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

[Chris Wilson]

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).
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre