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Message-ID: <73c1f2160908130721y3c14c4e7h1ff967bac867d7e6@mail.gmail.com>
Date: Thu, 13 Aug 2009 10:21:22 -0400
From: Brian Gerst <brgerst@...il.com>
To: Borislav Petkov <borislav.petkov@....com>
Cc: kjwinchester@...il.com, mikpe@...uu.se, mingo@...e.hu,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] x86: Clear incorrectly forced X86_FEATURE_LAHF_LM
flag
On Thu, Aug 13, 2009 at 8:31 AM, Borislav Petkov<borislav.petkov@....com> wrote:
> From: Kevin Winchester <kjwinchester@...il.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@...il.com>
> Signed-off-by: Borislav Petkov <borislav.petkov@....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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