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Re: [Qemu-devel] [PULL 15/28] migration: create new section to store glo

From: Juan Quintela
Subject: Re: [Qemu-devel] [PULL 15/28] migration: create new section to store global state
Date: Wed, 08 Jul 2015 14:51:55 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux)

Christian Borntraeger <address@hidden> wrote:
> Am 08.07.2015 um 14:25 schrieb Juan Quintela:
>> Christian Borntraeger <address@hidden> wrote:
>>> Am 08.07.2015 um 14:08 schrieb Juan Quintela:
>>>> Christian Borntraeger <address@hidden> wrote:
>>>>> Am 07.07.2015 um 15:08 schrieb Juan Quintela:
>>>>>> This includes a new section that for now just stores the current qemu 
>>>>>> state.
>>>>>> Right now, there are only one way to control what is the state of the
>>>>>> target after migration.
>>>>>> - If you run the target qemu with -S, it would start stopped.
>>>>>> - If you run the target qemu without -S, it would run just after
>>>>>> migration finishes.
>>>>>> The problem here is what happens if we start the target without -S and
>>>>>> there happens one error during migration that puts current state as
>>>>>> -EIO.  Migration would ends (notice that the error happend doing block
>>>>>> IO, network IO, i.e. nothing related with migration), and when
>>>>>> migration finish, we would just "continue" running on destination,
>>>>>> probably hanging the guest/corruption data, whatever.
>>>>>> Signed-off-by: Juan Quintela <address@hidden>
>>>>>> Reviewed-by: Dr. David Alan Gilbert <address@hidden>
>>>>> This is bisected to cause a regression on s390.
>>>>> A guest restarts (booting) after managedsave/start instead of continuing.
>>>>> Do you have any idea what might be wrong?
>>>> Can you check the new patch series that I sent.  There is a fix that
>>>> *could* help there.  *could* because I don't fully understand why it can
>>>> give you problems (and even only sometimes).  Current guess is that some
>>>> of the devices are testing the guest state on LOAD, so that patch.
>>>> Please, test.
>>> That patch does indeed fix my problem.
>>> I can see that virtio_init uses the runstate to set vm_running of the vdev. 
>>> This
>>> is used in virtio-net for several aspects.
>>> But I really dont understand why this causes the symptoms.
>>> So I am tempted to add
>>> a
>>> Tested-by: Christian Borntraeger <address@hidden>
>>> but I have a bad feeling on the "why" :-/
>> The other reason of that patch is that it removes the need that the
>> global_state is the last migration section.
> Hmm, we have some register_savevm here and there, but these seem
> to be called in device realize/init or machine init.
>> Could it be that you are
>> adding sections after you launch qemu?  Or that virtio devices are
>> generated later on s390 for any reason?
> Not that I am aware of. There can be hotplug of course, but I dont see
> why libvirt should do that during migration.

It can also happens if it plugs the devices after we have registerd the
global save state.  On x86_64 that don't happens, but not sure how
things are structured in s390.

If it was that, it has been fixed with the patch that I sent.

> Still looking.....

Thanks, Juan.

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