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

[Amit Shah <amit.shah@redhat.com>]

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.

> 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.

		Amit