linux-kernel - Re: [PATCH 1/1] x86: fix text

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Date:	Fri, 25 Apr 2008 13:18:35 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
CC:	Andi Kleen <andi@...stfloor.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, Jiri Slaby <jirislaby@...il.com>,
	David Miller <davem@...emloft.net>, zdenek.kabelac@...il.com,
	rjw@...k.pl, paulmck@...ux.vnet.ibm.com, akpm@...ux-foundation.org,
	linux-ext4@...r.kernel.org, herbert@...dor.apana.org.au,
	penberg@...helsinki.fi, clameter@....com,
	linux-kernel@...r.kernel.org, pageexec@...email.hu,
	Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: [PATCH 1/1] x86: fix text_poke

Mathieu Desnoyers wrote:
> 
> This epilogue should then be used on both branches of the condition,
> like this :
> 
> if (unlikely(imv_cond(var))) {
>   imv_cond_end();
>   ...
> } else {
>   imv_cond_end();
>    ...
> }
> 
> Where imv_cond_end() would look like this :
> 
> +/*
> + * Puts a test and branch make sure the %al register and ZF are not live
> + * anymore.
> + * All asm statements clobbers the flags, but add "cc" clobber just to be sure.
> + * Clobbers %al.
> + */
> +#define imv_cond_end()                                                 \
> +       do {                                                            \
> +               asm (".section __imv_cond_end,\"a\",@progbits\n\t"      \
> +                               _ASM_PTR "1f\n\t"                       \
> +                               ".previous\n\t"                         \
> +                               "1:\n\t"                                \
> +                               : : : "a", "cc");                       \
> +       } while (0)
> +
> 

As far as this is concerned, all you accomplish here is that gcc, if it 
wants to re-use the %al value, will copy it into another register before 
doing your imv_conv_end().

	-hpa
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