From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933560AbZFOSyX (ORCPT ); Mon, 15 Jun 2009 14:54:23 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1764528AbZFOSyN (ORCPT ); Mon, 15 Jun 2009 14:54:13 -0400 Received: from tomts20-srv.bellnexxia.net ([209.226.175.74]:49675 "EHLO tomts20-srv.bellnexxia.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1764849AbZFOSyM (ORCPT ); Mon, 15 Jun 2009 14:54:12 -0400 X-IronPort-Anti-Spam-Filtered: true X-IronPort-Anti-Spam-Result: AokFAAcwNkpMQWQl/2dsb2JhbACBT9VwhA0F Date: Mon, 15 Jun 2009 14:48:58 -0400 From: Mathieu Desnoyers To: "H. Peter Anvin" Cc: Linus Torvalds , Ingo Molnar , mingo@redhat.com, paulus@samba.org, acme@redhat.com, linux-kernel@vger.kernel.org, a.p.zijlstra@chello.nl, penberg@cs.helsinki.fi, vegard.nossum@gmail.com, efault@gmx.de, jeremy@goop.org, npiggin@suse.de, tglx@linutronix.de, linux-tip-commits@vger.kernel.org Subject: Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods Message-ID: <20090615184858.GD6520@Krystal> References: <20090615171845.GA7664@elte.hu> <4A369508.2090707@zytor.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Content-Disposition: inline In-Reply-To: <4A369508.2090707@zytor.com> X-Editor: vi X-Info: http://krystal.dyndns.org:8080 X-Operating-System: Linux/2.6.21.3-grsec (i686) X-Uptime: 14:47:15 up 107 days, 15:13, 3 users, load average: 1.41, 1.44, 1.15 User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * H. Peter Anvin (hpa@zytor.com) wrote: > Linus Torvalds wrote: > > > > On Mon, 15 Jun 2009, Ingo Molnar wrote: > >> A simple cr2 corruption would explain all those cc1 SIGSEGVs and > >> other user-space crashes i saw, with sufficiently intense sampling - > >> easily. > > > > Note that we could work around the %cr2 issue, since any corruption is > > always nicely "nested" (ie there are never any SMP issues with async > > writes to the register). > > > > So what we _could_ do is to have a magic value for %cr2, along with a "NMI > > sequence count", and if we see that value, we just return (without doing > > anything) from the page fault handler. > > > > Wouldn't it be simpler to just require the NMI handler to save and > restore %cr2 around any potentially faulting references? > > -hpa If we require that around the whole NMI handler execution, then we get all vmalloc + module text code references handled for free. This would be a nice-to-have.a And given nmi-handler is not such a frequent code path, we should not care that much about the performance hit of saving/restoring the cr2 register at each nmi entry/exit. Mathieu -- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68