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

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Date:	Tue, 10 May 2011 16:02:19 +0200
From:	Borislav Petkov <bp@...64.org>
To:	Ben Hutchings <ben@...adent.org.uk>
Cc:	Borislav Petkov <bp@...en8.de>, Greg KH <gregkh@...e.de>,
	"Rosenfeld, Hans" <Hans.Rosenfeld@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"stable@...nel.org" <stable@...nel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	"stable-review@...nel.org" <stable-review@...nel.org>,
	"alan@...rguk.ukuu.org.uk" <alan@...rguk.ukuu.org.uk>,
	"bp@...64.org" <bp@...64.org>,
	"Ostrovsky, Boris" <Boris.Ostrovsky@....com>
Subject: Re: [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?

Thanks.

-- 
Regards/Gruss,
Boris.

Advanced Micro Devices GmbH
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