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

[David Weinehall]

On Thu, Mar 19, 2015 at 03:22:42AM +0000, Song, Ruiling wrote:
> 
> > Yeah, MAP_FIXED sounds a bit more ambitious and though I think it would
> > work for OCL 2.0 pointer sharing, it's a little different than we were planning.
> > To summarize, we have three possible approaches, each with its own
> > problems:
> >   1) simple patch to avoid binding at address 0 in PPGTT:
> >      does impact the ABI (though generally not in a harmful way), and
> >      may not be possible with aliasing PPGTT with e.g. framebuffers
> >      bound at offset 0
> >   2) exposing PIN_BIAS to userspace
> >      Would allow userspace to avoid pinning any buffers at offset 0 at
> >      execbuf time, but still has the problem with previously bound buffers
> >      and aliasing PPGTT
> >   3) MAP_FIXED interface
> >      Flexible approach allowing userspace to manage its own virtual
> >      memory, but still has the same issues with aliasing PPGTT, and with
> >      shared contexts, which would have to negotiate between libraries
> > how to
> >      handle the zero page
> > 
> > For (1) and (2) the kernel pieces are really already in place, the main thing we
> > need is a new flag to userspace to indicate behavior.  I'd prefer (1) with a
> > context creation flag to indicate "don't bind at 0".
> > Execbuf would try to honor this, and userspace could check if any buffers
> > ended up at 0 in the aliasing PPGTT case by checking the resulting offsets
> > following the call.  I expect in most cases this would be fine.
> > 
> > It should be pretty easy to extend Ruiling's patch to use a context flag to
> > determine the behavior; is that something you can do?  Any objections to
> > this approach?
> 
> I am ok with adding a context flag to indicate "don't bind at 0". Any objections from others?
> The patch is not from me, it is from David. I am not familiar with KMD. David, could you help on this patch?

Yup, assuming, of course, that such an approach is acceptable.


Kind regards, David