Re: [PATCH v3] x86: clear incorrectly forced X86_FEATURE_LAHF_LM flag

Mikael Pettersson
2009-08-11T14:38:13Z

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@gmail.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