[Mesa-dev] [PATCH] anv: Always set the reloc value to match the presumed_offset

[Chris Wilson]

On Fri, May 05, 2017 at 12:33:54PM -0700, Jason Ekstrand wrote:
>    On Fri, May 5, 2017 at 5:09 AM, Chris Wilson <[1]chris at chris-wilson.co.uk>
>    wrote:
> 
>      It is a requirement, not just of using the NO_RELOC mode, that all
>      relocation values in the execobjects match their reloc.presumed_offset,
>      as the kernel will skip performing *any* relocation if the
>      presumed_offset
>      matches the target object. As anv is setting unknown relocations to -1
>      irrespective of the actual value, if the kernel placed the target at -1
>      (i.e. 0xfffffffff000 for 48bit GTT), it would happily skip the
>      relocation.
>      To prevent this, we need to always do the userspace relocations to
>      ensure
>      the values match. To improve further, set the unknown object offset to 0
>      (a valid location) on the offchance it is available and the migration
>      skipped.
> 
>    Is this a real issue?  I specifically chose -1 because it *wasn't*
>    page-aligned.  I don't think the kernel whacks off the bottom 12 bits
>    before doing the comparison to determine whether or not the relocation
>    needs to happen.

Yes. It is real.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre