Re: [PATCH v3] x86: clear incorrectly forced X86_FEATURE_LAHF_LMflag

[Borislav Petkov]

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@xxxxxxx>:
> >>> 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:
> >> â MSR C001_100Dh, bit 32 for revision D silicon.
> >> â 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.

Operating | Advanced Micro Devices GmbH
  System  | Karl-Hammerschmidt-Str. 34, 85609 Dornach b. MÃnchen, Germany
 Research | GeschÃftsfÃhrer: Thomas M. McCoy, Giuliano Meroni
  Center  | Sitz: Dornach, Gemeinde Aschheim, Landkreis MÃnchen
  (OSRC)  | Registergericht MÃnchen, HRB Nr. 43632

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/