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Re: -only-migrate and the two different uses of migration blockers

From: David Gibson
Subject: Re: -only-migrate and the two different uses of migration blockers
Date: Wed, 21 Jul 2021 16:32:28 +1000

On Tue, Jul 20, 2021 at 07:30:16AM +0200, Markus Armbruster wrote:
> "Dr. David Alan Gilbert" <dgilbert@redhat.com> writes:
> > * Markus Armbruster (armbru@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

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_!

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