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>

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index e2485b0..63fddcd 100644 (file)
--- 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);