[PATCH 2/2] x86: Clear incorrectly forced X86_FEATURE_LAHF_LM flag

[Borislav Petkov]

From: Kevin Winchester <kjwinchester@xxxxxxxxx>

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@xxxxxxxxx>
Signed-off-by: Borislav Petkov <borislav.petkov@xxxxxxx>
---
 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) {
+			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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