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

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Date:	Tue, 11 Aug 2009 16:37:56 +0200
From:	Mikael Pettersson <mikpe@...uu.se>
To:	Kevin Winchester <kjwinchester@...il.com>
Cc:	Borislav Petkov <borislav.petkov@....com>,
	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

Kevin Winchester writes:
 > Due to an erratum with certain AMD Athlon 64 processors, the BIOS may
 > need to force enable the LAHF_LM capability.  Unfortunately, in at
 > least one case, the BIOS does this even for processors that do not
 > support the functionality.
 > 
 > Add a specific check that will clear the feature bit for processors
 > known not to support the LAHF/SAHF instructions.
 > 
 > Signed-off-by: Kevin Winchester <kjwinchester@...il.com>
 > ---
 >  arch/x86/kernel/cpu/amd.c |    7 +++++++
 >  1 files changed, 7 insertions(+), 0 deletions(-)
 > 
 > diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
 > index e2485b0..63fddcd 100644
 > --- a/arch/x86/kernel/cpu/amd.c
 > +++ b/arch/x86/kernel/cpu/amd.c
 > @@ -400,6 +400,13 @@ static void __cpuinit init_amd(struct cpuinfo_x86 *c)
 >  		level = cpuid_eax(1);
 >  		if((level >= 0x0f48 && level < 0x0f50) || level >= 0x0f58)
 >  			set_cpu_cap(c, X86_FEATURE_REP_GOOD);
 > +
 > +		/*
 > +		 * Some BIOSes incorrectly force this feature, but only K8
 > +		 * revision D (model = 0x14) and later actually support it.
 > +		 */
 > +		if (c->x86_model < 0x14)
 > +			clear_cpu_cap(c, X86_FEATURE_LAHF_LM);
 >  	}
 >  	if (c->x86 == 0x10 || c->x86 == 0x11)
 >  		set_cpu_cap(c, X86_FEATURE_REP_GOOD);

But this only fixes /proc/cpuinfo, right?

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?

/Mikael
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