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#LyX 2.0 created this file. For more info see http://www.lyx.org/
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\begin_document
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\author -1930653948 "Amos Kong"
\author -1728740334 "Sasha Levin" [email protected]
\author -875410574 "Pawel Moll"
\author -608949062 "Rusty Russell,,,"
\author -385801441 "Cornelia Huck" [email protected]
\author 460276516 "Dmitry Fleytman" [email protected]
\author 1112500848 "Rusty Russell" [email protected]
\author 1531152142 "Paolo Bonzini,,,"
\author 1717892615 "Alexey Zaytsev,,,"
\author 1986246365 "Michael S. Tsirkin"
\end_header
\begin_body
\begin_layout Title
Virtio PCI Card Specification
\begin_inset Newline newline
\end_inset
v0.9.5 DRAFT
\begin_inset Newline newline
\end_inset
-
\end_layout
\begin_layout Author
Rusty Russell <[email protected]> IBM Corporation (Editor)
\end_layout
\begin_layout Date
2012 May 7.
\end_layout
\begin_layout Chapter
Purpose and Description
\end_layout
\begin_layout Standard
This document describes the specifications of the
\begin_inset Quotes eld
\end_inset
virtio
\begin_inset Quotes erd
\end_inset
family of
\emph on
PCI
\emph default
\begin_inset CommandInset nomenclature
LatexCommand nomenclature
symbol "PCI"
description "Peripheral Component Interconnect; a common device bus. See\\\\http://en.wikipedia.org/wiki/Peripheral Component Interconnect"
\end_inset
devices.
These
\change_deleted 1112500848 1370222658
are
\change_unchanged
devices are found in
\emph on
virtual
\emph default
\emph on
environments
\begin_inset CommandInset nomenclature
LatexCommand nomenclature
symbol "virtualized"
description "Environments where access to hardware is restricted (and often emulated) by a hypervisor."
\end_inset
\emph default
, 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.
\end_layout
\begin_layout Standard
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.
\end_layout
\begin_layout Description
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.
\begin_inset Foot
status open
\begin_layout Plain Layout
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.
\end_layout
\end_inset
\end_layout
\begin_layout Description
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 lines.
\end_layout
\begin_layout Description
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.
\begin_inset Foot
status open
\begin_layout Plain Layout
The Linux implementation further separates the PCI virtio code from the
specific virtio drivers: these drivers are shared with the non-PCI implementati
ons (currently lguest and S/390).
\end_layout
\end_inset
\end_layout
\begin_layout Description
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.
\end_layout
\begin_layout Section
Virtqueues
\end_layout
\begin_layout Standard
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.
\end_layout
\begin_layout Standard
Each virtqueue occupies two or more physically-contiguous pages (defined,
for the purposes of this specification, as 4096 bytes), and consists of
three parts:
\end_layout
\begin_layout Standard
\begin_inset Tabular
<lyxtabular version="3" rows="1" columns="4">
<features tabularvalignment="middle">
<column alignment="center" valignment="top" width="0">
<column alignment="center" valignment="top" width="0">
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<column alignment="center" valignment="top" width="0">
<row>
<cell alignment="center" valignment="top" topline="true" bottomline="true" leftline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
Descriptor Table
\end_layout
\end_inset
</cell>
<cell multicolumn="1" alignment="center" valignment="top" topline="true" bottomline="true" leftline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
Available Ring
\begin_inset space ~
\end_inset
\begin_inset space ~
\end_inset
\begin_inset space ~
\end_inset
\begin_inset space ~
\end_inset
\begin_inset space ~
\end_inset
\emph on
(padding)
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\end_inset
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\begin_layout Plain Layout
\end_layout
\end_inset
</cell>
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\begin_layout Plain Layout
Used Ring
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\end_inset
</cell>
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\end_inset
\end_layout
\begin_layout Standard
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.
\end_layout
\begin_layout Chapter
Specification
\end_layout
\begin_layout Section
PCI Discovery
\end_layout
\begin_layout Standard
Any PCI device with Vendor ID 0x1AF4, and Device ID 0x1000 through 0x103F
inclusive is a virtio device
\begin_inset Foot
status open
\begin_layout Plain Layout
The actual value within this range is ignored
\end_layout
\end_inset
.
