Re: [PATCH v4 9/9] docs/system: add basic virtio-gpu documentation

[Akihiko Odaki <akihiko.odaki@gmail.com>]

On 2023/08/09 11:11, Gurchetan Singh wrote:
> This adds basic documentation for virtio-gpu.
> 
> Suggested-by: Akihiko Odaki <akihiko.odaki@daynix.com>
> Signed-off-by: Gurchetan Singh <gurchetansingh@chromium.org>
> ---
> v2: - Incorporated suggestions by Akihiko Odaki
>      - Listed the currently supported capset_names (Bernard)
> 
> v3: - Incorporated suggestions by Akihiko Odaki and Alyssa Ross
> 
> v4: - Incorporated suggestions by Akihiko Odaki
> 
>   docs/system/device-emulation.rst   |   1 +
>   docs/system/devices/virtio-gpu.rst | 115 +++++++++++++++++++++++++++++
>   2 files changed, 116 insertions(+)
>   create mode 100644 docs/system/devices/virtio-gpu.rst
> 
> diff --git a/docs/system/device-emulation.rst b/docs/system/device-emulation.rst
> index 4491c4cbf7..1167f3a9f2 100644
> --- a/docs/system/device-emulation.rst
> +++ b/docs/system/device-emulation.rst
> @@ -91,6 +91,7 @@ Emulated Devices
>      devices/nvme.rst
>      devices/usb.rst
>      devices/vhost-user.rst
> +   devices/virtio-gpu.rst
>      devices/virtio-pmem.rst
>      devices/vhost-user-rng.rst
>      devices/canokey.rst
> diff --git a/docs/system/devices/virtio-gpu.rst b/docs/system/devices/virtio-gpu.rst
> new file mode 100644
> index 0000000000..d56524270d
> --- /dev/null
> +++ b/docs/system/devices/virtio-gpu.rst
> @@ -0,0 +1,115 @@
> +..
> +   SPDX-License-Identifier: GPL-2.0
> +
> +virtio-gpu
> +==========
> +
> +This document explains the setup and usage of the virtio-gpu device.
> +The virtio-gpu device paravirtualizes the GPU and display controller.
> +
> +Linux kernel support
> +--------------------
> +
> +virtio-gpu requires a guest Linux kernel built with the
> +``CONFIG_DRM_VIRTIO_GPU`` option.
> +
> +QEMU virtio-gpu variants
> +------------------------
> +
> +QEMU virtio-gpu device variants come in the following form:
> +
> + * ``virtio-vga[-BACKEND]``
> + * ``virtio-gpu[-BACKEND][-INTERFACE]``
> + * ``vhost-user-vga``
> + * ``vhost-user-pci``
> +
> +**Backends:** QEMU provides a 2D virtio-gpu backend, and two accelerated
> +backends: virglrenderer ('gl' device label) and rutabaga_gfx ('rutabaga'
> +device label).  There is a vhost-user backend that runs the graphics stack
> +in a separate process for improved isolation.
> +
> +**Interfaces:** QEMU further categorizes virtio-gpu device variants based
> +on the interface exposed to the guest. The interfaces can be classified
> +into VGA and non-VGA variants. The VGA ones are prefixed with virtio-vga
> +or vhost-user-vga while the non-VGA ones are prefixed with virtio-gpu or
> +vhost-user-gpu.
> +
> +The VGA ones always use the PCI interface, but for the non-VGA ones, the
> +user can further pick between MMIO or PCI. For MMIO, the user can suffix
> +the device name with -device, though vhost-user-gpu does not support MMIO.
> +For PCI, the user can suffix it with -pci. Without these suffixes, the
> +platform default will be chosen.
> +
> +This document uses the PCI interface in examples.

I think it's better to omit -pci.

