Re: live migration between qemu-kvm 1.0 and 0.15

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2012-03-27 17:59, Avi Kivity wrote:
> On 03/27/2012 11:55 AM, Jan Kiszka wrote:
>> On 2012-03-27 10:55, Vasilis Liaskovitis wrote:
>>> Hi,
>>>
>>> is live migration between qemu-kvm stable-0.15 and stable-1.0 trees possible?
>>> When I live migrate a VM from 1.0 to 0.15, the destination side 0.15 qemu-kvm
>>> exits with:
>>> (qemu) Unknown savevm section or instance 'i8259' 0
>>>
>>> That's expected, since commit "i8259:convert to qdev" 747c70af78f7088f182c87e683bdf847beead1e4
>>> introduces the i8259 device in the qdev tree.
>>>
>>> The other direction (live migrate from 0.15.1 to 1.0.0) seems to work fine.
>>> Are any other issues expected in this direction?
>>>
>>> The vmstate for i8259 has not changed between these trees afaict. If the
>>> qdev-ified i8259 is reverted from stable-1.0 tree (to restore live-migration
>>> compatibility between the versions), would you expect problems?
>>
>> The legacy instance IDs used in old versions are not written out by
>> newer ones. They are just accepted on reading to allow moving forward.
>> There are more devices in the tree that used those instance IDs, not
>> only the i8259. Reverting the qdev conversion may reactivate backward
>> migratability for 1.0->0.15 (unless there are others as well). But that
>> will definitely be over after 1.1 as inrevertible code depends on the
>> qdev conversion.
> 
> Is this true even for -M pc-0.15?

Yes. Alias IDs enable modeling according to qdev (back then) for devices
that wrote oddly numbered states in the past and porting them over to
the new format. Adding some compat write-out mode would probably be
feasible but would also require some thoughts and quite a bit work to
integrate this cleanly in vmstate.

I guess this just remained unnoticed because the introduction of the
alias ID concept and first conversions happened at a time when lots of
devices increased their vmstate version anyway.

Jan

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