Re: [Qemu-devel] [Qemu-ppc] [PATCH] target-ppc: Add @cpu_dt_id into migration stream

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 04/11/2014 12:41 AM, Alexander Graf wrote:
> 
> On 10.04.2014, at 16:35, Alexey Kardashevskiy <aik@ozlabs.ru> wrote:
> 
>> On 04/10/2014 10:10 PM, Alexander Graf wrote:
>>>
>>> On 08.04.14 03:26, Alexey Kardashevskiy wrote:
>>>> On 03/28/2014 12:07 AM, Alexey Kardashevskiy wrote:
>>>>> On 03/27/2014 11:57 PM, Peter Maydell wrote:
>>>>>> On 27 March 2014 12:49, Alexey Kardashevskiy <aik@ozlabs.ru> wrote:
>>>>>>> On 03/27/2014 11:37 PM, Andreas Färber wrote:
>>>>>>>> Am 27.03.2014 03:41, schrieb Alexey Kardashevskiy:
>>>>>>>>> This should prevent the destination guest from misbehaving when
>>>>>>>>> the threads number is different in "-smp" command.
>>>>>>>> Sorry, I don't understand. When migrating, surely -smp needs to be the
>>>>>>>> same on source and destination, so how can they differ?
>>>>>>>
>>>>>>> The idea is that "-smp" does not migrate and if we run source and
>>>>>>> destination guests with different numbers in -smp, we end up with weird
>>>>>>> machine
>>>>>> Yes, so don't do that. As I understand it:
>>>>>>  (1) if you don't run QEMU with the exact same command line
>>>>>>      and config at both ends then migration won't work
>>>>>>  (2) we don't guarantee to detect and cleanly fail if you
>>>>>>      don't do (1)
>>>>>>
>>>>>> It would probably be nice if we did detect config mismatches,
>>>>> Yep, we do not send the device tree (as libvirt does). Pure command line
>>>>> matching won't work.
>>>>>
>>>>>> but that seems to me like a problem we should be addressing
>>>>>> more globally than just for one particular config item for
>>>>>> one particular target...
>>>>
>>>> Ok. So. Let's assume I want to implement migration of "-smp" parameters.
>>>> What would be the correct way of doing this in terms of the current QOM
>>>> principles? Thanks.
>>>
>>> You don't. The migration protocol doesn't migrate configuration. If you
>>> want to start to transfer VM configuration (which I'd be all in for), do it
>>> properly and transfer _all_ configuration.
>>
>>
>> Then what is the purpose of many, many VMSTATE_.*_EQUAL?
> 
> Probably legacy from old vmstate layouts.


So this should not be used from now on?


>> And I do not want to send configuration by the proposed patch, I want to
>> make sure that the new guest is able to continue. Why exactly is this bad?
> 
> It's not bad, but we should solve this properly, not one field at a time.





-- 
Alexey