Re: [Qemu-devel] [PATCH v9 06/10] replay: fix save/load vm for non-empty queue

["Pavel Dovgalyuk" <dovgaluk@ispras.ru>]

> From: Paolo Bonzini [mailto:pbonzini@redhat.com]
> On 04/05/2017 14:34, Pavel Dovgalyuk wrote:
> >> 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.
> 
> Understood.  And what checkpoint will you be waiting for during drain,
> causing a deadlock?

Every checkpoint processes the queue, but none of them are invoked,
because iothread (which invokes all the checkpoints) is waiting for the end of the drain.

And we cannot add a checkpoint into the drain, because then vmstop will
affect the behavior of the guest.

Pavel Dovgalyuk