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From: "Pavel Dovgalyuk" <dovgaluk@ispras.ru>
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Date: Thu, 4 May 2017 15:34:10 +0300
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Subject: Re: [Qemu-devel] [PATCH v9 06/10] replay: fix save/load vm for
 non-empty queue
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To: 'Paolo Bonzini' <pbonzini@redhat.com>, 'Pavel Dovgalyuk' <pavel.dovgaluk@ispras.ru>, qemu-devel@nongnu.org
Cc: kwolf@redhat.com, peter.maydell@linaro.org, mst@redhat.com, jasowang@redhat.com, quintela@redhat.com, kraxel@redhat.com

> From: Paolo Bonzini [mailto:pbonzini@redhat.com]
> On 04/05/2017 14:10, Pavel Dovgalyuk wrote:
> >> From: Paolo Bonzini [mailto:pbonzini@redhat.com]
> >> On 04/05/2017 13:54, Pavel Dovgalyuk wrote:
> >>>> From: Paolo Bonzini [mailto:pbonzini@redhat.com]
> >>>> On 04/05/2017 13:13, Pavel Dovgalyuk wrote:
> >>>>>>> This patch does not allows saving/loading vmstate when
> >>>>>>> replay events queue is not empty. There is no reliable
> >>>>>>> way to save events queue, because it describes internal
> >>>>>>> coroutine state. Therefore saving and loading operations
> >>>>>>> should be deferred to another record/replay step.
> >>>>>>
> >>>>>> Can it actually be non-empty after bdrv_drain_all?
> >>>>>
> >>>>> drain/flush cannot succeed, because started requests are
> >>>>> prisoned in the replay events queue.
> >>>>
> >>>> But that would apply to loading only.  Saving should still be always
> >>>> possible.
> >>>
> >>> We can save it. But it wouldn't load correctly - replay queue will be empty after loading.
> >>
> >> When saving you can drain, and then the events queue should be empty.
> >> Or I am misunderstanding how it works, which is possible too.
> >
> > Drain will wait until the queue becomes empty.
> > Queue is processed only at checkpoints.
> > Checkpoints are met in iothread (at timers processing and so on).
> > But iothread is waiting for finishing the drain.
> 
> What checkpoint can hang the drain during a save?

No one. There are no checkpoins during vmsave and during drain.
Therefore the queue will never become empty.

Pavel Dovgalyuk