FR: Xen support

[DemiMarie]

Parts of virtio-GPU can straightforwardly (if not easily) be implemented on Xen, but mapping host-allocated buffers into the guest isn't supported yet and so there will need to be a workaround.

[talex5]

Currently (without GPU acceleration), buffers are allocated by guest applications and ideally would be shared with the host. But for some reason crosvm doesn't support that, so we allocate host memory and copy the data across manually. If Xen can pass guest memory to the host then that probably simplifies things:

wayland-proxy-virtwl/relay.ml

Lines 208 to 211 in 1c0cd6d

	(* wl_shm memory buffers are allocated by the client inside the guest and
	cannot be shared directly with the host. Instead, we allocate some host
	memory of the same size, map that into the guest, and copy the data across
	as needed.

[DemiMarie]

Xen can easily share guest memory with the host, but there are two caveats:

1. The current kernel API requires that the memory be allocated specifically for sharing, so a copy will still be needed. This can likely be fixed by a new ioctl that takes a FOLL_LONGTERM pin on the memory.
2. The host needs to know how to map the memory. crosvm doesn’t support that.