On Thu, May 15, 2014 at 08:49:48AM +0200, Greg Kurz wrote: > On Thu, 15 May 2014 11:34:25 +0530 > Amit Shah <amit.s...@redhat.com> wrote: > > > On (Wed) 14 May 2014 [17:41:38], Greg Kurz wrote: > > > There is a need to add some more fields to VirtIODevice that should be > > > migrated (broken status, endianness). The problem is that we do not > > > want to break compatibility while adding a new feature... This issue has > > > been addressed in the generic VMState code with the use of optional > > > subsections. As a *temporary* alternative to port the whole virtio > > > migration code to VMState, this patch mimics a similar subsectionning > > > ability for virtio. > > > > > > 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. > > > > Hmmm... you mean we support migration from a newer QEMU to an older one ?
In theory yes, that's why we have the -M switch. This area isn't well tested unfortunately, but there are downstreams that test and productize it. > > > All users of virtio_load()/virtio_save() need to be patched because the > > > subsections are streamed AFTER the device itself. > > > > Since all have the same fixup, I'm wondering if a new section can be > > added to the virtio-bus itself, which gets propagated to all devices > > upon load in the dest. > > > > That would be nice if possible. I will have a closer look. > > > Amit > > > > Thanks. > > -- > Gregory Kurz kurzg...@fr.ibm.com > gk...@linux.vnet.ibm.com > Software Engineer @ IBM/Meiosys http://www.ibm.com > Tel +33 (0)562 165 496 > > "Anarchy is about taking complete responsibility for yourself." > Alan Moore.
