From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S964854AbcGZS7s (ORCPT ); Tue, 26 Jul 2016 14:59:48 -0400 Received: from mx1.redhat.com ([209.132.183.28]:47496 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S964839AbcGZS7o (ORCPT ); Tue, 26 Jul 2016 14:59:44 -0400 Date: Tue, 26 Jul 2016 13:59:41 -0500 From: Josh Poimboeuf To: Brian Gerst Cc: Andy Lutomirski , Thomas Gleixner , Ingo Molnar , "H . Peter Anvin" , X86 ML , "linux-kernel@vger.kernel.org" , Linus Torvalds , Steven Rostedt , Kees Cook , Peter Zijlstra , Frederic Weisbecker , Byungchul Park Subject: Re: [PATCH 10/19] x86/dumpstack: add get_stack_info() interface Message-ID: <20160726185941.rpldgeban5m66r2d@treble> References: <88568c51f1a253210897e368262d0f5fa1d7e97a.1469136008.git.jpoimboe@redhat.com> <20160722235459.xtikpj263hroloqo@treble> <20160723140439.36bhw5aslcxlkf3f@treble> <20160726164751.44xcnegpw2x3v2o6@treble> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.6.0.1 (2016-04-01) X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.27]); Tue, 26 Jul 2016 18:59:43 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 26, 2016 at 01:49:06PM -0400, Brian Gerst wrote: > On Tue, Jul 26, 2016 at 12:47 PM, Josh Poimboeuf wrote: > > On Mon, Jul 25, 2016 at 05:09:44PM -0700, Andy Lutomirski wrote: > >> On Sat, Jul 23, 2016 at 7:04 AM, Josh Poimboeuf wrote: > >> > Am I correct in understanding that there can only be one level of NMI > >> > nesting at any given time? If so, could we make it easier on the > >> > unwinder by putting the nested NMI on a separate software stack, so the > >> > "next stack" pointers are always in the same place? Or am I just being > >> > naive? > >> > >> I think you're being naive :) > > > > Another dumb question: since NMIs are reentrant, have you considered > > removing the NMI IST entry, and instead just have NMIs keep using the > > current stack? > > > > The first NMI could then be switched to an NMI software stack, like IRQs > > (assuming there's a way to do that atomically!). And then determining > > the context of subsequent NMIs would be straightforward, and we'd no > > longer need to jump through all those horrible hoops in the entry code > > to deal with NMI nesting. > > > > Now you can tell me what else I'm missing... > > There are several places (most notably SYSCALL entry) where the kernel > stack pointer is unsafe/user controlled for a brief time. Since an > NMI can interrupt anywhere in the kernel, you have to use an IST to > protect against that case. Ah, that makes sense. Thanks. -- Josh