Re: [Beignet] Preventing zero GPU virtual address allocation

From: David Weinehall <david.weinehall@linux.intel.com>
To: "Song, Ruiling" <ruiling.song@intel.com>
Cc: "Yang, Rong R" <rong.r.yang@intel.com>,
	"intel-gfx@lists.freedesktop.org"
	<intel-gfx@lists.freedesktop.org>,
	"beignet@lists.freedesktop.org" <beignet@lists.freedesktop.org>,
	"Weinehall, David" <david.weinehall@intel.com>,
	"Vetter, Daniel" <daniel.vetter@intel.com>
Subject: Re: [Beignet] Preventing zero GPU virtual address allocation
Date: Thu, 19 Mar 2015 12:09:53 +0200	[thread overview]
Message-ID: <20150319100953.GA10425@boom> (raw)
In-Reply-To: <148B1B7A67D1C24B9EF0BE42EA4977062B7F7BB2@SHSMSX101.ccr.corp.intel.com>

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
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