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

[Alexander Graf]

Am 09.09.2013 um 04:40 schrieb Alexey Kardashevskiy <address@hidden>:

> 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?

Alex

> 
> 
> -- 
> Alexey