From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751447AbVH3QCW (ORCPT ); Tue, 30 Aug 2005 12:02:22 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751446AbVH3QCW (ORCPT ); Tue, 30 Aug 2005 12:02:22 -0400 Received: from ebiederm.dsl.xmission.com ([166.70.28.69]:24994 "EHLO ebiederm.dsl.xmission.com") by vger.kernel.org with ESMTP id S1751447AbVH3QCV (ORCPT ); Tue, 30 Aug 2005 12:02:21 -0400 To: Mikael Pettersson Cc: Andi Kleen , Alan Cox , Andrew Morton , linux-kernel@vger.kernel.org Subject: Re: [PATCH] i386, x86_64 Initial PAT implementation References: <1125413136.8276.14.camel@localhost.localdomain> <200508301650.23149.ak@suse.de> <17172.31726.480750.90360@alkaid.it.uu.se> From: ebiederm@xmission.com (Eric W. Biederman) Date: Tue, 30 Aug 2005 10:01:40 -0600 In-Reply-To: <17172.31726.480750.90360@alkaid.it.uu.se> (Mikael Pettersson's message of "Tue, 30 Aug 2005 17:31:58 +0200") Message-ID: User-Agent: Gnus/5.1007 (Gnus v5.10.7) Emacs/21.4 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Mikael Pettersson writes: > Andi Kleen writes: > > On Tuesday 30 August 2005 16:45, Alan Cox wrote: > > > On Llu, 2005-08-29 at 18:20 -0600, Eric W. Biederman wrote: > > > > ways. Currently this code only allows for an additional flavor > > > > of uncached access to physical memory addresses which should be hard > > > > to abuse, and should raise no additional aliasing problems. No > > > > attempt has been made to fix theoretical aliasing problems. > > > > > > Even an uncached/cached alias causes random memory corruption or an MCE > > > on x86 systems. In fact it can occur even for an alias not in theory > > > touched by the CPU if it happens to prefetch into or speculate the > > > address. > > > > > > Also be sure to read the PII Xeon errata - early PAT has a bug or two. > > > > > > We can always force cpu_has_pat == 0 on these machines. > > I don't think it is worth it to add any more complicated workarounds > > for old broken systems. > > I don't have the spec updates in front of me, but I believe the PAT > bug existed well into the P4 line. The workaround is simply to make > the high 4 PAT entries identical to the low 4 entries. (But I confess > to not having a clue as to whether it's still useful then or not.) It is still useful even if that is the case. Unless we need something more than WC we don't have a need for anything else. If you could track down the reference you are thinking of I would appreciate it. According to the errata I can find simply not using the high 4 entries is sufficient, to avoid the problem. Eric