Re: [Qemu-ppc] [Qemu-devel] [RFC PATCH] spapr: add initial ibm, client-architecture-support rtas call support

[Alexander Graf]

On 05.09.2013, at 12:16, Paul Mackerras wrote:

> On Wed, Sep 04, 2013 at 04:32:20PM -0500, Anthony Liguori wrote:
>> On Wed, Sep 4, 2013 at 8:37 AM, Alexander Graf <address@hidden> wrote:
>>> 
>>>> 
>>>>> So IMHO this whole thing should be orthogonal to -cpu.
>>>> 
>>>> Well, since we cannot change CPU class on the fly, yes, it should be a
>>>> "compatibility" flags/properties/methods/whatever of the default CPU for
>>>> the specific family.
>>> 
>>> Since it's machine global, it could just as well be a machine option, no? 
>>> Or can you have multiple CPUs with different compat modes in a single 
>>> system?
>> 
>> AFAIK, this has nothing to do with CPUs.
> 
> I'm not sure what you mean by that; it has to do with CPUs since it
> means changing the CPUs' behaviour, at least for user-mode programs.
> 
>>>>> Why? Just because you're on POWER7 as default doesn't mean you can't bump 
>>>>> to a newer compat later on, no?
>>>> 
>>>> Bump when exactly? And it won't be a new compat, it will be a native CPU. I
>>> 
>>> If you configure your guest to boot in POWER7-compat mode on your POWER8 
>>> box and it then tells you through ibm,client-architecture-support that it 
>>> can do POWER8, we can just remove all the compat bits and be happy, no?
> 
> Answering Alex here -- if we want to preserve the option of migrating
> to a POWER7 host in future, we would run the guest in POWER7 compat
> mode even if the current host is a POWER8 and the guest knows about
> POWER8.

Yes, so we boot the guest with compat mode set to POWER7, then the guest calls 
ibm,client-architecutre-support including POWER8 and then we can reconfigure 
the guest to be POWER8, right?


Alex