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
