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Date: Fri, 20 Dec 2002 15:50:37 -0800
From: "H. Peter Anvin" <hpa@transmeta.com>
Organization: Transmeta Corporation
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To: Jamie Lokier <lk@tantalophile.demon.co.uk>
CC: Linus Torvalds <torvalds@transmeta.com>,
       Ulrich Drepper <drepper@redhat.com>, bart@etpmod.phys.tue.nl,
       davej@codemonkey.org.uk, terje.eggestad@scali.com,
       matti.aarnio@zmailer.org, hugh@veritas.com, mingo@elte.hu,
       linux-kernel@vger.kernel.org
Subject: Re: Intel P6 vs P7 system call performance
References: <20021220120656.GA20674@bjl1.asuk.net> <Pine.LNX.4.44.0212200834590.2035-100000@home.transmeta.com> <20021220233825.GA22232@bjl1.asuk.net>
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Jamie Lokier wrote:
> 
> No, your "real" code sequence is wrong.
> 
> %ebx/%edi/%esi are preserved across sysenter/sysexit, whereas
> %ecx/%edx are call-clobbered registers in the i386 function call ABI.
> 
> This is not a coincidence.
> 
> So, getpid looks like this with the _smaller_ vsyscall code:
> 
>  	getpid():
>  		movl $__NR_getpid,%eax
>  		call *%gs:0x18
>  		ret

... or just...

getpid:
	movl $__NR_getpid, %eax
	jmp *%gs:0x18

This doesn't mess up the call/return stack, even, because the ret in the
stub matches the call to getpid.

	-hpa

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