[libvirt] s390: change default cpu model to host-model?

[Christian Borntraeger]

On the KVM forum I have discussed the default cpu model mode on s390.
Right now if the xml does not specify anything, libvirt defaults to 
not specifying anything on the qemu command line (no -cpu statement)
which is the equivalent of -cpu host for s390 which is equivalent to
host-passthrough. While this enables all features it does not provide
any migration safety by default.

So in fact we are kind of "broken" right now when it comes to safery.

So we discussed that it would make sense that an empty xml should actually
be defaulted to host-model, which results in - as of today - the same guest
features but in a migration safe way.

There is another change planned right now to actually make the cpu model
present in an xml if none was specified. So we could actually do this change
before, together  or after te other. Jiri and I think it probably makes most
sense to have both changes at the same time (in terms of libvirt version).

Does anyone see an issue with changing the default model mode to "host-model"
if the xml does not specify anything else?

Christian