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

[Daniel Vetter]

On Fri, Mar 13, 2015 at 04:58:47PM +0000, Chris Wilson wrote:
> 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.

Afaiui userspace only needs the guarantee that NULL is never a valid
address. Which means it's never a valid address for its own buffer
objects. I don't think it cares one bit what's actually there, it's not
mandatory to fault apparently. And faulting is what's not possible.

I guess the standard is like normal C: If you access a NULL pointer,
anything can happen (including garbage on the frontbuffer), the only
guarantee you need to make is that NULL is never a valid address. At least
if no one plays tricks ;-)
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch