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SubjectRe: [PATCH] x86: do not allow to optimize flag_is_changeable_p()
Yinghai Lu wrote:
> On Mon, Sep 29, 2008 at 11:14 PM, Jeremy Fitzhardinge <jeremy@goop.org> wrote:
>
>> Krzysztof Helt wrote:
>>
>>> From: Krzysztof Helt <krzysztof.h1@wp.pl>
>>>
>>> The flag_is_changeable_p() is used by
>>> has_cpuid_p() which can return different results
>>> in the code sequence below:
>>>
>>> if (!have_cpuid_p())
>>> identify_cpu_without_cpuid(c);
>>>
>>> /* cyrix could have cpuid enabled via c_identify()*/
>>> if (!have_cpuid_p())
>>> return;
>>>
>>> Otherwise, the gcc 3.4.6 optimizes these two calls
>>> into one which make the code not working correctly.
>>> Cyrix cpus have the CPUID instruction enabled but
>>> it is not detected due to the gcc optimization.
>>> Thus the ARR registers (mtrr like) are not detected
>>> on such a cpu.
>>>
>>>
>> If "asm volatile" changes the code and fixes the bug, it seems like
>> you're making use of an undocumented - or at least non-portable - behaviour.
>>
>> Does adding a "memory" clobber also fix the problem? That would have
>> better defined characteristics.
>>
>>
>
> how about
>
> if (!have_cpuid_p()) {
> identify_cpu_without_cpuid(c);
>
> /* cyrix could have cpuid enabled via c_identify()*/
> if (!have_cpuid_p())
> return;
> }
>

That doesn't help, does it? If gcc thinks it can get away with
evaluating have_cpuid_p() once, then that's the same as:

if (!have_cpuid_p()) {
identify_cpu_without_cpuid(c);

return;
}

even though identify_cpu_without_cpuid() can cause the cpu to suddenly
start supporting cpuid.

The trouble is that flag_is_changeable_p() doesn't have any obvious
global dependencies; it just takes a constant argument and returns a
result. The asm() needs to be updated to have a "memory" constraint as
a stand-in for the specific constraint of "cpu has switched into
cpuid-supporting state".

J


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