[Beignet] [Intel-gfx] Preventing zero GPU virtual address allocation

[Chris Wilson]

On Fri, Mar 13, 2015 at 10:27:38AM +0100, Daniel Vetter wrote:
> If supporting systems without full ppgtt is a requirement for you (still
> wonky on gen8 a bit, so might be a good strategy) then imo it's the
> PIN_BIAS idea I've laid out earlier in this thread. That one will work
> everywhere. softpin can unexpectedly fail without full ppgtt if the kernel
> decides to put something at a given spot, which imo means we should only
> expose it on full ppgtt systems.
> 
> And PIN_BIAS should be fairly easy to wire up since the internal logic is
> all there already. So "just" needs an execbuf flag, igt test and
> appropriate userspace to set that new bit.

It doesn't though. To provide the guarantee userspace is asking for
(which is that address 0 goes to a special, preferrably inaccessible,
page), you have to evict the first N pages in the GGTT. That is just as
likely to fail with an execbuffer flag as it would with an execobject flag.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre