Re: [PATCH 00/03] Preventing zero GPU virtual address allocation

[David Weinehall <david.weinehall@linux.intel.com>]

On Wed, May 20, 2015 at 06:00:43PM +0200, Daniel Vetter wrote:
> On Wed, May 20, 2015 at 03:14:06PM +0100, Chris Wilson wrote:
> > On Wed, May 20, 2015 at 03:09:43PM +0100, Chris Wilson wrote:
> > > On Wed, May 20, 2015 at 04:54:19PM +0300, David Weinehall wrote:
> > > > This patch series (one patch each for libdrm, the kernel, and beignet)
> > > > aims to provide a means to add a context-specific means to prevent
> > > > a mapping to GPU virtual address zero.  This is needed at least by
> > > > Beignet (possibly in other use-cases too, though I don't know of any
> > > > other) to allow use of address zero to represent NULL.
> > > 
> > > Urm, you cannot allow absolute addressing period. What happens to the
> > > object at 0 when the user reads from it or writes to it? You have to
> > > have an object at 0 for the user's NULL pointer access.
> > 
> > I'll mollify that: outside of full-ppgtt where you need to share the VM.
> 
> The description is misleading, the new flag doesn't prevent anything from
> getting mapped at 0 but only prevents any bo submitted through execbuf on
> the given context from being bound at address 0. If that would happen
> compute kernels using NULL checks for some things would fall over.
> 
> Essentially it applies the PIN_BIAS for all execbuf objects, which works
> even on ggtt execbufs.
> 
> Patches themselves look good, but we miss the igt to update the invalid
> ctx flags testcase. And a bare minimal function testcase (which checks the
> reloc offset with and without a ctx with this flag set) would be nice too.
> With that and an r-b from the beignet developers I'll pull this in.
> -Daniel

Yeah, I'll submit the test cases; I have both of them laying around
somewhere already :)


Kind regards, David
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