Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758536AbcDHRAH (ORCPT ); Fri, 8 Apr 2016 13:00:07 -0400 Received: from smtp.citrix.com ([66.165.176.89]:23602 "EHLO SMTP.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753723AbcDHRAG (ORCPT ); Fri, 8 Apr 2016 13:00:06 -0400 X-IronPort-AV: E=Sophos;i="5.24,454,1454976000"; d="scan'208";a="345847811" Subject: Re: [Xen-devel] [PATCH] x86/mm/pat: Fix BUG_ON in mmap_mem on QEMU/i386 To: Toshi Kani , Borislav Petkov References: <1459549185-14911-1-git-send-email-toshi.kani@hpe.com> <20160405110947.GB10109@pd.tnic> <1459869842.13914.39.camel@hpe.com> <1460133294.20338.82.camel@hpe.com> CC: , , , , , , From: David Vrabel Message-ID: <5707E392.2090106@citrix.com> Date: Fri, 8 Apr 2016 18:00:02 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Icedove/38.5.0 MIME-Version: 1.0 In-Reply-To: <1460133294.20338.82.camel@hpe.com> Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit X-DLP: MIA1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1686 Lines: 43 On 08/04/16 17:34, Toshi Kani wrote: > On Tue, 2016-04-05 at 09:24 -0600, Toshi Kani wrote: >> +xen-devl >> >> On Tue, 2016-04-05 at 13:09 +0200, Borislav Petkov wrote: >>> On Fri, Apr 01, 2016 at 04:19:45PM -0600, Toshi Kani wrote: >>>> > : >>>> >>>> When the system does not have much memory, 'high_memory' points to >>> >>> What does "much memory" mean, exactly? >> >> I meant to say when a 32-bit system does not have ZONE_HIGHMEM, >> __pa(high_memory) points to the maximum memory address + 1. >> >> I will remove this sentence since it is irrelevant to this BUG_ON. Even >> if a 32-bit system does have ZONE_HIGHMEM, slow_virt_to_phys() still >> returns 0 for high_memory because it is set to the maximum direct mapped >> address + 1 in this case. This address is not covered by page table, >> either. >> >> But this made me realized that this high_memory check can be harmful in >> such case, ie. __pa(high_memory) is not the maximum memory address when >> ZONE_HIGHMEM is present. >> >> I assume when this code block was originally added, legacy systems >> without MTRRs did not have ZONE_HIGHMEM. However, MTRRs are also >> disabled on Xen. Reactivating this code may cause an issue on Xen 32-bit >> guests with ZONE_HIGHMEM. >> >> Question to Xen folks: Does Xen support 32-bit guests with ZONE_HIGHMEM? >> >> If yes, a safer fix may be to remove this code block since it was >> deadcode anyway... > > I have not heard a confirmation from Xen folks, but I believe ZONE_HIGHMEM > is supported on 32-bit Xen guests. So, unless someone has an objection, I > am going to remove this code block in the next version of this patch. 32-bit Xen guests have highmem, yes. David