On Tue, Jul 20, 2021 at 07:30:16AM +0200, Markus Armbruster wrote: > "Dr. David Alan Gilbert" <dgilb...@redhat.com> writes: > > > * Markus Armbruster (arm...@redhat.com) wrote: > >> 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. > > > > That's interesting; it's the first time I've heard of anyone using it as > > 'best effort'. I've always regarded blockers as blocking. > > Me too, until I found this one.
Right, it may well have been the first usage this way, this fwnmi
stuff isn't super old.
> >> 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 :)
> >
> > Thanks.
> >
> > It almost feels like they need a way to temporarily hold off
> > 'completion' of migratio - i.e. the phase where we stop the CPU and
> > write the device data; mind you you'd also probably want it to stop
> > cold-migrates/snapshots?
>
> Yes, a proper way to delay 'completion' for a bit would be clearer, and
> wouldn't let -only-migrate interfere.
Right. If that becomes a thing, we should use it here. Note that
this one use case probably isn't a very strong argument for it,
though. The only problem here is slightly less that optimal error
reporting in a rare edge case (hardware fault occurs by chance at the
same time as a migration).
.... and, also, I half-suspect that the whole fwnmi feature exists
more to tick IBM RAS check boxes than because anyone will actually use
it.
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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