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

[Andreas Färber]

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 <address@hidden>
>>>> Signed-off-by: Alexey Kardashevskiy <address@hidden>
>>>
>>> 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.

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.

Andreas

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