From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:41834) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YseVV-0004tB-D0 for qemu-devel@nongnu.org; Wed, 13 May 2015 17:48:02 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1YseVT-000161-7s for qemu-devel@nongnu.org; Wed, 13 May 2015 17:48:01 -0400 Received: from mx1.redhat.com ([209.132.183.28]:39753) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YseVS-00015Q-WB for qemu-devel@nongnu.org; Wed, 13 May 2015 17:47:59 -0400 Date: Wed, 13 May 2015 23:47:48 +0200 From: "Michael S. Tsirkin" Message-ID: <20150513234516-mutt-send-email-mst@redhat.com> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <55539E7C.90004@de.ibm.com> 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: Christian Borntraeger Cc: Cornelia Huck , qemu-devel@nongnu.org, jjherne@linux.vnet.ibm.com 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. -- MST