Re: [Qemu-devel] Re: new->old version migration

[Anthony Liguori <anthony@codemonkey.ws>]

On 02/08/2011 12:42 AM, Michael S. Tsirkin wrote:
> On Mon, Feb 07, 2011 at 08:23:08PM -0600, Anthony Liguori wrote:
>    
>> On 02/07/2011 03:52 PM, Michael S. Tsirkin wrote:
>>      
>>> How does it? We need to know we are saving in 0.13
>>> format and skip the new subsection, otherwise
>>> 0.13 will see a subsection it does not recognize
>>> and exit.
>>>        
>> If you used subsections for flow control, presumably you would only
>> send the new savevm data if you had data buffered.
>>
>> If you add a qdev property to enable/disable flow control, then if
>> it's disabled, you naturally would never send the subsection because
>> you'd never buffer data.  So no explicit code is needed to support
>> migration.
>>      
> But the result is we get a new property that we can never remove
> as any qdev property is part of interface.
>
>    
>> The difficult case is when you truly need to change the savevm
>> version.  I don't think we have a proper fix for this because
>> versions are linear so the proposed patch certainly wouldn't be a
>> good way to do it.  if flow_control=0 causes savevm 3 to be used
>> instead of 4, and then the next_feature=0 causes savevm 4 to be used
>> instead of 5, the semantics of flow_control=0,next_feature=1 becomes
>> problematic.
>>
>> But as long as the feature has isolated state, we can solve the
>> problem robustly with subsections.
>>
>> Regards,
>>
>> Anthony Liguori
>>      
> I see. I'm unhappy with the facts that
> 1. if (feature) is spread all over the code instead
>     of just in migration
>    

Yeah, but it's no different than say msi enablement.

> 2. it is also obfuscated with if (flow_control)
>     instead of plain if (migrate to qemu<  0.14)
>     so removing it will be much harder
> 3. this forces anyone who wants
>     a VM compatible with qemu 0.13 to also lose data,
>     even if migration to 0.13 is never attempted.
>    

It depends on whether we consider flow control a feature or a bug.  If 
it's a bug, then we have to bump the live migration version and then 
live with the compatibility breakage.

Regards,

Anthony Liguori

>>> We also need API to add subsections without vmstate,
>>> because virtio serial wasn't yet converted.
>>>
>>>        
>