Native context support is implemented by diverting the libdrm_amdgpu functions
into new functions that use virtio-gpu.
VA allocations are done directly in the guest, using newly exposed libdrm_amdgpu
helpers (retrieved through dlopen/dlsym).
Guest <-> Host roundtrips can be expensive so we try to avoid them as much as
possible. When possible we also don't wait for the host reply in case where
it's not needed to get correct result.
Implicit sync works because virtio-gpu commands are submitted in order to the
host (there a single queue per device, shared by all the guest processes).
virtio-gpu also only supports one context per file description (but multiple
file descriptions per process) while amdgpu only allows one fd per process,
but multiple contexts per fd. This causes synchronization problems, because
virtio-gpu drops all sync primitive if they belong to the same fd/context/ring:
ie the amdgpu_ctx can't be expressed in virtio-gpu terms.
For now the solution is to only allocate a single amdgpu_ctx per application.
Contrary to radeonsi/radv, amdgpu_virtio can use libdrm_amdgpu directly: the
ones that don't rely on ioctl() are safe to use here.
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21658>
