Re: passing FDs across domains

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

On 11/14/2017 09:02 AM, Gerd Hoffmann wrote:
> On Mon, Nov 13, 2017 at 03:18:46PM +0100, Tomeu Vizoso wrote:
>> Hi,
>>
>> I'm looking at what would be the best way for Wayland clients running in a
>> VM to communicate with a Wayland compositor in the host.
>>
>> The base Wayland protocol is fairly self-contained, with little or no
>> references to objects outside the client and the server, so it lends itself
>> very well to virtualization.
>>
>> But there's a problem when passing references to pixel buffers around with
>> FD passing, as SCM_RIGHTS isn't currently implemented in AF_VSOCK.
>>
>> The Wayland project is willing to consider adding AF_VSOCK support to the
>> libwayland libraries alongside existing support for AF_UNIX, both on client
>> and server.
>>
>> Any opinions on whether adding SCM_RIGHTS support to AF_VSOCK is a good
>> idea?
> 
> Not going to work.  A file handle is a reference to a kernel object
> which can be pretty much anything (file, socket, timer, memory, ...) and
> you can't pass that across machine borders.  It's not working for
> AF_INET for the same reason.

The idea was to only allow passing FDs that point that objects that can 
be shared across domains. So shared memory buffers in this case.

>> Or other options for letting processes in the host to access shmem
>> buffers allocated within the guest?
>>
>> Regarding the mechanics of mapping guest buffers in the host, I was hoping
>> the approach described below would work:
>>
>> http://www.fp7-save.eu/papers/SCALCOM2016.pdf
> 
> Doesn't look that useful to me on a quick glance.
> 
> 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.

Thanks,

Tomeu