Re: passing FDs across domains

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

On 11/14/2017 10:33 AM, Gerd Hoffmann wrote:
>    Hi,
> 
> [ Cc'ing David Airlie and Marc-André Lureau ]
> 
>>> Lets step back and look at the problem you are trying to solve.
>>>
>>> It seems you want guest wayland applications appear seamless on the host
>>> wayland server, correct?
>>
>> Correct.
>>
>>> How does the wayland rendering workflow look like?  As far I know the
>>> wayland protocol doesn't include any rendering.  Rendering happens
>>> client side, into some buffer (one per window), which is then passed to
>>> the server for display compositing.  Correct?  So you basically want
>>> pass that buffer from guest to host?
>>
>> Correct.
>>
>>> What kind of shared memory is used by wayland?
>>> sysv shm?  gbm buffers / dmabufs?
>>
>> Typically, shared memory for CPU-rendered content, and dmabufs for
>> GPU-rendered content.
> 
> Ok.  I guess solving this for virtio-gpu (with virgl enabled) is easiest
> then.  Due to opengl rendering being offloaded to the host gpu the
> guest window content already is in a host gpu buffer.

Yes, besides, we already have virtio-gpu in place which can be improved 
as needed.

I'm more worried about CPU-rendered buffers, as the client is just 
putting in the socket the output of shm_open or similar. There I don't 
see any easy solution which is why I tried to get more "creative".

Thanks,

Tomeu

> So we "only" need to export that buffer as dmabuf and pass it to the
> wayland server.  That still doesn't look trivial though.  qemu must
> manage the dmabufs, so it must be involved somehow; a direct guest
> client -> host server wayland connection (alone) will not work.
> 
> I think we need either a wayland proxy in qemu which rewrites the buffer
> references, or the host wayland server must talk to both guest client
> (vsock could work for that) and qemu (for dmabuf management).  Or some
> mixed model, such as a separate wayland proxy server talking to qemu for
> dmabuf management.
> 
> I suspect in any case we need a wayland protocol extension for a new
> buffer type as passing buffer references from guest to host can't use
> filehandles.
> 
> cheers,
>    Gerd
>