Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932065AbbGWVVM (ORCPT ); Thu, 23 Jul 2015 17:21:12 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:45101 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754305AbbGWVVJ (ORCPT ); Thu, 23 Jul 2015 17:21:09 -0400 Date: Thu, 23 Jul 2015 23:20:42 +0200 From: Peter Zijlstra To: Linus Torvalds Cc: Andy Lutomirski , X86 ML , "linux-kernel@vger.kernel.org" , Willy Tarreau , Borislav Petkov , Thomas Gleixner , Steven Rostedt , Brian Gerst Subject: Re: Dealing with the NMI mess Message-ID: <20150723212042.GN25159@twins.programming.kicks-ass.net> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1257 Lines: 28 On Thu, Jul 23, 2015 at 01:38:33PM -0700, Linus Torvalds wrote: > And the "take them and disable them" is really simple. No "am I in an > NMI contect" thing (because that leads to the whole question about > "what is NMI context"). That's not the real rule anyway. > > No, make it very simple and straightforward. Make the test be "uhhuh, > I got a #DB in kernel mode, and interrupts were disabled - I know I'm > going to return with "ret", so I'm just going to have to disable this > breakpoint". > > Nothing clever. Nothing subtle. Nothing that needs "this range of > instructions is magical". No. Just a very simple rule: if the context > we return to is kernel mode and interrupts are disabled, we're using > 'ret', so we cannot suppress debug faults. > > Did I miss something? There were a lot of emails flying around, but I > *thought* I saw them all.. So the NMI could trigger userspace debug register faults, and simply disabling them would make the whole debug register thing entirely unreliable. -- 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/