From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:46700) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YspLU-0001m6-81 for qemu-devel@nongnu.org; Thu, 14 May 2015 05:22:25 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1YspLO-0006H0-Kd for qemu-devel@nongnu.org; Thu, 14 May 2015 05:22:24 -0400 Received: from e06smtp17.uk.ibm.com ([195.75.94.113]:34517) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YspLO-0006F3-Cd for qemu-devel@nongnu.org; Thu, 14 May 2015 05:22:18 -0400 Received: from /spool/local by e06smtp17.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 14 May 2015 10:22:16 +0100 Received: from b06cxnps3075.portsmouth.uk.ibm.com (d06relay10.portsmouth.uk.ibm.com [9.149.109.195]) by d06dlp02.portsmouth.uk.ibm.com (Postfix) with ESMTP id 54ECC2190061 for ; Thu, 14 May 2015 10:21:55 +0100 (BST) Received: from d06av08.portsmouth.uk.ibm.com (d06av08.portsmouth.uk.ibm.com [9.149.37.249]) by b06cxnps3075.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id t4E9MEQ765536132 for ; Thu, 14 May 2015 09:22:14 GMT Received: from d06av08.portsmouth.uk.ibm.com (localhost [127.0.0.1]) by d06av08.portsmouth.uk.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id t4E9MDUr026944 for ; Thu, 14 May 2015 03:22:13 -0600 Message-ID: <55546945.4050400@de.ibm.com> Date: Thu, 14 May 2015 11:22:13 +0200 From: Christian Borntraeger MIME-Version: 1.0 References: <1431528122-50960-1-git-send-email-cornelia.huck@de.ibm.com> <1431528122-50960-2-git-send-email-cornelia.huck@de.ibm.com> <20150513165438-mutt-send-email-mst@redhat.com> <20150513170335.2d662124.cornelia.huck@de.ibm.com> <20150513180005-mutt-send-email-mst@redhat.com> <55539E7C.90004@de.ibm.com> <20150513234516-mutt-send-email-mst@redhat.com> In-Reply-To: <20150513234516-mutt-send-email-mst@redhat.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH RFC 1/1] virtio: migrate config_vector List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Michael S. Tsirkin" Cc: Cornelia Huck , qemu-devel@nongnu.org, jjherne@linux.vnet.ibm.com Am 13.05.2015 um 23:47 schrieb Michael S. Tsirkin: > On Wed, May 13, 2015 at 08:57:00PM +0200, Christian Borntraeger wrote: >> Am 13.05.2015 um 18:14 schrieb Michael S. Tsirkin: >>>> - AFAICS, there's no easy way to add transport-specific subsections - >>>> and simply adding config_vector in ccw would break compatibility >>> >>> subsections break compatibility too. The only way around that is to set >>> a flag to skip migrating config_vector for old machine types. >> >> My main concern is about undetected compatibility issues. A subsection will >> tell the user that something went wrong. What happens if we just add a new >> qemu_put_byte in the stream. Will the savevm core always detect that we have >> too many or not enough bytes? If yes, adding new stuff in the stream will >> always be detected in some way as error we can go with just adding >> qemu_put_be16/qemu_get_be16 in virtio_ccw_save_config/virtio_ccw_load_config. >> Old/new QEMUs will then not be compatible - but thats probably ok as long as it >> errors out. >> >> My understanding was that we do not have a guarentee that this will be >> detected all the time and having random junk in some variables is a debugging >> nightmare. Is that correct? >> >> >> Christian > > It's not too bad - normally there's a bunch of strings that > helps you find out what's going on. > But if you really care about debuggability of migration streams, help move > forward dgilbert's RFC that switched to a self-delimiting format. > Just piling up random hacks in virtio seems like a wrong approach. > Thats not my question. PLEASE try to understand my question. I want a hard stop if migration changes in incompatible ways. If adding a qemu_put_byte in virtio_ccw gets detected we can just fix virtio_ccw AS YOU SUGGESTED. I just want to know if I can rely on that or not. Christian