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 <ag...@suse.de> 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. Paul.
