linux-kernel - Re: [PATCH v3] x86: clear incorrectly forced X86_FEATURE_LAHF

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Message-ID: <20090811160116.GD16173@aftab>
Date:	Tue, 11 Aug 2009 18:01:16 +0200
From:	Borislav Petkov <borislav.petkov@....com>
To:	Kevin Winchester <kjwinchester@...il.com>
CC:	Mikael Pettersson <mikpe@...uu.se>,
	Borislav Petkov <petkovbb@...glemail.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] x86: clear incorrectly forced X86_FEATURE_LAHF_LM
 flag

On Tue, Aug 11, 2009 at 12:55:03PM -0300, Kevin Winchester wrote:
> 2009/8/11 Borislav Petkov <borislav.petkov@....com>:
> > On Tue, Aug 11, 2009 at 04:37:56PM +0200, Mikael Pettersson wrote:
> >> Since the BIOS apparently wrote some MSR to get LAHF_LM incorrectly
> >> reported by CPUID, would it be possible to also correct that MSR so
> >> that applications that execute CPUID get the correct feature flags?
> >
> > That's a good catch, actually. We have to turn off that bit in the cpuid
> > leaf too if the CPU doesn't support the instructions so that cpuid info
> > is consistent. LAHF/SAHF support in 64bit mode has to be cpuid-checked
> > prior to using them so that info has to be correct.
> >
> > @Kevin: willing to try a patch or two?
> >
> 
> Sure, I'll give it a try this evening.  I assume that since Erratum 110 says:
> 
> --------------------------
> Suggested Workaround
> For processors which support the feature (as determined by the
> processor revision ID), BIOS should
> write a one to:
> • MSR C001_100Dh, bit 32 for revision D silicon.
> • MSR C001_1005h, bit 32 for revision E and later silicon.
> This will cause the extended feature flag in ECX[0] to be set.
> --------------------------
> 
> That writing a zero to those same MSRs would clear the feature flag?

Yep :). Patch coming up...

-- 
Regards/Gruss,
Boris.

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