From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932830AbaIEOKr (ORCPT ); Fri, 5 Sep 2014 10:10:47 -0400 Received: from g2t2354.austin.hp.com ([15.217.128.53]:59913 "EHLO g2t2354.austin.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932201AbaIEOKp (ORCPT ); Fri, 5 Sep 2014 10:10:45 -0400 Message-ID: <1409925614.28990.184.camel@misato.fc.hp.com> Subject: Re: [PATCH 1/5] x86, mm, pat: Set WT to PA4 slot of PAT MSR From: Toshi Kani To: Andy Lutomirski Cc: Henrique de Moraes Holschuh , "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , akpm@linuxfoundation.org, Arnd Bergmann , "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" , Juergen Gross , Stefan Bader , Konrad Rzeszutek Wilk Date: Fri, 05 Sep 2014 08:00:14 -0600 In-Reply-To: References: <1409855739-8985-1-git-send-email-toshi.kani@hp.com> <1409855739-8985-2-git-send-email-toshi.kani@hp.com> <20140904201123.GA9116@khazad-dum.debian.net> <5408C9C4.1010705@zytor.com> <20140904231923.GA15320@khazad-dum.debian.net> <1409876991.28990.172.camel@misato.fc.hp.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.8.5 (3.8.5-2.fc19) Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2014-09-04 at 17:51 -0700, Andy Lutomirski wrote: > On Thu, Sep 4, 2014 at 5:29 PM, Toshi Kani wrote: > > On Thu, 2014-09-04 at 16:34 -0700, Andy Lutomirski wrote: > >> On Thu, Sep 4, 2014 at 4:19 PM, Henrique de Moraes Holschuh > >> wrote: > >> > On Thu, 04 Sep 2014, H. Peter Anvin wrote: > >> >> On 09/04/2014 01:11 PM, Henrique de Moraes Holschuh wrote: > >> >> > I am worried of uncharted territory, here. I'd actually advocate for not > >> >> > enabling the upper four PAT entries on IA-32 at all, unless Windows 9X / XP > >> >> > is using them as well. Is this a real concern, or am I being overly > >> >> > cautious? > >> >> > >> >> It is extremely unlikely that we'd have PAT issues in 32-bit mode and > >> >> not in 64-bit mode on the same CPU. > >> > > >> > Sure, but is it really a good idea to enable this on the *old* non-64-bit > >> > capable processors (note: I don't mean x86-64 processors operating in 32-bit > >> > mode) ? > >> > > >> >> As far as I know, the current blacklist rule is very conservative due to > >> >> lack of testing more than anything else. > >> > > >> > I was told that much in 2009 when I asked why cpuid 0x6d8 was blacklisted > >> > from using PAT :-) > >> > >> At the very least, anyone who plugs an NV-DIMM into a 32-bit machine > >> is nuts, and not just because I'd be somewhat amazed if it even > >> physically fits into the slot. :) > > > > According to the spec, the upper four entries bug was fixed in Pentium 4 > > model 0x1. So, the remaining Intel 32-bit processors that may enable > > the upper four entries are Pentium 4 model 0x1-4. Should we disable it > > for all Pentium 4 models? > > Assuming that this is Pentium 4 erratum N46, then there may be another > option: use slot 7 instead of slot 4 for WT. Then, even if somehow > the blacklist screws up, the worst that happens is that a WT page gets > interpreted as UC. I suppose this could cause aliasing issues, but > can't cause problems for people who don't use the high entries in the > first place. That's a fine idea, but as Ingo also suggested, I am going to disable this feature on all Pentium 4 models. That should give us a safety margin. Using slot 4 has a benefit that it keeps the PAT setup consistent with Xen. Thanks, -Toshi