[PATCH v3 9/9] docs/system: add basic virtio-gpu documentation

[Gurchetan Singh]

From: 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)

v3: - Incorporated suggestions by Akihiko Odaki and Alyssa Ross

 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..2b8ecdac32
--- /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.
+
+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.
+
+Atleast 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="$XDG_RUNTIME_DIR/$WAYLAND_DISPLAY",
+       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
+.. _guest Wayland proxy: https://crosvm.dev/book/devices/wayland.html
-- 
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