Re: [PATCH v3 1/2] drm/virtio: Add window server support

[Tomeu Vizoso]

On 1 February 2018 at 17:36, Gerd Hoffmann <kraxel@xxxxxxxxxx> wrote:
> Hi,
>
> Sorry for joining the party late. Had a broken finger and was
> offline for a bunch of weeks (and a buif backlog afterwards ...).

Hi, no problem, hope it's fine now.

> > This is to allow clients running within VMs to be able to
> > communicate with a compositor in the host. Clients will use the
> > communication protocol that the compositor supports, and virtio-gpu
> > will assist with making buffers available in both sides, and
> > copying content as needed.
>
> Why not use virtio-vsock to run the wayland protocol? I don't like
> the idea to duplicate something with very simliar functionality in 
> virtio-gpu.

The reason for abandoning that approach was the type of objects that
could be shared via virtio-vsock would be extremely limited. Besides
that being potentially confusing to users, it would mean from the
implementation side that either virtio-vsock would gain a dependency on
the drm subsystem, or an appropriate abstraction for shareable buffers
would need to be added for little gain.

Another factor that was taken into account was that the complexity
required for implementing passing protocol data around was very small
when compared with the buffer sharing mechanism.

> > It is expected that a service in the guest will act as a proxy, 
> > interacting with virtio-gpu to support unmodified clients.
>
> If you have a guest proxy anyway using virtio-sock for the protocol 
> stream and virtio-gpu for buffer sharing (and some day 3d rendering
> too) should work fine I think.

If I understand correctly your proposal, virtio-gpu would be used for
creating buffers that could be shared across domains, but something
equivalent to SCM_RIGHTS would still be needed in virtio-vsock?

If so, that's what was planned initially, with the concern being that we
would be adding a bunch of complexity to virtio-vsock that would be only
used in this specific use case. Then we would also need to figure out
how virtio-vsock would be able to work with buffers from virtio-gpu
(either direct dependency or a new abstraction).

If the mechanics of passing presentation data were very complex, I think
this approach would have more merit. But as you can see from the code,
it isn't that bad.

> > When the client notifies the compositor that it can read from that
buffer,
> > the proxy should copy the contents from the SHM region to the
> > virtio-gpu resource and call DRM_VIRTGPU_TRANSFER_TO_HOST.
>
> What is the plan for the host side? I see basically two options. 
> Either implement the host wayland proxy directly in qemu. Or
> implement it as separate process, which then needs some help from
> qemu to get access to the buffers. The later would allow qemu running
> independant from the desktop session.

Regarding synchronizing buffers, this will stop becoming needed in
subsequent commits as all shared memory is allocated in the host and
mapped to the guest via KVM_SET_USER_MEMORY_REGION.

This is already the case for buffers passed from the compositor to the
clients (see patch 2/2), and I'm working on the equivalent for buffers
from the guest to the host (clients still have to create buffers with
DRM_VIRTGPU_RESOURCE_CREATE but they will be only backend by host memory
so no calls to DRM_VIRTGPU_TRANSFER_TO_HOST are needed).

But in the case that we still need a proxy for some reason on the host
side, I think it would be better to have it in the same process where
virtio-gpu is implemented. In crosvm's case it would be in a process
separate from the main VMM, as device processes are isolated from each
other with minijail (see
https://chromium.googlesource.com/chromiumos/platform/crosvm/ ).

Regards,

Tomeu