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? Alex