Re: [Qemu-devel] [RFC PATCH 0/4] Fix subsection ambiguity in the migration format

[Dor Laor <dlaor@redhat.com>]

On 08/01/2011 12:03 AM, Anthony Liguori wrote:
> On 07/31/2011 03:57 PM, Dor Laor wrote:
>> On 07/31/2011 11:43 PM, Anthony Liguori wrote:
>>>> ps: how hard is to finish the vmstate conversion? Can't we just assume
>>>> not converted code is not functional and just remove all of it?
>>>
>>> No. VMState is a solution looking for a problem. Many important device
>>
>> The initial target solved some rare bugs, that tend not to bite us with
>> virtio. On the way, it got enhanced with subsections that was a major
>> improvement.
>
> I should have qualified my statement. VMState did solve many real
> problems. I meant that at this point in time, we've gotten pretty much
> what we can get out it.
>
>>
>>> models are still not converted and ultimately, it doesn't solve the
>>> problem we're really trying to solve.
>>
>> From the start I supported Michael Tisrkin's idea for ASN.1 protocol.
>> The question is how visitors and ability to translate from one
>> representation to another will help us.
>
> Because with Visitors you can do:
>
> Devices -> internal QObject representation -> ASN.1 -> wire -> ASN.1 ->
> internal QObject representation -> Device.

I admit that QObject sounds more appealing than VMState, we can convert 
all into it. I'm not sure what's the difference between visitor and the 
load/save functions, potentially with enhanced parameters like name 
which can be part of QObject anyway.

>
> While it's in an internal representation, we can make large changes like
> translating entire device state structures to new formats, splitting one
> device into two, etc.
>
> It's sort of the ultimate mechanism to make compatibility changes. If
> you just go Devices -> ASN.1, you miss out on that.

What's important in ASN.1 is not the data representation itself but the 
ability to have a flexible protocol. We can have it with VMState and 
QObject as well. I do admit that QObject+ASN.1 will ease the way to make 
it right so you convinced me :).

I still don't see have using ASN.1 will easily join/split several 
devices into few and some other magics. Not that it is not possible but 
it is way too hard.

The main 'real' problems you're trying to solve are migration from one 
release to the other while most of our problems were forgotten fields 
here and there (floppy/ide/rtl/kvmclock/etc). I doubt that live 
migration of the same release worked on upstream for the random git 
head. Verifying save(i)== load(i)+save(i+1) is simple but no one 
executing it. Looks like we might be ready to go with your suggestion, 
I'm just worried that there are too many other non migration open 
issues. If the above work won't get complete we're better off with the 
current machine type + VMState + subsections. If it will be all 
completed, we're better with your suggestion.

>
> BTW, another really useful thing that Visitor would enable is the
> ability to read an individual device to a QObject and implement the
> equivalent of 'show devicename' which dumps the state of arbitrary
> devices via QMP. This could be very useful for debugging.
>
>> I do see value in it but I don't
>> think it is that important. If we have one real device serialization
>> method that is flexible enough we can stick with it w/o translation. If
>> we define qdev serialization into vmstate/asn.1/json/other and add some
>> capability negotiation and various other goodies it should be enough.
>>
>> btw: separating the live migration protocol from the machine state is
>> even more important if we take a gradual approach.
>
> Yeah, I think the critical technical requirement to achieve this is that
> the devices need to generate their own serialization format, and then
> another layer translates that to the "live migration protocol" format.
>
> Regards,
>
> Anthony Liguori
>
>>
>>>
>>> Regards,
>>>
>>> Anthony Liguori
>>>
>>>>
>>>>>
>>>>> Regards,
>>>>>
>>>>> Anthony Liguori
>>>>
>>>>
>>>>
>>>
>>>
>>
>>
>