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

[Song, Ruiling]

> 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?

> It does mean that shared contexts need to be handled specially, or won't get
> the 0 page protection, but I think Mesa wants this behavior too, and libva
> probably wouldn't mind, so you could just require new versions of those that
> set this flag when telling people what's supported for proper NULL pointer
> handling.
> 
> Any objections to that approach?
> 
> Thanks,
> Jesse