From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759256AbbIDO65 (ORCPT ); Fri, 4 Sep 2015 10:58:57 -0400 Received: from g9t5008.houston.hp.com ([15.240.92.66]:58593 "EHLO g9t5008.houston.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753447AbbIDO6z (ORCPT ); Fri, 4 Sep 2015 10:58:55 -0400 Message-ID: <1441378581.21638.9.camel@hpe.com> Subject: Re: Fwd: [PATCH] x86: Use larger chunks in mtrr_cleanup From: Toshi Kani To: "Luis R. Rodriguez" Cc: Prarit Bhargava , Stuart Hayes , tglx@linutronix.de, mingo@redhat.com, "H. Peter Anvin" , linux-kernel@vger.kernel.org, x86@kernel.org, mcgrof@do-not-panic.com, Toshi Kani , Jan Beulich , Juergen Gross , Roger Pau =?ISO-8859-1?Q?Monn=E9?= , xen-devel@lists.xensource.com Date: Fri, 04 Sep 2015 08:56:21 -0600 In-Reply-To: <20150904014040.GA8051@wotan.suse.de> References: <20150903184029.GV8051@wotan.suse.de> <1441308162.3277.20.camel@hpe.com> <20150903195134.GW8051@wotan.suse.de> <1441315902.3277.39.camel@hpe.com> <20150903220711.GX8051@wotan.suse.de> <1441319131.3277.54.camel@hpe.com> <20150903224556.GY8051@wotan.suse.de> <1441322474.3277.78.camel@hpe.com> <20150903235429.GZ8051@wotan.suse.de> <1441327726.3277.109.camel@hpe.com> <20150904014040.GA8051@wotan.suse.de> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.16.5 (3.16.5-1.fc22) Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 2015-09-04 at 03:40 +0200, Luis R. Rodriguez wrote: > On Thu, Sep 03, 2015 at 06:48:46PM -0600, Toshi Kani wrote: > > On Fri, 2015-09-04 at 01:54 +0200, Luis R. Rodriguez wrote: > > > On Thu, Sep 03, 2015 at 05:21:14PM -0600, Toshi Kani wrote: > > > > On Fri, 2015-09-04 at 00:45 +0200, Luis R. Rodriguez wrote: > > > > > On Thu, Sep 03, 2015 at 04:25:31PM -0600, Toshi Kani wrote: > > : > > > > > > On Xen, > > > > > > > > > > When Xen is used a platform firmware may still set up MTRR, even if > > > > > the hypervisor doesn't set up MTRR right ? So same issue and > > > > > question here. > > > > > > > > Right, I meant to say Xen guests. > > > > > > Ah but its import complicated than that. > > > > > > > In case of the Xen hypervisor, > > > > mtrr_type_lookup() returns a valid type as it runs on a platform. > > > > > > I am not sure if this happens today, I know MTRR is simply disabled by > > > the Xen Hypervisor on the CPU explicitly, it disable it so guests > > > reading the MTRR capabilities sees it as disabled when queried. > > > > Oh, I would not let the hypervisor to disable MTRRs... > > Commit 586ab6a055376ec3f3e1e8 ("x86/pvh: disable MTRR feature on cpuid for > Dom0") by Roger Pau Monné disables MTRR for PVH dom0, so that cpuid returns > that MTRR is disabled to guests. Oh, I see. It just clears the capability bit so that the kernel thinks MTRRs are disabled. That makes sense. > Then later on Linux as of commit 47591df50512 ("xen: Support Xen pv-domains > using PAT") added by Juergen as of v3.19 Linux guests can end up booting > without MTRR but with PAT now enabled. Nice! > > > Then since the Xen Linux guests cannot speak MTRR through the hypervisor > > > (for instance Xen guests cannot ask Xen hypervisor to mtrr_type_lookup() > > > for it) if PCI passthrough is used it could mean a guest might set up / > > > use incorrect info as well. > > > > > > If I undestand this correctly then I think we're in a pickle with Xen > > > unless we add hypervisor support and hypercall support for > > > mtrr_type_lookup(). > > > > I was under assumption that MTRRs are emulated and disabled on guests. > > Some "special" flavor Linux guests (with non-upstream code) have guest > MTRR hypercall support, for vanilla Xen and Linux they just never get MTRR > support. After Juergen's Linux changes though Xen guests can now get > shiny PAT support. Since MTRR hypercall support is not upstream and MTRR is > ancient I decided instead of adding MTRR hypercall support upstream to go > with converting all drivers to PAT interfaces, with the assumption there > would be no issues. > > > Isn't guest physical address virtualized? > > It is, there is a xen iotlb and stuff but that should ensure dom0 gets > to get proper access to devices, and if you use PCI passthrough you want > the best experience as well. > > > I know other proprietary VMMs on IA64, but know nothing about Xen... So, > > please disregard my comments to Xen. :-) > > No worries, no one knows all the answers, we work together to remove > cob webs off of these odd corners no one cares about :) Thanks for all the info! That helps. -Toshi