Re: [PATCH v3] x86: clear incorrectly forced X86_FEATURE_LAHF_LM flag

Borislav Petkov
2009-08-12T11:40:50Z

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@amd.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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