[libvirt] [PATCH] conf: Fix backwards migration of pSeries guests

[Andrea Bolognani]

On Thu, 2017-07-20 at 09:55 +0200, Peter Krempa wrote:
> > In any case, that made me realize that not sending the model,
> > even if automatically filled in, could cause issues in the
> > future if a new model is added and becomes the default, as
> > the guest ABI would not be preserved during migration then.
> 
> That is the main reason to fill all the values in right away. Since
> there apparently was a period, where a default would be used, but not
> recorded, it needs some trickery unfortunately.
> 
> In such case you basically standardize, that the now-filled-in default
> model (which can't ever change) if it was not provided by the user is to
> be dropped from the migratable XML.
> 
> Once you start assign a new default model, that then needs to be
> explicitly sent over, so that the migration will not be successufl
> unless the destination is able to use the new model.
> 
> This means basically, that a missing model name becomes assigned to a
> particular value.

That makes sense.

Doesn't it also mean that we don't really need to record
whether the user set the model name explicitly or not? We
can just skip formatting it if it's spapr-pci-host-bridge
and all versions of libvirt, past or future, will handle
that correctly.

-- 
Andrea Bolognani / Red Hat / Virtualization