[Bug 108700] 2x Media CPU power usage, or 15% perf drop when CPU bound GPU use-case is TDP limited

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=108700

--- Comment #7 from Eero Tamminen <eero.t.tamminen at intel.com> ---
We don't specifically enable any mitigations, just use drm-tip kernel defaults.
It seems to have enabled an additional one when it was rebased to 4.20-rc1:
  Spectre V2 : Spectre v2 cross-process SMT mitigation: Enabling STIBP


Threading in the listed test-cases:

* MediaSDK 50 stream transcode case has 250 threads

* I thought GfxBench Driver2 doesn't thread, as only single CPU is busy, but it
actually uses 3 threads of which 2 use as much CPU as they can, and apparently
kernel just sticks them to same core, so they seem hyperthreaded

-> I think that SMT mitigation is very likely cause for the drop instead of
i915.


Could you point out suitable drm-tip commit IDs before and after enabling the
mitigation so that I could verify it?

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