[tip:x86/urgent] x86: Clear incorrectly forced X86_FEATURE_LAHF_LM flag

tip-bot for Kevin Winchester
2009-08-11T11:59:36Z

Commit-ID: fbd8b1819e80ac5a176d085fdddc3a34d1499318
Gitweb: http://git.kernel.org/tip/fbd8b1819e80ac5a176d085fdddc3a34d1499318
Author: Kevin Winchester <kjwinchester@gmail.com>
AuthorDate: Mon, 10 Aug 2009 19:56:45 -0300
Committer: Ingo Molnar <mingo@elte.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@gmail.com>
Acked-by: Borislav Petkov <petkovbb@googlemail.com>
LKML-Reference: <4A80A5AD.2000209@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.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);