Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758789Ab1DNLFu (ORCPT ); Thu, 14 Apr 2011 07:05:50 -0400 Received: from smtp.ctxuk.citrix.com ([62.200.22.115]:40622 "EHLO SMTP.EU.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758738Ab1DNLFt (ORCPT ); Thu, 14 Apr 2011 07:05:49 -0400 X-IronPort-AV: E=Sophos;i="4.64,210,1301875200"; d="scan'208";a="5301241" Date: Thu, 14 Apr 2011 12:05:42 +0100 From: Stefano Stabellini X-X-Sender: sstabellini@kaball-desktop To: "H. Peter Anvin" CC: Stefano Stabellini , Yinghai Lu , "linux-kernel@vger.kernel.org" , "xen-devel@lists.xensource.com" , "konrad.wilk@oracle.com" , "jeremy@goop.org" , "mingo@elte.hu" , "H. Peter Anvin" Subject: Re: [PATCH 2/4] x86,xen: introduce x86_init.mapping.pagetable_reserve In-Reply-To: <4DA5EB46.6090302@linux.intel.com> Message-ID: References: <1302607192-21355-2-git-send-email-stefano.stabellini@eu.citrix.com> <4DA48EA9.70109@kernel.org> <4DA5EB46.6090302@linux.intel.com> User-Agent: Alpine 2.00 (DEB 1167 2008-08-23) MIME-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2578 Lines: 51 On Wed, 13 Apr 2011, H. Peter Anvin wrote: > On 04/13/2011 03:35 AM, Stefano Stabellini wrote: > > Yes, that would be ideal, but we cannot do that because we don't know > > exactly where is pgt_buf_end before allocating the pagetable pages and > > the pagetable pages need to be marked RO before being hooked into the > > pagetable. This is why we mark the whole range RO and after the > > pagetable allocation when we know for sure where is pgt_buf_end we > > modify the range pgt_buf_end-pgt_buf_top to RW. > > The hell? You have to fill the pages before you hook them into the page > tables anyway (this means writing!) and then you have to mark them RO as > you add them to the page tables... anything else doesn't make any sense > at all. Right. The problem is that at some point init_memory_mapping is going reach the pagetable pages area and map those pages too (I don't mean hooking the pagetable pages in the pagetable, I mean mapping them as normal memory that falls in the range of addresses passed to init_memory_mapping as argument). Some of those pages are already pagetable pages (they are in the range pgt_buf_start-pgt_buf_end) therefore they are going to be mapped RO and everything is fine. Some of these pages are not pagetable pages yet (they fall in the range pgt_buf_end-pgt_buf_top; for example the page at pgt_buf_end) so they are going to be mapped RW. When these pages become pagetable pages and are hooked into the pagetable, xen will find that the guest has already a RW mapping of them somewhere and fail the operation. In order to fix the issue I could mark all the pages in the entire range pgt_buf_start-pgt_buf_top as RO, but then once the pagetable allocation is completed only the range pgt_buf_start-pgt_buf_end is reserved by init_memory_mapping therefore the kernel is going to crash as soon as one of the pages in the range pgt_buf_end-pgt_buf_top is reused. Initially I suggested to add two hooks: one to allocate the pagetable pages memory and one to reserve the pagetable pages memory after the allocation: http://marc.info/?l=linux-kernel&m=130141955626268 Following Yinghai's suggestion I removed the first hook (currently unnecessary because we would use the same implementation on native and on xen) and modified the second one, that became x86_init.mapping.pagetable_reserve. -- 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/