From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:37803) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YsbqB-0008Aa-MI for qemu-devel@nongnu.org; Wed, 13 May 2015 14:57:12 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Ysbq7-0002S8-MC for qemu-devel@nongnu.org; Wed, 13 May 2015 14:57:11 -0400 Received: from e06smtp16.uk.ibm.com ([195.75.94.112]:43494) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Ysbq7-0002Ri-C0 for qemu-devel@nongnu.org; Wed, 13 May 2015 14:57:07 -0400 Received: from /spool/local by e06smtp16.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 13 May 2015 19:57:04 +0100 Received: from b06cxnps4076.portsmouth.uk.ibm.com (d06relay13.portsmouth.uk.ibm.com [9.149.109.198]) by d06dlp03.portsmouth.uk.ibm.com (Postfix) with ESMTP id 755021B08067 for ; Wed, 13 May 2015 19:57:48 +0100 (BST) Received: from d06av01.portsmouth.uk.ibm.com (d06av01.portsmouth.uk.ibm.com [9.149.37.212]) by b06cxnps4076.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id t4DIv2WO65536088 for ; Wed, 13 May 2015 18:57:02 GMT Received: from d06av01.portsmouth.uk.ibm.com (localhost [127.0.0.1]) by d06av01.portsmouth.uk.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id t4DIv1Gj006400 for ; Wed, 13 May 2015 12:57:02 -0600 Message-ID: <55539E7C.90004@de.ibm.com> Date: Wed, 13 May 2015 20:57:00 +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> In-Reply-To: <20150513180005-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" , Cornelia Huck Cc: qemu-devel@nongnu.org, jjherne@linux.vnet.ibm.com 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