Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756195AbYGIVMP (ORCPT ); Wed, 9 Jul 2008 17:12:15 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751967AbYGIVL7 (ORCPT ); Wed, 9 Jul 2008 17:11:59 -0400 Received: from out01.mta.xmission.com ([166.70.13.231]:46629 "EHLO out01.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751049AbYGIVL5 (ORCPT ); Wed, 9 Jul 2008 17:11:57 -0400 From: ebiederm@xmission.com (Eric W. Biederman) To: Jeremy Fitzhardinge Cc: "H. Peter Anvin" , Christoph Lameter , Ingo Molnar , Mike Travis , Andrew Morton , Jack Steiner , linux-kernel@vger.kernel.org, Arjan van de Ven References: <20080709165129.292635000@polaris-admin.engr.sgi.com> <20080709200757.GD14009@elte.hu> <48751B57.8030605@goop.org> <48751CF9.4020901@linux-foundation.org> <4875209D.8010603@goop.org> <487522A8.60906@zytor.com> <48752409.1030201@goop.org> Date: Wed, 09 Jul 2008 14:06:27 -0700 In-Reply-To: <48752409.1030201@goop.org> (Jeremy Fitzhardinge's message of "Wed, 09 Jul 2008 13:48:09 -0700") Message-ID: User-Agent: Gnus/5.110006 (No Gnus v0.6) Emacs/21.4 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-SA-Exim-Connect-IP: 24.130.11.59 X-SA-Exim-Mail-From: ebiederm@xmission.com X-Spam-DCC: XMission; sa04 1397; Body=1 Fuz1=1 Fuz2=1 X-Spam-Combo: ;Jeremy Fitzhardinge X-Spam-Relay-Country: X-Spam-Report: * -1.8 ALL_TRUSTED Passed through trusted hosts only via SMTP * 0.0 T_TM2_M_HEADER_IN_MSG BODY: T_TM2_M_HEADER_IN_MSG * -0.7 BAYES_20 BODY: Bayesian spam probability is 5 to 20% * [score: 0.0971] * -0.0 DCC_CHECK_NEGATIVE Not listed in DCC * [sa04 1397; Body=1 Fuz1=1 Fuz2=1] * 0.0 XM_SPF_Neutral SPF-Neutral Subject: Re: [RFC 00/15] x86_64: Optimize percpu accesses X-SA-Exim-Version: 4.2 (built Thu, 03 Mar 2005 10:44:12 +0100) X-SA-Exim-Scanned: Yes (on mgr1.xmission.com) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1590 Lines: 44 Jeremy Fitzhardinge writes: > H. Peter Anvin wrote: >> Thinking about this some more, I don't know if it would make sense to put the >> x86-64 stack canary at the *end* of the percpu area, and otherwise use >> negative offsets. That would make sure they were readily reachable from >> %rip-based references from within the kernel text area. > > If we can move the canary then a whole pile of options open up. But the problem > is that we can't. But we can pick an arbitrary point where %gs points at. Hmm. This whole thing is even sillier then I thought. Why can't we access per cpu vars as: %gs:(per_cpu__var - __per_cpu_start) ? If we can subtract constants and allow the linker to perform that resolution at link. A zero based per cpu segment becomes a moot issue. We may need to change the definition of PERCPU in vmlinux.lds.h to #define PERCPU(align) \ . = ALIGN(align); \ - __per_cpu_start = .; \ .data.percpu : AT(ADDR(.data.percpu) - LOAD_OFFSET) { \ + __per_cpu_start = .; \ *(.data.percpu) \ *(.data.percpu.shared_aligned) \ + __per_cpu_end = .; \ + } - } \ - __per_cpu_end = .; So that the linker knows __per_cpu_start and __per_cpu_end are in the same section but otherwise it sounds entirely reasonable. Just slightly trickier math at link time. Eric -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/