Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965356Ab0GPMaV (ORCPT ); Fri, 16 Jul 2010 08:30:21 -0400 Received: from mx1.redhat.com ([209.132.183.28]:1433 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965337Ab0GPMaR (ORCPT ); Fri, 16 Jul 2010 08:30:17 -0400 Message-ID: <4C405078.20707@redhat.com> Date: Fri, 16 Jul 2010 15:28:40 +0300 From: Avi Kivity User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.10) Gecko/20100621 Fedora/3.0.5-1.fc13 Thunderbird/3.0.5 MIME-Version: 1.0 To: Mathieu Desnoyers CC: LKML , Linus Torvalds , Andrew Morton , Ingo Molnar , Peter Zijlstra , Steven Rostedt , Steven Rostedt , Frederic Weisbecker , Thomas Gleixner , Christoph Hellwig , Li Zefan , Lai Jiangshan , Johannes Berg , Masami Hiramatsu , Arnaldo Carvalho de Melo , Tom Zanussi , KOSAKI Motohiro , Andi Kleen , Mathieu Desnoyers , akpm@osdl.org, "H. Peter Anvin" , Jeremy Fitzhardinge , "Frank Ch. Eigler" Subject: Re: [patch 2/2] x86 NMI-safe INT3 and Page Fault References: <20100714154923.947138065@efficios.com> <20100714155804.252253097@efficios.com> In-Reply-To: <20100714155804.252253097@efficios.com> 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: 2258 Lines: 47 On 07/14/2010 06:49 PM, Mathieu Desnoyers wrote: > Implements an alternative iret with popf and return so trap and exception > handlers can return to the NMI handler without issuing iret. iret would cause > NMIs to be reenabled prematurely. x86_32 uses popf and far return. x86_64 has to > copy the return instruction pointer to the top of the previous stack, issue a > popf, loads the previous esp and issue a near return (ret). > > It allows placing dynamically patched static jumps in asm gotos, which will be > used for optimized tracepoints, in NMI code since returning from a breakpoint > would be valid. Accessing vmalloc'd memory, which allows executing module code > or accessing vmapped or vmalloc'd areas from NMI context, would also be valid. > This is very useful to tracers like LTTng. > > This patch makes all faults, traps and exception safe to be called from NMI > context*except* single-stepping, which requires iret to restore the TF (trap > flag) and jump to the return address in a single instruction. Sorry, no kprobes > support in NMI handlers because of this limitation. This cannot be emulated > with popf/lret, because lret would be single-stepped. It does not apply to > "immediate values" because they do not use single-stepping. This code detects if > the TF flag is set and uses the iret path for single-stepping, even if it > reactivates NMIs prematurely. > You need to save/restore cr2 in addition, otherwise the following hits you - page fault - processor writes cr2, enters fault handler - nmi - page fault - cr2 overwritten I guess you would usually not notice the corruption since you'd just see a spurious fault on the page the NMI handler touched, but if the first fault happened in a kvm guest, then we'd corrupt the guest's cr2. But the whole thing strikes me as overkill. If it's 8k per-cpu, what's wrong with using a per-cpu pointer to a kmalloc() area? -- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain. -- 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/