From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761905AbXK2Srn (ORCPT ); Thu, 29 Nov 2007 13:47:43 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1762294AbXK2Sqz (ORCPT ); Thu, 29 Nov 2007 13:46:55 -0500 Received: from terminus.zytor.com ([198.137.202.10]:52324 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1762257AbXK2Sqy (ORCPT ); Thu, 29 Nov 2007 13:46:54 -0500 Message-ID: <474F08E1.2090806@zytor.com> Date: Thu, 29 Nov 2007 10:45:53 -0800 From: "H. Peter Anvin" User-Agent: Thunderbird 2.0.0.9 (X11/20071115) MIME-Version: 1.0 To: Linus Torvalds CC: Chuck Ebbert , Roland McGrath , Andrew Morton , linux-kernel@vger.kernel.org, Thomas Gleixner , Ingo Molnar Subject: Re: [PATCH x86/mm 6/6] x86-64 ia32 ptrace get/putreg32 current task References: <20071129003849.428E026F8E7@magilla.localdomain> <20071129004222.E49AD26F8E7@magilla.localdomain> <474EF824.3020806@redhat.com> <474F01F6.2030509@zytor.com> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org 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