From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932238AbWH0S1W (ORCPT ); Sun, 27 Aug 2006 14:27:22 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932241AbWH0S1V (ORCPT ); Sun, 27 Aug 2006 14:27:21 -0400 Received: from gw.goop.org ([64.81.55.164]:14788 "EHLO mail.goop.org") by vger.kernel.org with ESMTP id S932238AbWH0S1U (ORCPT ); Sun, 27 Aug 2006 14:27:20 -0400 Message-ID: <44F1E405.6090208@goop.org> Date: Sun, 27 Aug 2006 11:27:17 -0700 From: Jeremy Fitzhardinge User-Agent: Thunderbird 1.5.0.5 (X11/20060803) MIME-Version: 1.0 To: Andi Kleen CC: Arjan van de Ven , linux-kernel@vger.kernel.org, Chuck Ebbert <76306.1226@compuserve.com>, Zachary Amsden , Jan Beulich , Andrew Morton Subject: Re: [PATCH RFC 0/6] Implement per-processor data areas for i386. References: <20060827084417.918992193@goop.org> <44F1CC67.8040807@goop.org> <1156700663.3034.118.camel@laptopd505.fenrus.org> <200608272007.47741.ak@suse.de> In-Reply-To: <200608272007.47741.ak@suse.de> Content-Type: text/plain; charset=ISO-8859-15; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Andi Kleen wrote: > On AMD K7/K8 a segment register prefix is a single cycle penalty. > > I couldn't find anything in the Intel optimization manuals on it, but I assume > it's also not dramatic. > All I could find was: * avoid multiple prefixes (which was the least important guideline in instruction selection) * avoid using multiple segment registers (the pentium M only has one level of segment register renaming) * avoid prefixes which take the instruction length over 7 bytes None of these apply to the use of %gs to access PDA. Most of the discussion about prefixes is in avoiding the 0x66 16-bit prefix. J