Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753356AbbFOGUY (ORCPT ); Mon, 15 Jun 2015 02:20:24 -0400 Received: from mail.emea.novell.com ([130.57.118.101]:38939 "EHLO mail.emea.novell.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750885AbbFOGUR convert rfc822-to-8bit (ORCPT ); Mon, 15 Jun 2015 02:20:17 -0400 Message-Id: <557E8ABA02000078000849A8@mail.emea.novell.com> X-Mailer: Novell GroupWise Internet Agent 14.0.1 Date: Mon, 15 Jun 2015 07:20:10 +0100 From: "Jan Beulich" To: "Andy Lutomirski" Cc: "Luis R. Rodriguez" , "Bjorn Helgaas" , "Jej B" , "Toshi Kani" , "X86 ML" , "Andrew Morton" , , "Julia Lawall" , "xen-devel@lists.xenproject.org" , "Dave Airlie" , , "Juergen Gross" , "Luis Rodriguez" , "Borislav Petkov" , "Tomi Valkeinen" , "linux-fbdev" , "linux-kernel@vger.kernel.org" , , "linux-pci@vger.kernel.org" Subject: Re: [Xen-devel] RIP MTRR - status update for upcoming v4.2 References: <1434064996.11808.64.camel@misato.fc.hp.com> <557AAD910200007800084014@mail.emea.novell.com> In-Reply-To: Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 8BIT Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1574 Lines: 37 >>> On 13.06.15 at 01:15, 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. I buy the SMI part, but UEFI runtime calls are being done on page tables we construct and control, so attributes could be kept correct without relying on MTRRs. Jan -- 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/