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Date: Fri, 28 Mar 2014 13:16:16 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] migration: Fix possible bug for migrate
 cancel
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To: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: "quintela@redhat.com" <quintela@redhat.com>, "libvir-list@redhat.com" <libvir-list@redhat.com>, Yanqiangjun <yanqiangjun@huawei.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, "Zhaoyanbin (A)" <lefty.zhao@huawei.com>, Zengjunliang <zengjunliang@huawei.com>, "Gonglei (Arei)" <arei.gonglei@huawei.com>

Il 28/03/2014 12:30, Dr. David Alan Gilbert ha scritto:
>> > Another reason for doing it in libvirt is that the serialization
>> > between cancellation and completion of migration ultimately is
>> > controlled by libvirt's lock.  Doing this in QEMU makes it harder to
>> > reason about concurrency.
> I think you have to be careful when you talk about 'cancellation and completion
> of migration' - in that paragraph I don't think you mean the same thing
> as MIG_STATE_CANCELLED and MIG_STATE_COMPLETED, I think you're talking
> about the larger scale idea of completion after you take into account
> that the VM might be paused after qemu has gone to MIG_STATE_COMPLETED and
> libvirt might still decide it wants to give up and use the version on
> the source that's still paused.

Yes, exactly.  This is why I considered the possibility of adding a 
"cancelled" flag within libvirt.

Libvirt always uses -S on the destination, so it's always possible to 
cancel migration even after MIG_STATE_COMPLETED.

Paolo