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Subject[PATCH] 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>
---

While making this change, I noticed the clause above my code:

if((level >= 0x0f48 && level < 0x0f50) || level >= 0x0f58)

It does not seem concerned with the possibility that some of the
upper 16 bits of level will be non-zero. Is this intentional, or
should the upper 16 bits be masked off before the comparisons?

arch/x86/kernel/cpu/amd.c | 8 ++++++++
1 files changed, 8 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index e2485b0..a2f0fe4 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -400,6 +400,14 @@ 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 set this feature, but only
+ * Revision E (with Extended Model = 2) actually supports
+ * it.
+ */
+ if (!(level & 0x00020000))
+ 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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