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

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Date:	Wed, 1 Oct 2008 09:11:38 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Krzysztof Helt <krzysztof.h1@...pl>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Yinghai Lu <yhlu.kernel@...il.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: [PATCH] x86: do not allow to optimize flag_is_changeable_p()
	(rev. 2)


* Krzysztof Helt <krzysztof.h1@...pl> 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 before
> the second call to the have_cpuid_p() but
> it is not detected due to the gcc optimization.
> Thus the ARR registers (mtrr like) are not detected
> on such a cpu.
> 
> Signed-off-by: Krzysztof Helt <krzysztof.h1@...pl>

applied to tip/x86/core, thanks Krzysztof!

	Ingo
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