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

[Michael S. Tsirkin]

On Thu, May 15, 2014 at 08:49:48AM +0200, Greg Kurz wrote:
> 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 ?

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                                     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."
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