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

[Alexander Graf]

On 10.04.2014, at 16:35, Alexey Kardashevskiy <address@hidden> 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 <address@hidden> 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.

> 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.


Alex