[Beignet] Preventing zero GPU virtual address allocation

[Song, Ruiling]

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?

Ruiling
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