Virtio-GPU Venus — The Mesa 3D Graphics Library latest documentation

# Virtio-GPU Venus

Venus is a Virtio-GPU protocol for Vulkan command serialization. The protocol definition and codegen are hosted at venus-protocol. The renderer is hosted at virglrenderer.

## Requirements

The Venus renderer requires

- Vulkan 1.1

- VK_EXT_external_memory_dma_buf

- VK_EXT_image_drm_format_modifier

- VK_EXT_queue_family_foreign

from the host driver. However, it violates the spec in some places currently and also relies on implementation-defined behaviors in others. It is not expected to work on all drivers meeting the requirements. It has only been tested with

- ANV 21.1 or later

- RADV 21.1 or later (the host kernel must have `CONFIG_TRANSPARENT_HUGEPAGE` disabled because of this KVM issue)

- TURNIP 22.0 or later

- Mali r32p0 or later

The Venus driver requires supports for

- `VIRTGPU_PARAM_3D_FEATURES`

- `VIRTGPU_PARAM_CAPSET_QUERY_FIX`

- `VIRTGPU_PARAM_RESOURCE_BLOB`

- `VIRTGPU_PARAM_HOST_VISIBLE`

- `VIRTGPU_PARAM_CROSS_DEVICE`

- `VIRTGPU_PARAM_CONTEXT_INIT`

from the virtio-gpu kernel driver, unless vtest is used. That usually means the guest kernel should be at least 5.16 or have the parameters back ported, paired with hypervisors such as crosvm, or patched qemu.

## vtest

The simplest way to test Venus is to use virglrenderer’s vtest server. To build virglrenderer with Venus support and to start the vtest server,

    $ git clone https://gitlab.freedesktop.org/virgl/virglrenderer.git
    $ cd virglrenderer
    $ meson out -Dvenus=true
    $ meson compile -C out
    $ meson devenv -C out
    $ ./vtest/virgl_test_server --venus
    $ exit

In another shell,

    $ export VK_DRIVER_FILES=<path-to-virtio_icd.x86_64.json>
    $ export VN_DEBUG=vtest
    $ vulkaninfo
    $ vkcube

If the host driver of the system is not new enough, it is a good idea to build the host driver as well when building the Venus driver. Just remember to set `VK_DRIVER_FILES` when starting the vtest server so that the vtest server finds the locally built host driver.

## Virtio-GPU

The driver requires `VIRTGPU_PARAM_CONTEXT_INIT` from the virtio-gpu kernel driver, which was upstreamed in kernel 5.16.

crosvm is written in Rust. To build crosvm, make sure Rust has been installed and

    $ git clone --recurse-submodules \
          https://chromium.googlesource.com/chromiumos/platform/crosvm
    $ cd crosvm
    $ RUSTFLAGS="-L<path-to-virglrenderer>/out/src" cargo build \
          --features "x wl-dmabuf virgl_renderer virgl_renderer_next default-no-sandbox"

Note that crosvm must be built with `default-no-sandbox` or started with `--disable-sandbox` in this setup.

This is how one might want to start crosvm

    $ sudo LD_LIBRARY_PATH=<...> VK_DRIVER_FILES=<...> ./target/debug/crosvm run \
          --gpu vulkan=true \
          --gpu-render-server path=<path-to-virglrenderer>/out/server/virgl_render_server \
          --display-window-keyboard \
          --display-window-mouse \
          --net "host-ip 192.168.0.1,netmask=255.255.255.0,mac=12:34:56:78:9a:bc" \
          --rwdisk disk.img \
          -p root=/dev/vda1 \
          <path-to-bzImage>

assuming a working system is installed to partition 1 of `disk.img`. `sudo` or `CAP_NET_ADMIN` is needed to set up the TAP network device.

## Virtio-GPU and Virtio-WL

In this setup, the guest userspace uses Xwayland and a special Wayland compositor to connect guest X11/Wayland clients to the host Wayland compositor, using Virtio-WL as the transport. This setup is more tedious, but that should hopefully change over time.

For now, the guest kernel must be built from the `chromeos-5.10` branch of the Chrome OS kernel.

To build minigbm and to enable minigbm support in virglrenderer,

    $ git clone https://chromium.googlesource.com/chromiumos/platform/minigbm
    $ cd minigbm
    $ CFLAGS=-DDRV_<I915-or-your-driver> OUT=out DESTDIR=out/install make install
    $ cd ../virglrenderer
    $ meson configure out -Dminigbm_allocation=true
    $ meson compile -C out

Make sure a host Wayland compositor is running. Replace `--display-window-keyboard --display-window-mouse` by `--wayland-sock=<path-to-wayland-socket>` when starting crosvm.

In the guest, build and start Sommelier, the special Wayland compositor,

    $ git clone https://chromium.googlesource.com/chromiumos/platform2
    $ cd platform2/vm_tools/sommelier
    $ meson out -Dxwayland_path=/usr/bin/Xwayland -Dxwayland_gl_driver_path=/usr/lib/dri
    $ meson compile -C out
    $ sudo chmod 777 /dev/wl0
    $ ./out/sommelier -X --glamor
          --xwayland-gl-driver-path=<path-to-locally-built-gl-driver> \
          sleep infinity

## Optional Requirements

When virglrenderer is built with `-Dminigbm_allocation=true`, the Venus renderer might need to import GBM BOs. The imports will fail unless the host driver supports the formats, especially multi-planar ones, and the DRM format modifiers of the GBM BOs.

In the future, if virglrenderer’s `virgl_renderer_export_fence` is supported, the Venus renderer will require VK_KHR_external_fence_fd with `VK_EXTERNAL_FENCE_HANDLE_TYPE_SYNC_FD_BIT` from the host driver.

## VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT

The Venus renderer makes assumptions about `VkDeviceMemory` that has `VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT`. The assumptions are illegal and rely on the current behaviors of the host drivers. It should be possible to remove some of the assumptions and incrementally improve compatibilities with more host drivers by imposing platform-specific requirements. But the long-term plan is to create a new Vulkan extension for the host drivers to address this specific use case.

The Venus renderer assumes a device memory that has `VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT` can be exported as a mmapable dma-buf (in the future, the plan is to export the device memory as an opaque fd). It chains `VkExportMemoryAllocateInfo` to `VkMemoryAllocateInfo` without checking if the host driver can export the device memory.

The dma-buf is mapped (in the future, the plan is to import the opaque fd and call `vkMapMemory`) but the mapping is not accessed. Instead, the mapping is passed to `KVM_SET_USER_MEMORY_REGION`. The hypervisor, host KVM, and the guest kernel work together to set up a write-back or write-combined guest mapping (see `virtio_gpu_vram_mmap` of the virtio-gpu kernel driver). CPU accesses to the device memory are via the guest mapping, and are assumed to be coherent when the device memory also has `VK_MEMORY_PROPERTY_HOST_COHERENT_BIT`.

While the Venus renderer can force a `VkDeviceMemory` external, it does not force a `VkImage` or a `VkBuffer` external. As a result, it can bind an external device memory to a non-external resource.