From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1763287AbXKTCSx (ORCPT ); Mon, 19 Nov 2007 21:18:53 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1762909AbXKTCQq (ORCPT ); Mon, 19 Nov 2007 21:16:46 -0500 Received: from terminus.zytor.com ([198.137.202.10]:47634 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1762910AbXKTCQp (ORCPT ); Mon, 19 Nov 2007 21:16:45 -0500 Message-ID: <4742435F.6010803@zytor.com> Date: Mon, 19 Nov 2007 18:15:59 -0800 From: "H. Peter Anvin" User-Agent: Thunderbird 2.0.0.9 (X11/20071115) MIME-Version: 1.0 To: Christoph Lameter CC: ak@suse.de, akpm@linux-foundation.org, travis@sgi.com, Mathieu Desnoyers , linux-kernel@vger.kernel.org Subject: Re: [rfc 37/45] x86_64: Support for fast per cpu operations References: <20071120011132.143632442@sgi.com> <20071120011340.569486037@sgi.com> <47423FB7.20800@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 Christoph Lameter wrote: > On Mon, 19 Nov 2007, H. Peter Anvin wrote: > >> There was, at some point, discussion about using the gcc TLS mechanism, which >> should permit even better code to be generated. Unfortunately, it would > > How would that be possible? Oh. You mean the discussion where I mentioned > using the thread attribute? > >> require gcc to be able to reference %gs instead of %fs (and vice versa for >> i386), which I don't think is available in anything except maybe the most >> cutting-edge version of gcc. > > Right. That is why we do it in ASM here. > >> However, if we're doing a masssive revampt it would be good to get an idea of >> how to migrate to that model eventually, or why it doesn't make sense at all. > > If you can tell me what the difference would be then we can discuss it. > AFAICT there is no difference. Both use a segment register. > As far as I can tell from a *very* brief look at your code (which means I might have misread it), these are the differences: - gcc uses %fs:0 to contain a pointer to itself. - gcc uses absolute offsets from the thread pointer, rather than adding %rip. The %rip-based form is actually more efficient, but it does affect the usable range off the base pointer. -hpa