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    SubjectRe: [PATCH 2/2] x86: Clear incorrectly forced X86_FEATURE_LAHF_LM flag
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    On Thu, Aug 13, 2009 at 8:31 AM, Borislav Petkov<borislav.petkov@amd.com> wrote:
    > From: Kevin Winchester <kjwinchester@gmail.com>
    >
    > 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.
    >
    > Borislav: turn off cpuid bit.
    >
    > Signed-off-by: Kevin Winchester <kjwinchester@gmail.com>
    > Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>
    > ---
    >  arch/x86/kernel/cpu/amd.c |   16 ++++++++++++++++
    >  1 files changed, 16 insertions(+), 0 deletions(-)
    >
    > diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
    > index e2485b0..9cd6fc7 100644
    > --- a/arch/x86/kernel/cpu/amd.c
    > +++ b/arch/x86/kernel/cpu/amd.c
    > @@ -400,6 +400,22 @@ 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) {

    Shouldn't you test that the flag is actually set before trying to clear it?

    > +                       u64 val;
    > +
    > +                       clear_cpu_cap(c, X86_FEATURE_LAHF_LM);
    > +                       if (!rdmsrl_amd_safe(0xc001100d, &val)) {
    > +                               val &= ~(1ULL << 32);
    > +                               wrmsr_amd_safe(0xc001100d, (u32) val,
    > +                                                          (u32)(val >> 32));
    > +                       }
    > +               }
    > +
    >        }
    >        if (c->x86 == 0x10 || c->x86 == 0x11)
    >                set_cpu_cap(c, X86_FEATURE_REP_GOOD);
    > --
    > 1.6.3.3
    >
    >
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