[Qemu-devel] unplug_request and migration

[David Gibson]

Hi Dave & Juan,

I'm hoping one of you can answer this.

I'm currently grappling with (amongst other things) a pseries machine
racing a hot unplug operation with a migrate.  There's various issues
with what interim state we need, and which bits of it need to be
migrated that I'm still investigating.  But, there's a more general
question that I'm guessing must have already been addressed for x86.

For any "soft" unplug device - i.e. using ->unplug_request, rather
than ->unplug, giving a device_del command will just ask the guest
nicely to release the device, with the completion of the unplug
happening only if and when the guest indicates it's ready for the
device to go away.  AFAICT, the device_del command will return as soon
as the request is made, but if the guest is busy, the completion of
the hot unplug could take arbitrarily long.

So, what happens if there's a migration in between the unplug_request
and the guest completing the unplug?  How does libvirt (or whatever)
know whether to include the device on the destination machine command
line?

-- 
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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