linux-kernel - [PATCH] x86: clear incorrectly forced X86_FEATURE_LAHF

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Date:	Sat, 08 Aug 2009 08:53:30 -0300
From:	Kevin Winchester <kjwinchester@...il.com>
To:	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
CC:	Yinghai Lu <yinghai@...nel.org>,
	Andreas Herrmann <andreas.herrmann3@....com>,
	LKML <linux-kernel@...r.kernel.org>
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@...il.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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