[Qemu-devel] Re: Qemu-KVM Livate Migration 0.12.2 -> 0.12.3/4 broken?

[Jan Kiszka <jan.kiszka@web.de>]

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Juan Quintela wrote:
> Jan Kiszka <jan.kiszka@web.de> wrote:
>> Juan Quintela wrote:
>>> Lack of "proper" subsections.  IDE is something like:
>>>
>>> const VMStateDescription vmstate_ide_drive = {
>>>     .version_id = 4,
>>> ....
>>> }
>>>
>>> static const VMStateDescription vmstate_bmdma = {
>>>     .name = "ide bmdma",
>>>     .version_id = 4,
>>> ...
>>> }
>>>
>>> const VMStateDescription vmstate_ide_pci = {
>>>     .name = "ide",
>>>     .version_id = 4,
>>> ....
>>>         VMSTATE_STRUCT_ARRAY(bmdma, PCIIDEState, 2, 0,
>>>                      vmstate_bmdma, BMDMAState),
>>>         VMSTATE_IDE_DRIVES(bus[0].ifs, PCIIDEState),
>>>         VMSTATE_IDE_DRIVES(bus[1].ifs, PCIIDEState),
>>> ....
>>> }
>>>
>>>
>>> Notice that everything is at version 4.  It used to be everything at
>>> version 3.  Now the problem is that when migrating from v3 -> v4.  We
>>> put in one place v3, But we only have a version number at the toplevel,
>>> rest of "subsections" don't sent a version number.  There is no way to
>>> fix it in the general case.  We can hack something around for ide, but
>>> that will just be a hack, or we can backport marcelo change and port it
>>> as a proper subsection (that is my plan).  I expect to have time at the
>>> end of next time to work on this.
>> BTW, the IDE subsystem is yet lacking a proper vmstate section split-up
>> along qdev boundaries (ie. vmstate_ide_pci should not contain drive
>> structures). Do you plan to address this as well?
> 
> Not for Friday, and not for 0.12.

For sure. I missed that this was only a 0.12 issue.

> 
> That is 0.13 material, and have to get one agreement on how to go.
> We can go for:
> - good structure
> - backward compatibility
> 
> I can't see any good way to get both at this stage :(  But I am open to
> sugestions.

Based on recent experiments with vmstate to enhance the hpet, I'm fairly
optimistic that we can have both (just the code complexity suffers a
bit): Split up the drive sections for new versions, but keep the legacy
fields with attached .field_exists() filters for reading of old
versions. But I may also underestimate issues of this particular case.

> 
> Later, Juan.
> 
> PD. BTW, very good work with printing the vmstate, that was one of the goals
>     when we added it, that was the next step after porting everything to
>     vmstate :)
> 

I'm sorry for stealing you the pleasure to add it. :)

Jan


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