lists.openwall.net   lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  linux-cve-announce  PHC 
Open Source and information security mailing list archives
 
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Powered by blists - more mailing lists

Powered by Openwall GNU/*/Linux Powered by OpenVZ