The device must also have a Revision ID of 0 to match this specification.
\end_layout
\begin_layout Standard
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).
\end_layout
\begin_layout Standard
\begin_inset Tabular
<lyxtabular version="3" rows="13" columns="3">
<features tabularvalignment="bottom">
<column alignment="center" valignment="top" width="0">
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Subsystem Device ID
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Virtio Device
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Specification
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1
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network card
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Appendix C
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2
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</cell>
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block device
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Appendix D
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3
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console
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Appendix E
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4
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entropy source
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Appendix F
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5
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memory ballooning
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Appendix G
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6
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ioMemory
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-
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7
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rpmsg
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Appendix H
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8
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SCSI host
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Appendix I
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9
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9P transport
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\change_inserted -1728740334 1365526571
Appendix J
\change_deleted -1728740334 1365526568
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mac80211 wlan
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\change_inserted -608949062 1347429535
rproc serial
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-
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\change_inserted 1112500848 1363749786
virtio CAIF
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</lyxtabular>
\end_inset
\end_layout
\begin_layout Section
Device Configuration
\end_layout
\begin_layout Standard
To configure the device, we use the first I/O region of the PCI device.
This contains a
\emph on
virtio header
\emph default
followed by a
\emph on
device-specific region.
\end_layout
\begin_layout Standard
There may be different widths of accesses to the I/O region; the
\begin_inset Quotes eld
\end_inset
natural
\begin_inset Quotes erd
\end_inset
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.
\end_layout
\begin_layout Standard
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).
\end_layout
\begin_layout Subsection
Device Initialization Sequence
\begin_inset CommandInset label
LatexCommand label
name "sub:Device-Initialization-Sequence"
\end_inset
\end_layout
\begin_layout Standard
We start with an overview of device initialization, then expand on the details
of the device and how each step is preformed.
\end_layout
\begin_layout Enumerate
Reset the device.
This is not required on initial start up.
\end_layout
\begin_layout Enumerate
The ACKNOWLEDGE status bit is set: we have noticed the device.
\end_layout
\begin_layout Enumerate
The DRIVER status bit is set: we know how to drive the device.
\end_layout
\begin_layout Enumerate
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.
\end_layout
\begin_layout Enumerate
The subset of Device Feature Bits understood by the driver is written to
the device.
\end_layout
\begin_layout Enumerate
The DRIVER_OK status bit is set.
\end_layout
\begin_layout Enumerate
The device can now be used (ie.
buffers added to the virtqueues)
\begin_inset Foot
status open
\begin_layout Plain Layout
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.
\end_layout
\end_inset
\end_layout
\begin_layout Standard
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).
\end_layout
\begin_layout Standard
We now cover the fields required for general setup in detail.
\end_layout
\begin_layout Subsection
Virtio Header
\end_layout
\begin_layout Standard
The virtio header looks as follows:
\end_layout
\begin_layout Standard
\begin_inset Tabular
<lyxtabular version="3" rows="4" columns="9">
<features tabularvalignment="middle">
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<column alignment="left" valignment="top" width="0">
<column alignment="left" valignment="top" width="0">
<column alignment="left" valignment="top" width="0">
<column alignment="left" valignment="top" width="0">
<column alignment="left" valignment="top" width="0">
<column alignment="left" valignment="top" width="0">
<column alignment="left" valignment="top" width="0">
<column alignment="left" valignment="top" width="0">
<row>
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\begin_inset Text
\begin_layout Plain Layout
Bits
\end_layout
\end_inset
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32
\end_layout
\end_inset
</cell>
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32
\end_layout
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32
\end_layout
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</cell>
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16
\end_layout
\end_inset
</cell>
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\begin_layout Plain Layout
16
\end_layout
\end_inset
</cell>
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16
\end_layout
\end_inset
</cell>
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8
\end_layout
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8
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</cell>
</row>
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\begin_layout Plain Layout
Read/Write
\end_layout
\end_inset
</cell>
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\begin_layout Plain Layout
R
\end_layout
\end_inset
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R+W
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\end_inset
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R+W
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\end_inset
</cell>
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R
\end_layout
\end_inset
</cell>