linux-kernel - [PATCH v3] x86: clear incorrectly forced X86_FEATURE_LAHF

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Date:	Mon, 10 Aug 2009 19:56:45 -0300
From:	Kevin Winchester <kjwinchester@...il.com>
To:	Borislav Petkov <borislav.petkov@....com>
CC:	Borislav Petkov <petkovbb@...glemail.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH v3] x86: clear incorrectly forced X86_FEATURE_LAHF_LM flag

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 |    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);
-- 
1.6.4


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