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From: Alex Williamson <alex.williamson@redhat.com>
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Date: Mon, 13 Dec 2010 11:59:16 -0700
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Subject: [Qemu-devel] Re: [PATCH] rtl8139: IO memory is not part of vmstate
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: qemu-devel@nongnu.org, quintela@redhat.com

On Mon, 2010-12-13 at 20:54 +0200, Michael S. Tsirkin wrote:
> On Mon, Dec 13, 2010 at 11:00:44AM -0700, Alex Williamson wrote:
> > On Mon, 2010-12-13 at 19:50 +0200, Michael S. Tsirkin wrote:
> > > On Mon, Dec 13, 2010 at 10:43:22AM -0700, Alex Williamson wrote:
> > > > So, unfortunately, I stand by my original patch.
> > > 
> > > What about the one that put -1 in saved index for a hotplugged device?
> > 
> > There are still examples that don't work even without hotplug (example 2
> > and example 3 after the reboot).  That hack limits the damage, but still
> > leaves a latent bug for reboot and doesn't address the non-hotplug
> > scenarios.  So, I don't think it's worthwhile to pursue, and we
> > shouldn't pretend we can use it to avoid bumping the version_id.
> > Thanks,
> > 
> > Alex
> 
> I guess when we bump it we tell users: migration is completely
> borken to the old version, don't even try it.
> 
> Is there a way for libvirt to discover such incompatibilities
> and avoid the migration?

I don't know if libvirt has a way to query this in advance.  If a
migration is attempted, the target will report:

savevm: unsupported version 5 for '0000:00:03.0/rtl8139' v4

And the source will continue running.  We waste plenty of bits getting
to that point, but hopefully libvirt understands that it failed.
Thanks,

Alex