Re: [Qemu-devel] [PATCH RFC 1/8] virtio: add subsections to the migration stream

[Greg Kurz]

On Thu, 15 May 2014 11:34:25 +0530
Amit Shah <address@hidden> 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 ?

> > 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                                     address@hidden
                                                 address@hidden
Software Engineer @ IBM/Meiosys                  http://www.ibm.com
Tel +33 (0)562 165 496

"Anarchy is about taking complete responsibility for yourself."
        Alan Moore.