Re: [Qemu-devel] [PATCH RFC 1/1] virtio: migrate config_vector

[Christian Borntraeger <borntraeger@de.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