Comment 7 for bug 1538620

Sean, I'm not sure if I understand your question, but the call will fail if both are passed in for LIberty Cinder:
https://github.com/openstack/cinder/blob/stable/liberty/cinder/api/contrib/volume_actions.py#L101

I believe the concern was that Mitaka+ Nova with Liberty- Cinder will always fail the attach if Nova passes host+instance_id at the same time. Since the Nova patch was reverted, it does seem unlikely that anyone (Nova or other) would pass both in, and I'm not sure what harm it would do if they did and both were accepted (Mitaka cinder only). I don't see an issue in manager.py:attach_volume()

But...For Nova to use this the Cinder code needs to be microversioned. So either back this patch out or fix with microversion logic. Here's the patch that adds logic for pre-Mitaka (Microversion 3.0 or before) to continue to throw an exception for both host and instance_uuid, and allows new microversion 3.1 to accept both:
https://review.openstack.org/#/c/300684/2