[Mesa-dev] [Mesa-stable] [PATCH] i965: Fix buffer overruns in MSAA MCS buffer clearing.

[Ian Romanick]

On 04/15/2014 03:50 PM, Kenneth Graunke wrote:
> On 04/15/2014 10:08 AM, Mike Stroyan wrote:
>> I would go farther than requiring the returned bo->size to be covered by
>> backing pages.
>> There really should be backing pages for every page mapped by
>> drm_intel_gem_bo_map.
>> No mapped addresses should be affecting memory outside of an object's
>> backing pages.
>>
>> If tiling can result in access to unallocated memory it might even lead
>> to corruption of data in pages
>> used by different processes.  That would break GL_ARB_robustness
>> requirements.
> 
> It shouldn't, now that proper PPGTT support has landed in the kernel -
> only data belonging to a single process is mapped into its address
> space.  So, a process can accidentally read or trash other portions of
> its own memory, but never any other memory.  This seems acceptable, as
> it's how things work with normal CPU programs.

It is fine... for now.  GL_ARB_robust_buffer_access makes stronger
guarantees about out-of-buffer accesses.  Reads outside a buffer (e.g.,
accessing too large an array element for an array contained in a UBO)
can only return data from elsewhere in the buffer or zero.  WebGL
implementers really want this.

I don't think that's relevant here because the fences only apply to CPU
access, yeah?

> That said, with older kernels, where we had only Global GTT or the
> aliasing PPGTT (using the PPGTT mechanism but storing the exact same
> mappings at the GGTT), you could definitely see/stomp other people's
> memory.  Which is awful, and one reason we think PPGTT support is so
> important.
> 
> --Ken
> 
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