Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754495AbbGWUw5 (ORCPT ); Thu, 23 Jul 2015 16:52:57 -0400 Received: from wtarreau.pck.nerim.net ([62.212.114.60]:6960 "EHLO 1wt.eu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754477AbbGWUwz (ORCPT ); Thu, 23 Jul 2015 16:52:55 -0400 Date: Thu, 23 Jul 2015 22:52:07 +0200 From: Willy Tarreau To: Linus Torvalds Cc: Andy Lutomirski , X86 ML , "linux-kernel@vger.kernel.org" , Borislav Petkov , Thomas Gleixner , Peter Zijlstra , Steven Rostedt , Brian Gerst Subject: Re: Dealing with the NMI mess Message-ID: <20150723205207.GA3052@1wt.eu> References: Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.4.2.3i Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1528 Lines: 38 On Thu, Jul 23, 2015 at 01:38:33PM -0700, Linus Torvalds wrote: > On Thu, Jul 23, 2015 at 1:21 PM, Andy Lutomirski wrote: > > > > 2. Forbid IRET inside NMIs. Doable but maybe not that pretty. > > > > We haven't considered: > > > > 3. Forbid faults (other than MCE) inside NMI. > > I'd really prefer #2. #3 depends on us getting many things right, and > never introducing new cases in the future. > > #2, in contrast, seems to be fairly localized. Yes, RF is an issue, > but returning to user space with RF clear doesn't really seem to be > all that problematic. What's the worst case that can happen with RF cleared when returing to user space ? My understanding is that it's just that we risk to break again on an instruction that had a break point set and which already triggered the breakpoint, right ? If so the problem probably is whether there's a risk of looping again without ever getting a chance to execute this instruction normally. But if the NMIs don't bomb as fast as we can process them, at some point the instruction should get a chance to be executed, so the problem doesn't seem dramatic. That makes me think that I have no idea what happens if we try to step-trace "int 2", I don't even know if we pass through the NMI handler. Willy -- 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/