On Mon, Apr 28, 2014 at 09:06:42PM +0200, Paolo Bonzini wrote: > Il 28/04/2014 17:46, Eduardo Habkost ha scritto: > >>So I couldn't indeed think of uses of "-cpu host" together with > >>migration. But if you're sure there is none, block it in QEMU. > >>There's no reason why this should be specific to libvirt. > > > >It's not that there are no useful use cases, it is that we simply can't > >guarantee it will work because (by design) "-cpu host" enables features > >QEMU doesn't know about (so it doesn't know if migration is safe or > >not). And that's the main use case for "-cpu host". > > True, but in practice QEMU and KVM support is added in parallel, and > we already have full support for Broadwell (IIRC) and are starting to > add Skylake features. > > >So, what about doing the following on QEMU 2.1: > > > > * "-cpu host,migratable=yes": > > * No invtsc > > * Migration not blocked > > * No feature flag unknown to QEMU will be enabled > > * "-cpu host,migratable=no": > > * invtsc enabled > > * Unknown-to-QEMU features enabled > > * Migration blocked > > * "-cpu host": > > * Same behavior as "-cpu host,migratable=yes" > > * Release notes indicating that "-cpu host,migratable=no" is > > required if the user doesn't care about migration and wants new > > (unknown to QEMU) features to be available > > > >I was going to propose making "migratable=no" the default after a few > >releases, as I expect the majority of existing users of "-cpu host" to > >not care about migration. But I am not sure, because there's less harm > >in _not_ getting new bleeding edge features enabled, and those users > >(and management software) can start using "migratable=no" if they really > >want the new unmigratable features. > > Makes sense. Basically "-cpu host,migratable=yes" is close to > libvirt's host-model and Alex Graf's proposed "-cpu best". Should we > call it "-cpu best" and drop migratability of "-cpu host"? "-cpu best" is different from the modes above. It means "use the best existing CPU model (from the pre-defined table) that can run on this host". And it would have the same ambiguities we found on "-cpu host": if a CPU model in the table have invtsc enabled, should it be considered a candidate for "-cpu best", or not? -- Eduardo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
