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Vhost-user-gpu Protocol
:Licence: This work is licensed under the terms of the GNU GPL,
version 2 or later. See the COPYING file in the top-level
.. contents:: Table of Contents
The vhost-user-gpu protocol is aiming at sharing the rendering result
of a virtio-gpu, done from a vhost-user slave process to a vhost-user
master process (such as QEMU). It bears a resemblance to a display
server protocol, if you consider QEMU as the display server and the
slave as the client, but in a very limited way. Typically, it will
work by setting a scanout/display configuration, before sending flush
events for the display updates. It will also update the cursor shape
and position.
The protocol is sent over a UNIX domain stream socket, since it uses
socket ancillary data to share opened file descriptors (DMABUF fds or
shared memory). The socket is usually obtained via
Requests are sent by the *slave*, and the optional replies by the
Wire format
Unless specified differently, numbers are in the machine native byte
A vhost-user-gpu message (request and reply) consists of 3 header
fields and a payload.
| request | flags | size | payload |
:request: ``u32``, type of the request
:flags: ``u32``, 32-bit bit field:
- Bit 2 is the reply flag - needs to be set on each reply
:size: ``u32``, size of the payload
Payload types
Depending on the request type, **payload** can be:
| scanout-id | x | y |
:scanout-id: ``u32``, the scanout where the cursor is located
:x/y: ``u32``, the cursor postion
| pos | hot_x | hot_y | cursor |
:pos: a ``VhostUserGpuCursorPos``, the cursor location
:hot_x/hot_y: ``u32``, the cursor hot location
:cursor: ``[u32; 64 * 64]``, 64x64 RGBA cursor data (PIXMAN_a8r8g8b8 format)
| scanout-id | w | h |
:scanout-id: ``u32``, the scanout configuration to set
:w/h: ``u32``, the scanout width/height size
| scanout-id | x | y | w | h | data |
:scanout-id: ``u32``, the scanout content to update
:x/y/w/h: ``u32``, region of the update
:data: RGB data (PIXMAN_x8r8g8b8 format)
| scanout-id | x | y | w | h | fdw | fwh | stride | flags | fourcc |
:scanout-id: ``u32``, the scanout configuration to set
:x/y: ``u32``, the location of the scanout within the DMABUF
:w/h: ``u32``, the scanout width/height size
:fdw/fdh/stride/flags: ``u32``, the DMABUF width/height/stride/flags
:fourcc: ``i32``, the DMABUF fourcc
C structure
In QEMU the vhost-user-gpu message is implemented with the following struct:
.. code:: c
typedef struct VhostUserGpuMsg {
uint32_t request; /* VhostUserGpuRequest */
uint32_t flags;
uint32_t size; /* the following payload size */
union {
VhostUserGpuCursorPos cursor_pos;
VhostUserGpuCursorUpdate cursor_update;
VhostUserGpuScanout scanout;
VhostUserGpuUpdate update;
VhostUserGpuDMABUFScanout dmabuf_scanout;
struct virtio_gpu_resp_display_info display_info;
uint64_t u64;
} payload;
} QEMU_PACKED VhostUserGpuMsg;
Protocol features
None yet.
As the protocol may need to evolve, new messages and communication
changes are negotiated thanks to preliminary
Message types
:id: 1
:request payload: N/A
:reply payload: ``u64``
Get the supported protocol features bitmask.
:id: 2
:request payload: ``u64``
:reply payload: N/A
Enable protocol features using a bitmask.
:id: 3
:request payload: N/A
:reply payload: ``struct virtio_gpu_resp_display_info`` (from virtio specification)
Get the preferred display configuration.
:id: 4
:request payload: ``VhostUserGpuCursorPos``
:reply payload: N/A
Set/show the cursor position.
:id: 5
:request payload: ``VhostUserGpuCursorPos``
:reply payload: N/A
Set/hide the cursor.
:id: 6
:request payload: ``VhostUserGpuCursorUpdate``
:reply payload: N/A
Update the cursor shape and location.
:id: 7
:request payload: ``VhostUserGpuScanout``
:reply payload: N/A
Set the scanout resolution. To disable a scanout, the dimensions
width/height are set to 0.
:id: 8
:request payload: ``VhostUserGpuUpdate``
:reply payload: N/A
Update the scanout content. The data payload contains the graphical bits.
The display should be flushed and presented.
:id: 9
:request payload: ``VhostUserGpuDMABUFScanout``
:reply payload: N/A
Set the scanout resolution/configuration, and share a DMABUF file
descriptor for the scanout content, which is passed as ancillary
data. To disable a scanout, the dimensions width/height are set
to 0, there is no file descriptor passed.
:id: 10
:request payload: ``VhostUserGpuUpdate``
:reply payload: empty payload
The display should be flushed and presented according to updated
region from ``VhostUserGpuUpdate``.
Note: there is no data payload, since the scanout is shared thanks
to DMABUF, that must have been set previously with