[PATCH v2 9/9] docs/system: add basic virtio-gpu documentation

[Gurchetan Singh <gurchetansingh@chromium.org>]

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)

 docs/system/device-emulation.rst   |  1 +
 docs/system/devices/virtio-gpu.rst | 98 ++++++++++++++++++++++++++++++
 2 files changed, 99 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..f359584033
--- /dev/null
+++ b/docs/system/devices/virtio-gpu.rst
@@ -0,0 +1,98 @@
+..
+   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 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.
+
+Theses backends can be further 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 PCI interface, but for the non-VGA ones, you can
+further pick simple MMIO or PCI. For MMIO, you can suffix the device
+name with -device though vhost-user-gpu apparently does not support
+MMIO. For PCI, you can suffix it with -pci. Without these suffixes, the
+platform default will be chosen.  The syntax of  available combinations
+is listed below.
+
+ * ``virtio-vga[-BACKEND]``
+ * ``virtio-gpu[-BACKEND][-INTERFACE]``
+ * ``vhost-user-vga``
+ * ``vhost-user-pci``
+
+This document uses the PCI variant in examples.
+
+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, 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 directly to the host with minimal modification.
+
+The crosvm book provides directions on how to build a `gfxstream-enabled
+rutabaga`_ and launch a `guest Wayland compositor`_.
+
+This device does require host blob support (``hostmem`` field below), but not
+all capsets (``capset_names`` below) have to enabled 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.
+
+.. parsed-literal::
+    -device virtio-gpu-rutabaga-pci,capset_names=gfxstream-vulkan:cross-domain,\\
+      hostmem=8G,wayland_socket_path="$XDG_RUNTIME_DIR/$WAYLAND_DISPLAY"
+
+.. _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
+.. _guest Wayland compositor: https://crosvm.dev/book/devices/wayland.html
-- 
2.41.0.585.gd2178a4bd4-goog