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Re: [Qemu-devel] [Bug?] BQL about live migration

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [Bug?] BQL about live migration
Date: Fri, 3 Mar 2017 13:48:16 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0

On 03/03/2017 13:00, Dr. David Alan Gilbert wrote:
> Ouch that's pretty nasty; I remember Paolo explaining to me a while ago that
> their were times when run_on_cpu would have to drop the BQL and I worried 
> about it,
> but this is the 1st time I've seen an error due to it.
> Do you know what the migration state was at that point? Was it 
> I'm thinking perhaps we should stop 'cont' from continuing while migration is 
> in
> MIGRATION_STATUS_CANCELLING.  Do we send an event when we hit CANCELLED - so 
> that
> perhaps libvirt could avoid sending the 'cont' until then?

No, there's no event, though I thought libvirt would poll until
"query-migrate" returns the cancelled state.  Of course that is a small
consolation, because a segfault is unacceptable.

One possibility is to suspend the monitor in qmp_migrate_cancel and
resume it (with add_migration_state_change_notifier) when we hit the
CANCELLED state.  I'm not sure what the latency would be between the end
of migrate_fd_cancel and finally reaching CANCELLED.


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