drm/virtgpu api: create context init feature

drm/virtgpu api: create context init feature

This change allows creating contexts of depending on set of
context parameters.  The meaning of each of the parameters
is listed below:

1) VIRTGPU_CONTEXT_PARAM_CAPSET_ID

This determines the type of a context based on the capability set
ID.  For example, the current capsets:

VIRTIO_GPU_CAPSET_VIRGL
VIRTIO_GPU_CAPSET_VIRGL2

define a Gallium, TGSI based "virgl" context.  We only need 1 capset
ID per context type, though virgl has two due a bug that has since
been fixed.

The use case is the "gfxstream" rendering library and "venus"
renderer.

gfxstream doesn't do Gallium/TGSI translation and mostly relies on
auto-generated API streaming.  Certain users prefer gfxstream over
virgl for GLES on GLES emulation.  {gfxstream vk}/{venus} are also
required for Vulkan emulation.  The maximum capset ID is 63.

The goal is for guest userspace to choose the optimal context type
depending on the situation/hardware.

2) VIRTGPU_CONTEXT_PARAM_NUM_RINGS

This tells the number of independent command rings that the context
will use.  This value may be zero and is inferred to be zero if
VIRTGPU_CONTEXT_PARAM_NUM_RINGS is not passed in.  This is for backwards
compatibility for virgl, which has one big giant command ring for all
commands.

The maxiumum number of rings is 64.  In practice, multi-queue or
multi-ring submission is used for powerful dGPUs and virtio-gpu
may not be the best option in that case (see PCI passthrough or
rendernode forwarding).

3) VIRTGPU_CONTEXT_PARAM_POLL_RING_IDX_MASK

This is a mask of ring indices for which the DRM fd is pollable.
For example, if VIRTGPU_CONTEXT_PARAM_NUM_RINGS is 2, then the mask
may be:

[ring idx]  |  [1 << ring_idx] | final mask
-------------------------------------------
    0              1                1
    1              2                3

The "Sommelier" guest Wayland proxy uses this to poll for events
from the host compositor.

Signed-off-by: Gurchetan Singh <gurchetansingh@chromium.org>
Acked-by: Lingfeng Yang <lfy@google.com>
Acked-by: Nicholas Verne <nverne@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20210921232024.817-3-gurchetansingh@chromium.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>