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Re: [Qemu-ppc] [Qemu-devel] [RFC PATCH] spapr: add initial ibm, client-a

From: Alexander Graf
Subject: Re: [Qemu-ppc] [Qemu-devel] [RFC PATCH] spapr: add initial ibm, client-architecture-support rtas call support
Date: Thu, 5 Sep 2013 15:04:37 +0200

On 05.09.2013, at 13:55, Paul Mackerras wrote:

> On Thu, Sep 05, 2013 at 12:19:09PM +0200, Alexander Graf wrote:
>> 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?
> Not if we want to be able to migrate to a real POWER7 later.  If we
> tell the guest it's a POWER8, it will start using POWER8 features, and
> then break when we migrate it to a POWER7 where those features don't
> exist.  If we run the POWER8 in POWER7 compatibility mode (and more
> importantly, the device tree says it's a 2.06 architecture processor),
> it should only use POWER7 features and then work just fine when
> migrated to a real POWER7.

Ah, ok. So we want to be able to specify 2 things:

  - current compat level
  - max compat level

That way we can boot a POWER8 virtual machine in POWER7 compat mode straight on 
boot, but allow that machine to later reconfigure itself if it isn't configured 
to POWER7 as max compat level.

The current level is mostly a convenience thing to not have to reboot SLOF 
every time you boot up a guest. The max level is what gives you compatibility 
throughout a cluster of machines.

Or do I miss anything here?


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