linux-kernel - Re: [KJ PATCH] Replacing memcpy(dest,src,PAGE_SIZE) with copy_page(dest,src) in arch/i386/kernel/machine

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Date:	Tue, 12 Jun 2007 09:52:07 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Shani Moideen <shani.moideen@...ro.com>, kexec@...ts.infradead.org,
	linux-kernel@...r.kernel.org, kernel-janitors@...ts.osdl.org
Subject: Re: [KJ PATCH] Replacing memcpy(dest,src,PAGE_SIZE) with copy_page(dest,src)
 in arch/i386/kernel/machine_kexec.c

Eric W. Biederman wrote:
> Shani Moideen <shani.moideen@...ro.com> writes:
> 
>> Hi,
>> Replacing memcpy(dest,src,PAGE_SIZE) with copy_page(dest,src) in
>> arch/i386/kernel/machine_kexec.c.
> 
> Please no.
> 
> People get creative in copy_page (especially mmx_copy_page),
> and this code path need something simple and stupid, that
> will work every time, especially when things are messed up
> elsewhere.
> 

memcpy() is pretty creative, too.  If you want something really dumb and
robust, you should probably create an inline:

/*
 * Eric sez: his code path need something simple and stupid, that
 * will work every time, especially when things are messed up
 * elsewhere.
 */
static inline void braindead_copy_page(void *dst, void *src)
{
	unsigned long ctr = PAGE_SIZE >> 2;

	asm volatile("cld; rep; movsl"
			: "+D" (dst), "+S" (src), "+c" (ctr)
			: : "memory");
}

(FWIW, the "cld" is supposed to be redundant, as DF=0 is supposed to be
guaranteed by the ABI.)

	-hpa
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