Comment 12 for bug 1697095

There's been no further work on how arches are handled by the maas provider. Valid arches are restricted to the set of enlisted machines managed by maas. If a manual machine is added to the model, that machine's arch is not added to the allowed set. In general for maas provider, only machines managed by maas are available to be used with arch or tag constraints.

For the use case at hand, a placement directive is perhaps the best we can do at the moment. ie when a manual machine is added, it gets a juju machine id. Then you can do:

$ juju deploy myapp --to 5

Not the same as "pick any free s390 machine" for sure, but maybe good enough for now.