By the way you are not adding the aliases for Rutabaga so please do so. 
You can find the table in: softmmu/qdev-monitor.c

> +
> +virtio-gpu 2d
> +-------------
> +
> +The default 2D backend only performs 2D operations. The guest needs to
> +employ a software renderer for 3D graphics.
> +
> +Typically, the software renderer is provided by `Mesa`_ or `SwiftShader`_.
> +Mesa's implementations (LLVMpipe, Lavapipe and virgl below) work out of box
> +on typical modern Linux distributions.
> +
> +.. parsed-literal::
> +    -device virtio-gpu-pci
> +
> +.. _Mesa: https://www.mesa3d.org/
> +.. _SwiftShader: https://github.com/google/swiftshader
> +
> +virtio-gpu virglrenderer
> +------------------------
> +
> +When using virgl accelerated graphics mode in the guest, OpenGL API calls
> +are translated into an intermediate representation (see `Gallium3D`_). The
> +intermediate representation is communicated to the host and the
> +`virglrenderer`_ library on the host translates the intermediate
> +representation back to OpenGL API calls.
> +
> +.. parsed-literal::
> +    -device virtio-gpu-gl-pci
> +
> +.. _Gallium3D: https://www.freedesktop.org/wiki/Software/gallium/
> +.. _virglrenderer: https://gitlab.freedesktop.org/virgl/virglrenderer/
> +
> +virtio-gpu rutabaga
> +-------------------
> +
> +virtio-gpu can also leverage `rutabaga_gfx`_ to provide `gfxstream`_
> +rendering and `Wayland display passthrough`_.  With the gfxstream rendering
> +mode, GLES and Vulkan calls are forwarded to the host with minimal
> +modification.
> +
> +The crosvm book provides directions on how to build a `gfxstream-enabled
> +rutabaga`_ and launch a `guest Wayland proxy`_.
> +
> +This device does require host blob support (``hostmem`` field below). The
> +``hostmem`` field specifies the size of virtio-gpu host memory window.
> +This is typically between 256M and 8G.
> +
> +At least one capset (see colon separated ``capset_names`` below) must be
> +specified when starting the device.  The currently supported
> +``capset_names`` are ``gfxstream-vulkan`` and ``cross-domain`` on Linux
> +guests. For Android guests, ``gfxstream-gles`` is also supported.
> +
> +The device will try to auto-detect the wayland socket path if the
> +``cross-domain`` capset name is set.  The user may optionally specify
> +``wayland_socket_path`` for non-standard paths.
> +
> +The ``wsi`` option can be set to ``surfaceless`` or ``headless``.
> +Surfaceless doesn't create a native window surface, but does copy from the
> +render target to the Pixman buffer if a virtio-gpu 2D hypercall is issued.
> +Headless is like surfaceless, but doesn't copy to the Pixman buffer.
> +Surfaceless is the default if ``wsi`` is not specified.
> +
> +.. parsed-literal::
> +    -device virtio-gpu-rutabaga-pci,capset_names=gfxstream-vulkan:cross-domain,
> +       hostmem=8G,wayland_socket_path=/tmp/nonstandard/mock_wayland.sock,
> +       wsi=headless
> +
> +.. _rutabaga_gfx: https://github.com/google/crosvm/blob/main/rutabaga_gfx/ffi/src/include/rutabaga_gfx_ffi.h
> +.. _gfxstream: https://android.googlesource.com/platform/hardware/google/gfxstream/
> +.. _Wayland display passthrough: https://www.youtube.com/watch?v=OZJiHMtIQ2M
> +.. _gfxstream-enabled rutabaga: https://crosvm.dev/book/appendix/rutabaga_gfx.html

The build procedure looks almost good, but a command for building gfxstream:
meson -Ddefault_library=static build/

This results in a warning:
WARNING: Running the setup command as `meson [options]` instead of 
`meson setup [options]` is ambiguous and deprecated.

The same goes for the command for guest-side libraries.

> +.. _guest Wayland proxy: https://crosvm.dev/book/devices/wayland.html