docs/system: add basic virtio-gpu documentation

This adds basic documentation for virtio-gpu.

Suggested-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Signed-off-by: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Alyssa Ross <hi@alyssa.is>
Tested-by: Emmanouil Pitsidianakis <manos.pitsidianakis@linaro.org>
Tested-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Emmanouil Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Antonio Caggiano <quic_acaggian@quicinc.com>
Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>

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

docs/system/devices/virtio-gpu.rst

@@ -0,0 +1,112 @@
+..
+   SPDX-License-Identifier: GPL-2.0-or-later
+
+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.
+
+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
+
+.. _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
+
+.. _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 virtio-gpu capability set ("capset") must be specified when
+starting the device.  The currently capsets supported are ``gfxstream-vulkan``
+and ``cross-domain`` for Linux guests. For Android guests, the experimental
+``x-gfxstream-gles`` and ``x-gfxstream-composer`` capsets are 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,gfxstream-vulkan=on,cross-domain=on,
+       hostmem=8G,wayland-socket-path=/tmp/nonstandard/mock_wayland.sock,
+       wsi=headless
+
+.. _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
+.. _guest Wayland proxy: https://crosvm.dev/book/devices/wayland.html