From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755296AbaEOTw2 (ORCPT ); Thu, 15 May 2014 15:52:28 -0400 Received: from terminus.zytor.com ([198.137.202.10]:45287 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752100AbaEOTw0 (ORCPT ); Thu, 15 May 2014 15:52:26 -0400 Message-ID: <53751AC8.6040902@zytor.com> Date: Thu, 15 May 2014 12:51:36 -0700 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0 MIME-Version: 1.0 To: Keir Fraser CC: David Vrabel , xen-devel@lists.xenproject.org, x86@kernel.org, linux-kernel@vger.kernel.org, Dave Hansen , Ingo Molnar , Mel Gorman , Boris Ostrovsky , Thomas Gleixner Subject: Re: [Xen-devel] [PATCH 7/9] x86: skip check for spurious faults for non-present faults References: <1397571337-20409-1-git-send-email-david.vrabel@citrix.com> <1397571337-20409-8-git-send-email-david.vrabel@citrix.com> <53750A96.2020201@zytor.com> <53751403.1010109@gmail.com> In-Reply-To: <53751403.1010109@gmail.com> X-Enigmail-Version: 1.6 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/15/2014 12:22 PM, Keir Fraser wrote: >> >> Are we chasing hardware errata here? Or did someone go off and *assume* >> that the x86 hardware architecture work a certain way? Or is there >> something way more subtle going on? > > See Intel Developer's Manual Vol 3 Section 4.10.4.3, 3rd bullet... This > is expected behaviour, probably to make copy-on-write faults faster. > Hm, yes. My memory of this comes from before these formal rules were written down... I guess there is some wiggle room in there, presumably as you say, for performance reasons (or implementation leeway, which is another way to say performance.) This does make a P bit switch architecturally different from W or NX, so I'm okay with that, but I would like the patch adjusted in the following ways: 1. Put in an explicit comment about the architectural difference between the P bit on one hand and an W and NX on the other; an SDM reference is good, and *why* this makes the specific filtering correct. 2. Please use the standard format for multiline comments; /* * blah * blah */ With that this should be okay. -hpa