From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:53451) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1VHYCz-0003yW-Q9 for qemu-devel@nongnu.org; Thu, 05 Sep 2013 07:58:50 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1VHYCu-0007sM-T9 for qemu-devel@nongnu.org; Thu, 05 Sep 2013 07:58:45 -0400 Date: Thu, 5 Sep 2013 21:55:01 +1000 From: Paul Mackerras Message-ID: <20130905115501.GC18222@iris.ozlabs.ibm.com> References: <1378289952-32703-1-git-send-email-aik@ozlabs.ru> <6F7696D8-AC02-482C-9682-A39107C346ED@suse.de> <52271C41.7070005@ozlabs.ru> <522730BA.3000405@ozlabs.ru> <20130905101637.GB18222@iris.ozlabs.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Subject: Re: [Qemu-devel] [RFC PATCH] spapr: add initial ibm, client-architecture-support rtas call support List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Alexander Graf Cc: Nikunj A Dadhania , Alexey Kardashevskiy , qemu-devel , "open list:PowerPC" , Anthony Liguori , Andreas =?iso-8859-1?Q?F=E4rber?= 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 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.