linux-kernel - Re: [PATCH] x86: do not allow to optimize flag_is_changeable

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Message-ID: <86802c440809292334o557d2158ob2c52aebd3caf0b3@mail.gmail.com>
Date:	Mon, 29 Sep 2008 23:34:51 -0700
From:	"Yinghai Lu" <yinghai@...nel.org>
To:	"Jeremy Fitzhardinge" <jeremy@...p.org>
Cc:	"Krzysztof Helt" <krzysztof.h1@...zta.fm>,
	linux-kernel@...r.kernel.org, "Ingo Molnar" <mingo@...e.hu>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86: do not allow to optimize flag_is_changeable_p()

On Mon, Sep 29, 2008 at 11:14 PM, Jeremy Fitzhardinge <jeremy@...p.org> wrote:
> Krzysztof Helt wrote:
>> From: Krzysztof Helt <krzysztof.h1@...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;
        }


YH
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