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

From: Markus Armbruster
Subject: Re: -only-migrate and the two different uses of migration blockers
Date: Tue, 20 Jul 2021 07:30:16 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.2 (gnu/linux)

"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.

>> 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.

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