Re: [Qemu-devel] [RFC PATCH] spapr: support time base offset migration

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 09/09/2013 03:50 PM, Alexander Graf wrote:
> 
> 
> Am 09.09.2013 um 04:40 schrieb Alexey Kardashevskiy <aik@ozlabs.ru>:
> 
>> On 09/06/2013 01:11 AM, Alexander Graf wrote:
>>>
>>> On 05.09.2013, at 16:26, Benjamin Herrenschmidt wrote:
>>>
>>>> On Thu, 2013-09-05 at 16:14 +0200, Andreas Färber wrote:
>>>>
>>>>> Are you thinking of POWER8 having a different frequency than POWER8 in
>>>>> compat mode? Because migration from one -cpu to another is not supported
>>>>> elsewhere.
>>>>>
>>>>> Even if we want to migrate from one POWER7 revision to another, we
>>>>> should let the destination use the revision of the source (guest ABI!),
>>>>> via property if need be. Anything else will lead to confusion as to what
>>>>> is supported and what is not. That -cpu host is the default for
>>>>> convenience shouldn't relieve admins/libvirt to think about sensible
>>>>> setups like they have to on x86.
>>>>
>>>> Besides POWER8 uses 512Mhz too :-) It's been architected so it's
>>>> unlikely to change from now on.
>>>
>>> Ok, ditch the tb frequency then. We can always default to 512Mhz when it's absent.
>>
>>
>> QEMU uses what kvmppc_get_tbfreq() returns. And ppc_tb_t carries this
>> value. It does not use 512MHz automatically but migration should always
>> assume 512MHz :-/
>>
>> If we remove it, I would like to add assert(tb_freq!=512MHz) into
>> timebase_pre_save() and timebase_post_load(), is that ok?
> 
> Good point. This would break TCG migration, right?


In TCG we do not need any timebase offset at all, the time should migrate
as is, no? :)

It could break something like power5 migration under PR KVM, do not know...


-- 
Alexey