From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:42398) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UHaRG-0006Ib-Fk for qemu-devel@nongnu.org; Mon, 18 Mar 2013 09:49:27 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1UHaRA-0004af-DZ for qemu-devel@nongnu.org; Mon, 18 Mar 2013 09:49:22 -0400 Received: from mail.avalus.com ([2001:41c8:10:1dd::10]:60446) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UHaRA-0004aR-7J for qemu-devel@nongnu.org; Mon, 18 Mar 2013 09:49:16 -0400 Date: Mon, 18 Mar 2013 13:49:01 +0000 From: Alex Bligh Message-ID: <7AC8953FE45335FB794B6DFE@Ximines.local> In-Reply-To: <51471767.8030604@redhat.com> References: <1363609123-20748-1-git-send-email-alex@alex.org.uk> <51471767.8030604@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Content-Disposition: inline Subject: Re: [Qemu-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes. Reply-To: Alex Bligh List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini Cc: Ian Campbell , Stefano Stabellini , George Dunlap , Ian Jackson , qemu-devel@nongnu.org, xen-devel , Anthony Liguori , Alex Bligh --On 18 March 2013 14:32:23 +0100 Paolo Bonzini wrote: > How does migration work with this change? The target may read stale > data from the page cache. Fix the kernel bug please. Presumably the same way as if writeback caching is selected. I presume that must fsync() / fdatasync() all the data to disk, and a barrier will produce one of those. It would be great to fix the kernel bug (and I have submitted code), but the fix is pretty intrusive (see the link I posted) and there appears to be little interest in taking it forward. Certainly my kernel hacking skills are not adequate to the task. The current position is that booting a Xen domU which does disk I/O (Ubuntu cloud image used as the test case) with an NFS root crashes dom0 absolutely repeatably, and kills all other guests. Unless and until there is a kernel fix for that, Xen is in essence unusable with HVM and network based disk backend. So we need a workaround in the meantime which doesn't require a kernel fix. -- Alex Bligh