[Feature request] Multiple X servers on one graphics card?

Alan Cox alan at lxorguk.ukuu.org.uk
Tue Aug 2 03:26:04 PDT 2011

> But I believe this is a problem of all approaches which provide
> multiple hardware-accelerated (or Xv-enabled) seats on a single GPU,
> no matter if based on multiple DRM devices, on Xephyr or Xnest
> with some kind of OpenGL or DRI passthrough, or on Wayland:
> If one has direct access to the graphics engine, he also can access
> any video memory he wants.

Not always. It's a bit more complicated than that. Some hardware supports
write only memory spaces, some hardware supports contexts in the GTT). On
other cards you need some kind of security model and verifier to handle
this. I don't think reloading the GTT is enough on most of the cards
because the display scanout for all the framebuffers is needed all the

> Hence, that's no argument against multiple DRM devices on a single card,
> because the other solutions suffer from the same problem.

Displaylink USB devices don't have this problem - they have others. Ditto
mulitple graphics cards doesn't.

> In the long term, it needs to be fixed,
> but in a classroom environment, that's not my primary concern
> (and I believe 90 % of all multiseat installations
> will be classroom or home environments).

Certainly with such security limits it won't catch on in many places, and
I suspect it wouldn't be much use in many classroom environments students
being what they are!


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