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SubjectRe: [Stable-review] [12/28] x86, cpu: Clean up AMD erratum 400 workaround
On Tue, May 10, 2011 at 08:58:59AM -0400, Ben Hutchings wrote:
> I read that:
> "
> Support for Always Running APIC timer (ARAT) was introduced in
> commit db954b5898dd3ef3ef93f4144158ea8f97deb058. This feature
> allows us to avoid switching timers from LAPIC to something else
> (e.g. HPET) and go into timer broadcasts when entering deep
> C-states.
> AMD processors don't provide a CPUID bit for that feature but
> they also keep APIC timers running in deep C-states (except for
> cases when the processor is affected by erratum 400). Therefore
> we should set ARAT feature bit on AMD CPUs.
> "
> This implies that the HPET was previously used during deep C-states, and
> that all this erratum checking is about deciding whether the CPU has
> ARAT. So what bug is being fixed by using ARAT instead of the HPET?

That's a good question, actually. The original upstream commit
b87cf80af3ba4b4c008b4face3c68d604e1715c6 wasn't tagged for stable and
Boris O. didn't send it to stable either, as I'm being told.

Maybe Greg has an idea?



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