We appear to use migration blockers in two ways: (1) Prevent migration for an indefinite time, typically due to use of some feature that isn't compatible with migration.
(2) Delay migration for a short time. Option -only-migrate is designed for (1). It interferes with (2). Example for (1): device "x-pci-proxy-dev" doesn't support migration. It adds a migration blocker on realize, and deletes it on unrealize. With -only-migrate, device realize fails. Works as designed. Example for (2): spapr_mce_req_event() makes an effort to prevent migration degrate the reporting of FWNMIs. It adds a migration blocker when it receives one, and deletes it when it's done handling it. This is a best effort; if migration is already in progress by the time FWNMI is received, we simply carry on, and that's okay. However, option -only-migrate sabotages the best effort entirely. While this isn't exactly terrible, it may be a weakness in our thinking and our infrastructure. I'm bringing it up so the people in charge are aware :)
