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

From: Daniel Vetter <daniel@ffwll.ch>
To: Chris Wilson <chris@chris-wilson.co.uk>,
	Daniel Vetter <daniel@ffwll.ch>,
	"Song, Ruiling" <ruiling.song@intel.com>,
	"Vetter, Daniel" <daniel.vetter@intel.com>,
	"intel-gfx@lists.freedesktop.org"
	<intel-gfx@lists.freedesktop.org>,
	"Yang, Rong R" <rong.r.yang@intel.com>,
	"beignet@lists.freedesktop.org" <beignet@lists.freedesktop.org>,
	"Weinehall, David" <david.weinehall@intel.com>
Subject: Re: [Intel-gfx] Preventing zero GPU virtual address allocation
Date: Thu, 5 Mar 2015 16:27:59 +0100	[thread overview]
Message-ID: <20150305152758.GI18775@phenom.ffwll.local> (raw)
In-Reply-To: <20150305130121.GA18784@nuc-i3427.alporthouse.com>

On Thu, Mar 05, 2015 at 01:01:21PM +0000, Chris Wilson wrote:
> On Thu, Mar 05, 2015 at 01:52:51PM +0100, Daniel Vetter wrote:
> > On Thu, Mar 05, 2015 at 02:56:52AM +0000, Song, Ruiling wrote:
> > > Hi Daniel,
> > > 
> > > OpenCL language support NULL pointer, using zero as the NULL pointer is
> > > the obvious way. That is zero will be treated as invalid address.  Then
> > > it requires drm won't allocate zero to drm buffer. And David in CC
> > > list has help us make a patch, please see attached. The logic is only
> > > for ppgtt, and he said zero offset is used under ggtt. My question is
> > > what is offset zero used under ggtt? Will it make sure zero is not
> > > allocatable to drm buffer object?
> > 
> > The code in i915_gem_execbuf.c already supports an optional bias to avoid
> > putting a buffer into the first few kb. See __EXEC_OBJECT_NEEDS_BIAS. I
> > suggest you expose this to userspace, which also address your issue that
> > you didn't add an abi revision flag.
> 
> A better API would be to allow userspace to request a buffer to place at
> a specific point in the VM and fail if that is not possible aka
> soft-pinning. Then OCL could assign a bo to offset 0 and detect writes
> to the NULL address if it so desired. With full-ppgtt, userspace can be
> sure of being able to evict any location in its VM and so also allows
> graceful detection of scenarios under which it cannot provide the NULL
> address safety feature (and opt not to run, or just bury its head in the
> sand).

I recommended exposing the PIN_BIAS since that will work without full
ppgtt too. And yeah for full ppgtt we could just use svm where userspace
controls the address, but since that's still a bit out we might need a
quick interim solution?
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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