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
