Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754243AbbFLXP0 (ORCPT ); Fri, 12 Jun 2015 19:15:26 -0400 Received: from mail-la0-f44.google.com ([209.85.215.44]:36332 "EHLO mail-la0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752019AbbFLXPX (ORCPT ); Fri, 12 Jun 2015 19:15:23 -0400 MIME-Version: 1.0 In-Reply-To: <557AAD910200007800084014@mail.emea.novell.com> References: <1434064996.11808.64.camel@misato.fc.hp.com> <557AAD910200007800084014@mail.emea.novell.com> From: Andy Lutomirski Date: Fri, 12 Jun 2015 16:15:00 -0700 Message-ID: Subject: Re: [Xen-devel] RIP MTRR - status update for upcoming v4.2 To: Jan Beulich Cc: Juergen Gross , Tomi Valkeinen , =?UTF-8?B?VmlsbGUgU3lyasOkbMOk?= , 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 Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1331 Lines: 31 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. --Andy -- 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/