On Fri, Jul 03, 2015 at 11:10:09AM +0100, Marc Zyngier wrote: > On 03/07/15 10:34, Peter Maydell wrote: > > On 3 July 2015 at 09:28, Marc Zyngier <marc.zyngier@xxxxxxx> wrote: > >> On 03/07/15 09:12, Peter Maydell wrote: > >>> I would still like to see the proponents of this patch say > >>> what their model is for userspace support of cross-host migration, > >>> if we're abandoning the model the current API envisages. > >> > >> I thought we had discussed this above, and don't really see this as a > >> departure from the current model: > >> > >> - "-cpu host" results in "GENERIC" being used: VM can only be migrated > >> to the exact same HW (no cross-host migration). MIDR should probably > >> become RO. > >> - "-cpu host" results in "A57" (for example): VM can be migrated to a > >> variety of A57 platforms, and allow for some fuzzing on the revision (or > >> accept any revision). > >> - "-cpu a57" forces an A57 model to be emulated, always. It is always > >> possible to migrate such a VM on any host. > >> > >> I think only the first point is new, but the last two are what we have > >> (or what we should have). > > > > Right, but the implicit idea of this GENERIC patch seems to > > be that new host CPU types don't get their own KVM_ARM_TARGET_* > > constant, and are thus forever unable to do cross-host migration. > > It's not clear to me why we'd want to have new CPUs be second > > class citizens like that. > > I certainly don't want to see *any* CPU be a second class citizen. But > let's face it, we're adding more and more targets that don't implement > anything new, and just satisfy themselves with the generic implementation. > > I see it as an incentive to provide something useful (tables of all the > registers with default values?) so that cross-host migration becomes a > reality instead of the figment of our imagination (as it is now). If it > wasn't already ABI, I'd have removed the existing targets until we have > something meaningful to put there. What we're doing now certainly seems silly, because we're adding kernel patches without bringing anything to the table... > > Now, I also have my own doubts about cross-host migration (timers > anyone?). But I don't see the above as a change in policy. More as a way > to outline the fact that we currently don't have the right level of > information/infrastructure to support it at all. > The one thing that I've lost track of here (sorry) is whether we're enforcing the inability to do cross-host migration with the generic target when this patch is merged or do we leave this up to the graces of userspace? Thanks, -Christoffer -- 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
