Re: [PATCH v2] x86: clear incorrectly forced X86_FEATURE_LAHF_LM flag

Borislav Petkov
2009-08-10T13:12:36Z

Hi,

On Sat, Aug 08, 2009 at 08:17:43PM -0300, Kevin Winchester wrote:
> 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@gmail.com>
> ---
> arch/x86/kernel/cpu/amd.c | 8 ++++++++
> 1 files changed, 8 insertions(+), 0 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
> index e2485b0..7b52787 100644
> --- a/arch/x86/kernel/cpu/amd.c
> +++ b/arch/x86/kernel/cpu/amd.c
> @@ -400,6 +400,14 @@ 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 set this feature, but only Revisions
> + * D (Extended Model = 1) and E (Extended Model = 2) actually
> + * support it.
> + */
> + if (!(level & 0x00030000))
> + clear_cpu_cap(c, X86_FEATURE_LAHF_LM);

The LAHF/SAHF instructions are supported on all K8s from revision D
upwards. Revisions D start their CPU model numbering at 0x14 so your
check should rather be

if (c->x86_model < 0x14)
clear_cpu_cap(c, X86_FEATURE_LAHF_LM);

Please correct the comment above too for future reference.

Thanks.

--
Regards/Gruss,
Boris.

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