linux-kernel - [tip:x86/urgent] x86: Clear incorrectly forced X86_FEATURE_LAHF

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Date:	Tue, 11 Aug 2009 11:36:39 GMT
From:	tip-bot for Kevin Winchester <kjwinchester@...il.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...hat.com,
	petkovbb@...glemail.com, kjwinchester@...il.com,
	tglx@...utronix.de, mingo@...e.hu
Subject: [tip:x86/urgent] x86: Clear incorrectly forced X86_FEATURE_LAHF_LM flag

Commit-ID:  fbd8b1819e80ac5a176d085fdddc3a34d1499318
Gitweb:     http://git.kernel.org/tip/fbd8b1819e80ac5a176d085fdddc3a34d1499318
Author:     Kevin Winchester <kjwinchester@...il.com>
AuthorDate: Mon, 10 Aug 2009 19:56:45 -0300
Committer:  Ingo Molnar <mingo@...e.hu>
CommitDate: Tue, 11 Aug 2009 13:34:54 +0200

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>
Acked-by: Borislav Petkov <petkovbb@...glemail.com>
LKML-Reference: <4A80A5AD.2000209@...il.com>
Signed-off-by: Ingo Molnar <mingo@...e.hu>


---
 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);
--
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