive buffers.
Device Operation
For output, a buffer containing the characters is placed in the
port's transmitq.[footnote:
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.
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.
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.
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.
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.
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) */
Appendix F: Entropy Device
The virtio entropy device supplies high-quality randomness for
guest use.
Subsystem Device ID 4
Virtqueues 0:requestq.
Feature bits None currently defined
Device configuration layout None currently defined.
Device Initialization
The virtqueue is initialized
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.
Appendix G: 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.
Subsystem Device ID 5
Virtqueues 0:inflateq. 1:deflateq. 2:statsq.[footnote:
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.
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;
Device Initialization
The inflate and deflate virtqueues are identified.
If the VIRTIO_BALLOON_F_STATS_VQ feature bit is negotiated:
Identify the stats virtqueue.
Add one empty buffer to the stats virtqueue and notify the
Device operation begins immediately.
Device Operation
Memory Ballooning The device is driven by the receipt of a
configuration change interrupt.
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
To supply memory to the balloon (aka. inflate):
The driver constructs an array of addresses of unused memory
pages. These addresses are divided by 4096[footnote:
This is historical, and independent of the guest page size
] and the descriptor describing the resulting 32-bit array is
added to the inflateq.
To remove memory from the balloon (aka. deflate):
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.
If the VIRTIO_BALLOON_F_MUST_TELL_HOST feature is set, the
guest may not use these requested pages until that descriptor
in the deflateq has been used by the device.
Otherwise, the guest may begin to re-use pages previously given
to the balloon before the device has acknowledged their
withdrawl. [footnote:
In this case, deflation advice is merely a courtesy
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.[footnote:
As updates to configuration space are not atomic, this field
isn't particularly reliable, but can be used to diagnose buggy
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:
The device pushes the buffer onto the used ring and sends an
The driver pops the used buffer and discards it.
The driver collects memory statistics and writes them into a
new buffer.
The driver adds the buffer to the virtqueue and notifies the
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 {
u16 tag;
u64 val;
} __attribute__((packed));
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).