[libvirt] [RFC v2 1/2] qemu_migration: don't allow ABI changes for persistent migration

[Peter Krempa]

On Tue, Mar 07, 2017 at 18:45:23 +0100, Pavel Hrdina wrote:
> So far there is probably no change that is allowed to be done
> by the VIR_DOMAIN_DEF_PARSE_ABI_UPDATE flag that would break
> guest ABI but this may change in the future.
> 
> The other cases where this flag is used is only when we are
> defining new domain or adding new device into a domain.
> 
> Signed-off-by: Pavel Hrdina <phrdina at redhat.com>
> ---
> 
> This patch is a product of a discussion about the last patch in v1 [1].
> Currently we allow ABI changes for persistent migration however it might
> be something that user don't expect to be done.
> 
> Technically it defines new domain on the destination which would fall
> into the same category as defining new domain from scratch without
> migration but it may be unexpected behavior because for live migration
> we don't allow ABI changes (for obvious reasons).
> 
> At first I though that this is correct and we are doing the right thing,
> but now I'm not so sure about that and IMHO it would be probably better
> to not do ABI updates in this case like we don't do if libvirtd is
> restarted (for example because of an update) and also it would be
> consistent with the live migration.

This flag was invented so that libvirt could finally properly track the
total memory size of the VM when NUMA is used. Prior to that the
individual node sizes would not have to total up to the size declared in
the <memory> element. While migrating a live VM we can't change this but
other code expects that the size is correct.

This patch would break this particular case. In general I'm not against
a change, but the recalculation of the memory sizes should not be
broken.
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