Re: [Intel-gfx] Preventing zero GPU virtual address allocation

Song, Ruiling
2015-03-19T03:22:42Z

> 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
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