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

Kevin Winchester
2009-08-13T22:53:17Z

Brian Gerst wrote:
> On Thu, Aug 13, 2009 at 10:54 AM, Kevin
> Winchester<kjwinchester@gmail.com> wrote:
>> 2009/8/13 Brian Gerst <brgerst@gmail.com>:
>>> On Thu, Aug 13, 2009 at 8:31 AM, Borislav Petkov<borislav.petkov@amd.com> wrote:
>>>> From: Kevin Winchester <kjwinchester@gmail.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@gmail.com>
>>>> Signed-off-by: Borislav Petkov <borislav.petkov@amd.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.
>
> Testing a bit is cheap and MSR accesses are not.
>
>> - The extra instructions might actually cause more problems if the
>> flag is not set.
>
> These MSRs don't exist on older cpus and will cause a fault, which is
> handled at additional cost.
>

I stand corrected. I was unaware of this, so I guess testing the flag first
would be a good idea.

Thanks,

--
Kevin Winchester