Re: [Stable-review] [12/28] x86, cpu: Clean up AMD erratum 400workaround

[Borislav Petkov]

On Tue, May 10, 2011 at 08:13:05AM -0700, Greg KH wrote:
> On Tue, May 10, 2011 at 04:02:19PM +0200, Borislav Petkov wrote:
> > 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?
> 
> I added it because it was needed to get another errata patch to apply
> properly, and I saw no reason not to apply other "quirks" like this to
> older kernels, right?
> 
> Or should I not have done that?

Nah, I wouldn't say that. While this patch doesn't directly fix a
regression, it does away with the erratum 400 workaround where we switch
to HPET so it is more or less a regression fix for current and future
machines which aren't affected by that erratum. Besides reading the
LAPIC is much cheaper so we should be getting even a slight performance
bump with it.

So no, I don't see a problem with the patch being in stable.

Thanks.

-- 
Regards/Gruss,
Boris.

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