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

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 04/08/2014 04:53 AM, Andreas Färber wrote:
> Am 07.04.2014 05:27, schrieb Alexey Kardashevskiy:
>> 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.
> 
> The response is very simple: -cpu host is not supported for migration.
> Same as for x86 hosts.

Is there any good reason to limit ourselves on POWERPC?

> As you say, the domain config is transferred by libvirt:
> If you use -cpu POWER7, you can migrate from POWER7 to POWER8 and back;
> if you use -cpu POWER8, you can only migrate on POWER8.

-cpu other that "host" is not supported by HV KVM, only "compat" which
upstream QEMU does not have yet. So you are saying that the migration is
not supported by upstream QEMU for at least SPAPR. Well, ok, it is dead
anyway so I am fine :)



-- 
Alexey