From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755482AbXGOCyg (ORCPT ); Sat, 14 Jul 2007 22:54:36 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751440AbXGOCy2 (ORCPT ); Sat, 14 Jul 2007 22:54:28 -0400 Received: from tomts16-srv.bellnexxia.net ([209.226.175.4]:54532 "EHLO tomts16-srv.bellnexxia.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751350AbXGOCy2 (ORCPT ); Sat, 14 Jul 2007 22:54:28 -0400 Date: Sat, 14 Jul 2007 22:54:25 -0400 From: Mathieu Desnoyers To: Alexey Dobriyan Cc: akpm@linux-foundation.org, linux-kernel@vger.kernel.org Subject: Re: [patch 1/2] F00F bug fixup for i386 - use immediate values Message-ID: <20070715025425.GA26164@Krystal> References: <20070714012642.499375439@polymtl.ca> <20070714012743.511046887@polymtl.ca> <20070714072239.GA5822@martell.zuzino.mipt.ru> <20070715024931.GA25152@Krystal> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Content-Disposition: inline In-Reply-To: <20070715024931.GA25152@Krystal> X-Editor: vi X-Info: http://krystal.dyndns.org:8080 X-Operating-System: Linux/2.6.21.3-grsec (i686) X-Uptime: 22:52:06 up 7 days, 16:57, 2 users, load average: 0.72, 0.55, 0.39 User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org * Mathieu Desnoyers (mathieu.desnoyers@polymtl.ca) wrote: > * Alexey Dobriyan (adobriyan@gmail.com) wrote: > > On Fri, Jul 13, 2007 at 09:26:43PM -0400, Mathieu Desnoyers wrote: > > > Use the faster immediate values for F00F bug handling in do_page_fault. > > > > > --- linux-2.6-lttng.orig/arch/i386/mm/fault.c > > > +++ linux-2.6-lttng/arch/i386/mm/fault.c > > > @@ -492,7 +493,7 @@ > > > /* > > > * Pentium F0 0F C7 C8 bug workaround. > > > */ > > > - if (boot_cpu_data.f00f_bug) { > > > + immediate_if (&f00f_bug_fix) { > > > > This code is not called during normal pagefaults and even during invalid > > userspace accesses. > > > > Out of curiosity, I inserted printk() at this place to see where I was > > wrong. I got only two hits: > > > > Checking if this processor honours the WP bit even in supervisor mode... do_page_fault: > > Freeing unused kernel memory: 116k freed > > do_page_fault: > > > > Resume: nobody gives a fuck about performance of this particular if, > > so conversion it totally pointless. > > > > Interesting investigation, let's push it further: > > instrumenting the f00f test site with a printk, I get: > > [ 0.000000] Checking if this processor honours the WP bit even in > supervisor mode... TEST: would test f00f bug at vadd ffecc000, eip c011928e > [ 0.000000] Ok. > ... and (whenever xdm restarts) : > [ 64.768165] TEST: would test f00f bug at vadd 00000000, eip c0237596 > [ 64.787136] TEST: would test f00f bug at vadd 0000004c, eip c02375a2 > > Those EIPs are: > > 0xc011928e : mov %cl,0xffecd000(%edx) > -> Will trigger fixup_exception. > > 0xc0237596 <__copy_from_user_ll+53>: rep movsl %ds:(%esi),%es:(%edi) > 0xc02375a2 <__copy_from_user_ll+65>: mov 0x20(%esi),%eax > -> Those look like user-space programs that gave NULL pointers to kernel > system calls. > > I agree with you that this is not a "hot path". It was mostly a > straightforward test conversion. > > Mathieu Actually, what is even more weird is that my system is configured to output the segfaults, i.e. a small user-space program that does a memory read of *(char*)NULL causes: [ 1413.164212] null[3740]: segfault at 00000000 eip 0804836a esp bffde2c0 error 4 But there is no message for the __copy_from_user_ll fault, so I wonder 1 - has the process been killed ? 2 - Is it just the printk that is missing ? Mathieu -- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68