From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1763434AbYDPOZH (ORCPT ); Wed, 16 Apr 2008 10:25:07 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755252AbYDPOY5 (ORCPT ); Wed, 16 Apr 2008 10:24:57 -0400 Received: from pentafluge.infradead.org ([213.146.154.40]:60687 "EHLO pentafluge.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754525AbYDPOY4 (ORCPT ); Wed, 16 Apr 2008 10:24:56 -0400 Date: Wed, 16 Apr 2008 07:24:16 -0700 From: Arjan van de Ven To: Mathieu Desnoyers Cc: Ingo Molnar , Peter Zijlstra , prasad@linux.vnet.ibm.com, linux-kernel@vger.kernel.org, tglx@linutronix.de, Christoph Hellwig , "Frank Ch. Eigler" Subject: Re: [RFC PATCH 1/2] Marker probes in futex.c Message-ID: <20080416072416.7b9ff3d2@laptopd505.fenrus.org> In-Reply-To: <20080416140009.GA13968@Krystal> References: <20080415115314.GA6975@in.ibm.com> <1208260942.6395.6.camel@twins> <20080415123233.GA19797@Krystal> <1208264190.6395.21.camel@twins> <20080415131744.GA5248@elte.hu> <20080415134705.GB22351@Krystal> <20080415164814.GA15842@elte.hu> <20080415213832.GC7873@Krystal> <20080416131751.GI6304@elte.hu> <20080416064630.22428aea@laptopd505.fenrus.org> <20080416140009.GA13968@Krystal> Organization: Intel X-Mailer: Claws Mail 3.2.0 (GTK+ 2.12.5; i386-redhat-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-SRS-Rewrite: SMTP reverse-path rewritten from by pentafluge.infradead.org See http://www.infradead.org/rpr.html Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 16 Apr 2008 10:00:09 -0400 Mathieu Desnoyers wrote: > > > > If we want to support NMI context and have the ability to > > > > instrument preemptable code without too much headache, we must > > > > insure that every modification will leave the code in a > > > > "correct" state and that we do not grow the size of any > > > > reachable instruction. Also, we must insure gcc did not put > > > > code between these instructions. Modifying non-relocatable > > > > instructions would also be a pain, since we would have to deal > > > > with instruction pointer relocation in the breakpoint code when > > > > the code modification is being done. > > > > you also need to make sure no cpu is executing that code ever.. > > but you already deal with that right? > > > > By "insure that every modification will leave the code in a "correct" > state", I mean that at any given time before, during or after the code > modification, if an NMI comes on any CPU and try to run the modified > code, it should have a valid version of the code to execute. Does it > make more sense ? I understand your words. My concern is that I don't quite understand how you guarantee that you'll not be executing the code you're modifying. Just saying "it's consistent before and after" sounds nice but probably isn't enough to be safe. > Not only does the compare and jmp need to be consecutive, but the movb > $0x0,%al also does. I *could* try to detect specific code inserted in > between, but I really have to make sure I don't get burned by the > compiler inserting a jmp there. I wonder if just sticking in 2 barriers around your code make gcc stop moving stuff too much -- If you want to reach me at my work email, use arjan@linux.intel.com For development, discussion and tips for power savings, visit http://www.lesswatts.org