Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756997AbZIDQBe (ORCPT ); Fri, 4 Sep 2009 12:01:34 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752452AbZIDQBe (ORCPT ); Fri, 4 Sep 2009 12:01:34 -0400 Received: from claw.goop.org ([74.207.240.146]:60354 "EHLO claw.goop.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751192AbZIDQBd (ORCPT ); Fri, 4 Sep 2009 12:01:33 -0400 Message-ID: <4AA139DF.3050902@goop.org> Date: Fri, 04 Sep 2009 09:01:35 -0700 From: Jeremy Fitzhardinge User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.1) Gecko/20090814 Fedora/3.0-2.6.b3.fc11 Lightning/1.0pre Thunderbird/3.0b3 MIME-Version: 1.0 To: "H. Peter Anvin" CC: Tejun Heo , mingo@redhat.com, linux-kernel@vger.kernel.org, jeremy.fitzhardinge@citrix.com, stable@kernel.org, tglx@linutronix.de, mingo@elte.hu, linux-tip-commits@vger.kernel.org Subject: Re: [tip:x86/asm] x86/i386: Make sure stack-protector segment base is cache aligned References: <4AA01893.6000507@goop.org> <4AA02687.9080406@zytor.com> <4AA02B02.7080101@goop.org> <4AA031DE.2070109@zytor.com> <4AA080A0.7010804@kernel.org> <4AA08283.5020306@kernel.org> <4AA08B09.50503@zytor.com> In-Reply-To: <4AA08B09.50503@zytor.com> X-Enigmail-Version: 0.97a Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1179 Lines: 26 On 09/03/09 20:35, H. Peter Anvin wrote: > On 09/03/2009 07:59 PM, Tejun Heo wrote: > >> Another question. Other than saving and loading an extra segment >> register on kernel entry/exit, whether using the same or different >> segment registers doesn't look like would make difference >> performance-wise. If I'm interpreting the wording in the optimization >> manual correctly, it means that each non-zero segment based memory >> access will be costly regardless of which specific segment register is >> in use and there's no way we can merge segment based dereferences for >> stackprotector and percpu variables. >> >> > It's correct that it doesn't make any difference for access, only for load. > Well, to be completely precise, restore. When returning to usermode, the "pop %seg" is slightly faster if you're restoring a null selector, which is typically the case for %fs as 32-bit usermode doesn't use it. J -- 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/