linux-kernel - Re: [PATCH v2] x86: clear incorrectly forced X86_FEATURE_LAHF

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Date:	Mon, 10 Aug 2009 15:12:19 +0200
From:	Borislav Petkov <borislav.petkov@....com>
To:	Kevin Winchester <kjwinchester@...il.com>
CC:	Borislav Petkov <petkovbb@...glemail.com>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Yinghai Lu <yinghai@...nel.org>,
	Andreas Herrmann <andreas.herrmann3@....com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] x86: clear incorrectly forced X86_FEATURE_LAHF_LM
 flag

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@...il.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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