Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754171AbbGWVq4 (ORCPT ); Thu, 23 Jul 2015 17:46:56 -0400 Received: from wtarreau.pck.nerim.net ([62.212.114.60]:7373 "EHLO 1wt.eu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753923AbbGWVqv (ORCPT ); Thu, 23 Jul 2015 17:46:51 -0400 Date: Thu, 23 Jul 2015 23:46:03 +0200 From: Willy Tarreau To: Steven Rostedt Cc: Linus Torvalds , Andy Lutomirski , X86 ML , "linux-kernel@vger.kernel.org" , Borislav Petkov , Thomas Gleixner , Peter Zijlstra , Brian Gerst Subject: Re: Dealing with the NMI mess Message-ID: <20150723214603.GD3052@1wt.eu> References: <20150723173105.6795c0dc@gandalf.local.home> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150723173105.6795c0dc@gandalf.local.home> 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: 1573 Lines: 36 On Thu, Jul 23, 2015 at 05:31:05PM -0400, Steven Rostedt wrote: > On Thu, 23 Jul 2015 14:08:59 -0700 > Linus Torvalds wrote: > > > On Thu, Jul 23, 2015 at 1:49 PM, Andy Lutomirski wrote: > > > > > > Issue A: to return with RF clear, we need to disarm the breakpoint. > > > If it's limited to the duration of the NMI, that's easy. If not, when > > > do we re-arm? New prepare_exit_to_usermode hook? Hmm, setting ti > > > flags during context switch may target the wrong task. > > > > We don't re-arm it. > > > > Let me get this straight. The idea is in the #DB handler to detect that > it was triggered in NMI context, and if so, simply disarm that > breakpoint permanently, right? > > Nothing should be adding hw breakpoints to NMI code anyway. Sounds > perfectly reasonable to me. Of course, how we tell we are in NMI > brings back all the races as we had in the nesting code. We can check > the per-cpu variable that is set with nmi_enter() and cleared at > nmi_exit() but what happens if the breakpoint is outside those calls. > We can check the stack pointer, but then we are back to userspace > fooling us. Maybe add the DF trick again? Can't the back link of the TSS tell us where we come from ? At least it should not be manipulable from user-space. 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/