Re: [Qemu-devel] [RFC PATCH] target-ppc: enable migration within the same CPU family

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 04/04/2014 11:28 PM, Alexander Graf wrote:
> On 04/04/2014 07:17 AM, Alexey Kardashevskiy wrote:
>> On 03/24/2014 04:28 PM, Alexey Kardashevskiy wrote:
>>> Currently only migration fails if CPU version is different even a bit.
>>> For example, migration from POWER7 v2.0 to POWER7 v2.1 fails because of
>>> that. Since there is no difference between CPU versions which could
>>> affect migration stream, we can safely enable it.
>>>
>>> This adds a helper to find the closest POWERPC family class (i.e. first
>>> abstract class in hierarchy).
>>>
>>> This replaces VMSTATE_UINTTL_EQUAL statement with a custom handler which
>>> checks if the source and destination CPUs belong to the same family and
>>> fails if they are not.
>>>
>>> This adds a PVR reset to the default value as it will be overwritten
>>> by VMSTATE_UINTTL_ARRAY(env.spr, PowerPCCPU, 1024).
>>>
>>> Since the actual migration format is not changed by this patch,
>>> @version_id of vmstate_ppc_cpu does not have to be changed either.
>>>
>>> Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
>>> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
>>
>> Ping?
> 
> Can't we just always allow migration to succeed? It's a problem of the tool
> stack above if it allows migration to an incompatible host, no?

This is not how libvirt works. It simply sends the source XML, reconstructs
a guest on the destination side and then migrates. hoping that the
migration will fail is something (which only QEMU has knowledge of) is
incompatible. The new guest will start with "-cpu host" (as the source) but
it will create diffrent CPU class and do different things. If we do not
check PVR (and cpu_dt_id and chip_id - the latter is coming soon) and
migrate power8->power7, we can easily get a broken guest.

How would I fix this CPU parameters (i.e. PVR/cpu_dt_id) check in the
existing libvirt?


-- 
Alexey