From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751416AbbFMGhq (ORCPT ); Sat, 13 Jun 2015 02:37:46 -0400 Received: from mail-wi0-f177.google.com ([209.85.212.177]:35149 "EHLO mail-wi0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750728AbbFMGhi (ORCPT ); Sat, 13 Jun 2015 02:37:38 -0400 Date: Sat, 13 Jun 2015 08:37:32 +0200 From: Ingo Molnar To: Andy Lutomirski Cc: Jan Beulich , Juergen Gross , Tomi Valkeinen , Ville =?iso-8859-1?Q?Syrj=E4l=E4?= , Dave Airlie , "xen-devel@lists.xenproject.org" , linux-fbdev , X86 ML , Andrew Morton , Jej B , Bjorn Helgaas , "Luis R. Rodriguez" , linux-media@vger.kernel.org, Luis Rodriguez , "linux-pci@vger.kernel.org" , Toshi Kani , Borislav Petkov , Julia Lawall , "linux-kernel@vger.kernel.org" , ville.syrjala@linux.intel.com Subject: Re: [Xen-devel] RIP MTRR - status update for upcoming v4.2 Message-ID: <20150613063731.GB12612@gmail.com> References: <1434064996.11808.64.camel@misato.fc.hp.com> <557AAD910200007800084014@mail.emea.novell.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Andy Lutomirski wrote: > On Jun 12, 2015 12:59 AM, "Jan Beulich" wrote: > > > > >>> On 12.06.15 at 01:23, wrote: > > > There are two usages on MTRRs: > > > 1) MTRR entries set by firmware > > > 2) MTRR entries set by OS drivers > > > > > > We can obsolete 2), but we have no control over 1). As UEFI firmwares > > > also set this up, this usage will continue to stay. So, we should not > > > get rid of the MTRR code that looks up the MTRR entries, while we have > > > no need to modify them. > > > > > > Such MTRR entries provide safe guard to /dev/mem, which allows privileged > > > user to access a range that may require UC mapping while the /dev/mem driver > > > blindly maps it with WB. MTRRs converts WB to UC in such a case. > > > > But it wouldn't be impossible to simply read the MTRRs upon boot, store the > > information, disable MTRRs, and correctly use PAT to achieve the same effect > > (i.e. the "blindly maps" part of course would need fixing). > > This may crash and burn badly when we call a UEFI function or an SMI happens. I > think we should just leave the MTRRs alone. Not to mention suspend/resume, reboot and other goodies where the firmware might pop up expecting intact MTRRs. Btw., doesn't a lack of MTRRs imply UC? So is 'crash and burn' possible in most cases? Isn't it just 'executes slower than before'? Thanks, Ingo