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

[Greg KH <gregkh@suse.de>]

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?

thanks,

greg k-h