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

From: Paul Mackerras
Subject: Re: [Qemu-ppc] [Qemu-devel] [RFC PATCH] spapr: add initial ibm, client-architecture-support rtas call support
Date: Thu, 5 Sep 2013 21:55:01 +1000
User-agent: Mutt/1.5.21 (2010-09-15)

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.


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