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Message-ID: <d1af83620908130754i7db4bd33p26cd54d0c30d68a5@mail.gmail.com>
Date: Thu, 13 Aug 2009 11:54:05 -0300
From: Kevin Winchester <kjwinchester@...il.com>
To: Brian Gerst <brgerst@...il.com>
Cc: Borislav Petkov <borislav.petkov@....com>, mikpe@...uu.se,
mingo@...e.hu, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] x86: Clear incorrectly forced X86_FEATURE_LAHF_LM
flag
2009/8/13 Brian Gerst <brgerst@...il.com>:
> On Thu, Aug 13, 2009 at 8:31 AM, Borislav Petkov<borislav.petkov@....com> wrote:
>> From: Kevin Winchester <kjwinchester@...il.com>
>>
>> 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@...il.com>
>> Signed-off-by: Borislav Petkov <borislav.petkov@....com>
>> ---
>> 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) {
>
> Shouldn't you test that the flag is actually set before trying to clear it?
>
Possibly. If there were some concern that:
- The extra instructions would cause a performance impact, and the
test was significantly faster than the clear.
- The extra instructions might actually cause more problems if the
flag is not set.
Then we would certainly want to test it first. In my opinion, a few
simple instructions to clear the flag and the CPUID bit will not
affect performance, and clearing a flag that is already cleared should
not cause any additional problems, so I would not bother testing the
flag first. That results in fewer lines of code to change.
--
Kevin Winchester
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