=======================
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
directory.
.. contents:: Table of Contents
Introduction
============
The vhost-user-gpu protocol is aiming at sharing the rendering result
of a virtio-gpu, done from a vhost-user back-end process to a vhost-user
front-end process (such as QEMU). It bears a resemblance to a display
server protocol, if you consider QEMU as the display server and the
back-end 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
``VHOST_USER_GPU_SET_SOCKET``.
Requests are sent by the *back-end*, and the optional replies by the
*front-end*.
Wire format
===========
Unless specified differently, numbers are in the machine native byte
order.
A vhost-user-gpu message (request and reply) consists of 3 header
fields and a payload.
+---------+-------+------+---------+
| request | flags | size | payload |
+---------+-------+------+---------+
Header
------
: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:
VhostUserGpuCursorPos
^^^^^^^^^^^^^^^^^^^^^
+------------+---+---+
| scanout-id | x | y |
+------------+---+---+
:scanout-id: ``u32``, the scanout where the cursor is located
:x/y: ``u32``, the cursor position
VhostUserGpuCursorUpdate
^^^^^^^^^^^^^^^^^^^^^^^^
+-----+-------+-------+--------+
| 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)
VhostUserGpuScanout
^^^^^^^^^^^^^^^^^^^
+------------+---+---+
| scanout-id | w | h |
+------------+---+---+
:scanout-id: ``u32``, the scanout configuration to set
:w/h: ``u32``, the scanout width/height size
VhostUserGpuUpdate
^^^^^^^^^^^^^^^^^^
+------------+---+---+---+---+------+
| 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)
VhostUserGpuDMABUFScanout
^^^^^^^^^^^^^^^^^^^^^^^^^
+------------+---+---+---+---+-----+-----+--------+-------+--------+
| 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
``VHOST_USER_GPU_GET_PROTOCOL_FEATURES`` and
``VHOST_USER_GPU_SET_PROTOCOL_FEATURES`` requests.
Communication
=============
Message types
-------------
``VHOST_USER_GPU_GET_PROTOCOL_FEATURES``
:id: 1
:request payload: N/A
:reply payload: ``u64``
Get the supported protocol features bitmask.
``VHOST_USER_GPU_SET_PROTOCOL_FEATURES``
:id: 2
:request payload: ``u64``
:reply payload: N/A
Enable protocol features using a bitmask.
``VHOST_USER_GPU_GET_DISPLAY_INFO``
:id: 3
:request payload: N/A
:reply payload: ``struct virtio_gpu_resp_display_info`` (from virtio specification)
Get the preferred display configuration.
``VHOST_USER_GPU_CURSOR_POS``
:id: 4
:request payload: ``VhostUserGpuCursorPos``
:reply payload: N/A
Set/show the cursor position.
``VHOST_USER_GPU_CURSOR_POS_HIDE``
:id: 5
:request payload: ``VhostUserGpuCursorPos``
:reply payload: N/A
Set/hide the cursor.
``VHOST_USER_GPU_CURSOR_UPDATE``
:id: 6
:request payload: ``VhostUserGpuCursorUpdate``
:reply payload: N/A
Update the cursor shape and location.
``VHOST_USER_GPU_SCANOUT``
: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.
``VHOST_USER_GPU_UPDATE``
: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.
``VHOST_USER_GPU_DMABUF_SCANOUT``
: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.
``VHOST_USER_GPU_DMABUF_UPDATE``
: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
``VHOST_USER_GPU_DMABUF_SCANOUT``.
