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

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Date:	Mon, 29 Sep 2008 11:17:34 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Krzysztof Helt <krzysztof.h1@...zta.fm>
CC:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Yinghai Lu <yhlu.kernel@...il.com>
Subject: Re: [PATCH] x86: do not allow to optimize flag_is_changeable_p()

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.
> 

I suspect we should also out-of-line this function.  It's actually 
relatively sizable and certainly there is no point in inlining it.

	-hpa
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