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

[Tomeu Vizoso <tomeu.vizoso@collabora.com>]

[Tomasz wants to comment, adding him to CC]

On 2/5/18 9:19 AM, Tomeu Vizoso wrote:
> On 1 February 2018 at 17:36, Gerd Hoffmann <kraxel@redhat.com> 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
>