Am 15.05.2014 11:52, schrieb Michael S. Tsirkin: > On Thu, May 15, 2014 at 11:20:18AM +0200, Andreas Färber wrote: >> Am 15.05.2014 09:04, schrieb Greg Kurz: >>> On Thu, 15 May 2014 12:16:35 +0530 >>> Amit Shah <amit.s...@redhat.com> wrote: >>>> On (Thu) 15 May 2014 [09:23:51], Michael S. Tsirkin wrote: >>>>> On Thu, May 15, 2014 at 11:34:25AM +0530, Amit Shah wrote: >>>>>> On (Wed) 14 May 2014 [17:41:38], Greg Kurz wrote: >>>>>>> Since each virtio device is streamed in its own section, the idea is to >>>>>>> stream subsections between the end of the device section and the start >>>>>>> of the next sections. This allows an older QEMU to complain and exit >>>>>>> when fed with subsections: >>>>>>> >>>>>>> Unknown savevm section type 5 >>>>>>> Error -22 while loading VM state >>>>>> >>>>>> Please make this configurable -- either via configure or device >>>>>> properties. That avoids having to break existing configurations that >>>>>> work without this patch. >> >> Since backwards migration is not supported upstream, wouldn't it be >> easiest to just add support for the subsection marker and skipping to >> the end of section in that downstream? > > Backwards and forwards migration need to be supported, > customers told us repeatedly. So some downstreams support this > and not supporting it upstream just means downstreams need > to do their own thing. > > As importantly, ping-pong migration is the only > reliable way to stress migration. > > So if we want to test cross-version we need it to work > both way. > > Finally, the real issue and difficulty with cross-version migration is > making VM behave in a backwards compatible way. Serializing in a > compatible way is a trivial problem, or would be if the code wasn't a > mess :) Once you do the hard part, breaking migration because of the > trivial serialization issue is just silly. And special-casing forward > migration does not make code simpler, it really only leads to > proliferation of hacks and lack of symmetry. > > So yes it's a useful feature, and no not supporting it does > not help anyway.
It seems you misunderstand. I was not saying it's not useful. My point is that VMStateSubsections added in newer versions (and thus not existing in older versions) need to be handled for any VMState-converted devices. So why can't we make that work for virtio too? Andreas -- SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg, Germany GF: Jeff Hawn, Jennifer Guild, Felix Imendörffer; HRB 16746 AG Nürnberg
