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

Kevin Winchester
2009-08-11T16:02:07Z

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?

--
Kevin Winchester