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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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