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

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Date:	Wed, 12 Aug 2009 13:40:38 +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 09:15:50PM -0300, Kevin Winchester wrote:
> Borislav Petkov wrote:
> > 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...
> > 
> 
> I have been attempting to read those MSRs through the /dev/cpu/0/msr device
> file, without any success.  Is it possible that my CPU will not have those
> MSRs?  And if so, then maybe my original assumption about the BIOS forcing
> on the LAHF_LM feature is wrong.
> 
> In any case, clearing the feature flag (and thus fixing /proc/cpuinfo) is
> still the right thing to do.
> 
> Do you have any other suggestions for how I would affect that CPUID flag?

Before we do that though I'd like to verify that the BIOS is falsely
setting that bit. Can you run this small c program on your machine and
send me the result:

#include <stdio.h>

int main()
{

	unsigned int ecx = 0;

	asm volatile("cpuid"
		     : "=c" (ecx)
		     : "a" (0x80000001));

	printf("0x8000_0001_ecx = 0x%08x\n", ecx);

	return 0;
}


-- 
Regards/Gruss,
Boris.

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