lkml.org 
[lkml]   [2008]   [Sep]   [30]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] x86: do not allow to optimize flag_is_changeable_p()
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.

J

> Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
> ---
>
> I have tested the 6x86MX cpu with the CPUID
> disabled. I have used linux-next tree (20080819)
> and Yinghai Lu's patch:
>
> x86: identify_cpu_without_cpuid v2
>
> http://marc.info/?l=linux-kernel&m=122138380004347&w=2
>
> The patch below is required to make the patch
> above working correctly.
>
> diff -urp linux-orig/arch/x86/kernel/cpu/common.c linux-2.6.27/arch/x86/kernel/cpu/common.c
> --- linux-orig/arch/x86/kernel/cpu/common.c 2008-09-29 07:11:54.000000000 +0200
> +++ linux-2.6.27/arch/x86/kernel/cpu/common.c 2008-09-29 18:07:27.667392725 +0200
> @@ -124,18 +124,18 @@ static inline int flag_is_changeable_p(u
> {
> u32 f1, f2;
>
> - asm("pushfl\n\t"
> - "pushfl\n\t"
> - "popl %0\n\t"
> - "movl %0,%1\n\t"
> - "xorl %2,%0\n\t"
> - "pushl %0\n\t"
> - "popfl\n\t"
> - "pushfl\n\t"
> - "popl %0\n\t"
> - "popfl\n\t"
> - : "=&r" (f1), "=&r" (f2)
> - : "ir" (flag));
> + asm volatile ("pushfl\n\t"
> + "pushfl\n\t"
> + "popl %0\n\t"
> + "movl %0,%1\n\t"
> + "xorl %2,%0\n\t"
> + "pushl %0\n\t"
> + "popfl\n\t"
> + "pushfl\n\t"
> + "popl %0\n\t"
> + "popfl\n\t"
> + : "=&r" (f1), "=&r" (f2)
> + : "ir" (flag));
>
> return ((f1^f2) & flag) != 0;
> }
>
> ----------------------------------------------------------------------
> Dzwon taniej na zagraniczne komorki!
> Sprawdz >> http://link.interia.pl/f1f26
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at http://www.tux.org/lkml/
>



\
 
 \ /
  Last update: 2008-09-30 08:17    [W:0.257 / U:0.024 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site