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

[Alexey Kardashevskiy]

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?


-- 
Alexey