linux-kernel - Re: [PATCH x86/mm 6/6] x86-64 ia32 ptrace get/putreg32 current task

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Message-ID: <474F08E1.2090806@zytor.com>
Date:	Thu, 29 Nov 2007 10:45:53 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Chuck Ebbert <cebbert@...hat.com>,
	Roland McGrath <roland@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH x86/mm 6/6] x86-64 ia32 ptrace get/putreg32 current task

Linus Torvalds wrote:
> 
> However, you also say:
> 
>> It is advantageous for user space to use the register the kernel 
>> typically won't, in order to speed up system call entry/exit.
> 
> but I'm not seeing the reason for that one. Care to comment more? (Yes, 
> there is often a latency from segment reload to use, but the reload 
> latency for system call exit *should* be entirely covered by the cost of 
> doing the system call return itself, no?)
> 

I do seem to recall that some processor implementations can load a NULL 
segment faster than a non-NULL segment.  This was significant enough 
that we wanted to use %fs in x86-64 userspace, as opposed to the 
original ABI which used %gs both in userspace and in the kernel.

	-hpa
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