Re: [Qemu-devel] [PATCH 6/6] spapr: fix migration of ICP objects from/to older QEMU

[Cédric Le Goater]

On 05/15/2017 03:16 PM, Greg Kurz wrote:
> On Mon, 15 May 2017 14:22:32 +0200
> Cédric Le Goater <address@hidden> wrote:
> 
>> On 05/15/2017 01:40 PM, Greg Kurz wrote:
>>> Commit 5bc8d26de20c ("spapr: allocate the ICPState object from under
>>> sPAPRCPUCore") moved ICP objects from the machine to CPU cores. This
>>> is an improvement since we no longer allocate ICP objects that will
>>> never be used. But it has the side-effect of breaking migration of
>>> older machine types from older QEMU versions.
>>>
>>> This patch introduces a compat flag in the sPAPR machine class so
>>> that all pseries machine up to 2.9 go on with the previous behavior
>>> of pre-allocating ICP objects.  
>>
>> I think this is a quite elegant way to a handle the migration 
>> regression. Thanks for taking care of it.
>>
>> Have you tried to simply reparent the ICPs objects to OBJECT(spapr) 
>> instead of the OBJECT(cpu)  ? 
>>
> 
> Do you mean to reparent unconditionally to OBJECT(spapr) for all
> machine versions ? 

only in the case of smc->must_pre_allocate_icps

> I'm not sure this would be beneficial, but I might be missing 
> something...

I think that we would not need to allocate the legacy_icps array. 
Parenting the icp object to the spapr machine should be enough. 
I might be wrong. my expertise on the migration stream is very 
basic.

C.