Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755069AbYGIVsR (ORCPT ); Wed, 9 Jul 2008 17:48:17 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751574AbYGIVsF (ORCPT ); Wed, 9 Jul 2008 17:48:05 -0400 Received: from gw.goop.org ([64.81.55.164]:57961 "EHLO mail.goop.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751502AbYGIVsD (ORCPT ); Wed, 9 Jul 2008 17:48:03 -0400 Message-ID: <48753207.7070308@goop.org> Date: Wed, 09 Jul 2008 14:47:51 -0700 From: Jeremy Fitzhardinge User-Agent: Thunderbird 2.0.0.14 (X11/20080501) MIME-Version: 1.0 To: Mike Travis CC: "Eric W. Biederman" , Ingo Molnar , Andrew Morton , "H. Peter Anvin" , Christoph Lameter , Jack Steiner , linux-kernel@vger.kernel.org Subject: Re: [RFC 00/15] x86_64: Optimize percpu accesses References: <20080709165129.292635000@polaris-admin.engr.sgi.com> <4875301D.20805@sgi.com> In-Reply-To: <4875301D.20805@sgi.com> X-Enigmail-Version: 0.95.6 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2509 Lines: 56 Mike Travis wrote: > Eric W. Biederman wrote: > >> I just took a quick look at how stack_protector works on x86_64. Unless there is >> some deep kernel magic that changes the segment register to %gs from the ABI specified >> %fs CC_STACKPROTECTOR is totally broken on x86_64. We access our pda through %gs. >> >> Further -fstack-protector-all only seems to detect against buffer overflows and >> thus corruption of the stack. Not stack overflows. So it doesn't appear especially >> useful. >> >> So we don't we kill the broken CONFIG_CC_STACKPROTECTOR. Stop trying to figure out >> how to use a zero based percpu area. >> >> That should allow us to make the current pda a per cpu variable, and use %gs with >> a large offset to access the per cpu area. And since it is only the per cpu accesses >> and the pda accesses that will change we should not need to fight toolchain issues >> and other weirdness. The linked binary can remain the same. >> >> Eric >> > > Hi Eric, > > There is one pda op that I was not able to remove. Most likely it can be recoded > but it was a bit over my expertise. Most likely the "pda_offset(field)" can be > replaced with "per_cpu_var(field)" [per_cpu__##field], but for "_proxy_pda.field" > I wasn't sure about. > > include/asm-x86/pda.h: > > /* > * This is not atomic against other CPUs -- CPU preemption needs to be off > * NOTE: This relies on the fact that the cpu_pda is the *first* field in > * the per cpu area. Move it and you'll need to change this. > */ > #define test_and_clear_bit_pda(bit, field) \ > ({ \ > int old__; \ > asm volatile("btr %2,%%gs:%c3\n\tsbbl %0,%0" \ > : "=r" (old__), "+m" (_proxy_pda.field) \ > : "dIr" (bit), "i" (pda_offset(field)) : "memory");\ > asm volatile("btr %2,%%gs:%1\n\tsbbl %0,%0" \ : "=r" (old__), "+m" (per_cpu_var(var)) \ : "dIr" (bit) : "memory");\ but it barely seems worthwhile if we really can't use test_and_clear_bit. 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/