On Tue, Jun 23, 2015 at 11:23:16PM +0200, Michael S. Tsirkin wrote: > On Tue, Jun 23, 2015 at 05:42:04PM +0100, Daniel P. Berrange wrote: > > On Tue, Jun 23, 2015 at 06:33:05PM +0200, Michael S. Tsirkin wrote: > > > On Tue, Jun 23, 2015 at 05:25:55PM +0100, Daniel P. Berrange wrote: > > > > On Tue, Jun 23, 2015 at 06:15:51PM +0200, Andreas Färber wrote: > > > > > Am 23.06.2015 um 17:58 schrieb Eduardo Habkost: > > > > > > On Tue, Jun 23, 2015 at 05:32:42PM +0200, Michael S. Tsirkin wrote: > > > > > >> On Tue, Jun 23, 2015 at 12:08:28PM -0300, Eduardo Habkost wrote: > > > > > >>> On Tue, Jun 23, 2015 at 02:32:00PM +0200, Andreas Färber wrote: > > > > > >>>> Am 08.06.2015 um 22:18 schrieb Jiri Denemark: > > > > > >>>>>> To help libvirt in the transition, a x86-cpu-model-dump script > > > > > >>>>>> is provided, > > > > > >>>>>> that will generate a config file that can be loaded using > > > > > >>>>>> -readconfig, based on > > > > > >>>>>> the -cpu and -machine options provided in the command-line. > > > > > >>>>> > > > > > >>>>> Thanks Eduardo, I never was a big fan of moving (or copying) > > > > > >>>>> all the CPU > > > > > >>>>> configuration data to libvirt, but now I think it actually > > > > > >>>>> makes sense. > > > > > >>>>> We already have a partial copy of CPU model definitions in > > > > > >>>>> libvirt > > > > > >>>>> anyway, but as QEMU changes some CPU models in some machine > > > > > >>>>> types (and > > > > > >>>>> libvirt does not do that) we have no real control over the > > > > > >>>>> guest CPU > > > > > >>>>> configuration. While what we really want is full control to > > > > > >>>>> enforce > > > > > >>>>> stable guest ABI. > > > > > >>>> > > > > > >>>> That sounds like FUD to me. Any concrete data points where QEMU > > > > > >>>> does not > > > > > >>>> have a stable ABI for x86 CPUs? That's what we have the pc*-x.y > > > > > >>>> machines > > > > > >>>> for. > > > > > >>> > > > > > >>> What Jiri is saying that the CPUs change depending on -mmachine, > > > > > >>> not > > > > > >>> that the ABI is broken by a given machine. > > > > > >>> > > > > > >>> The problem here is that libvirt needs to provide CPU models whose > > > > > >>> runnability does not depend on the machine-type. If users have a > > > > > >>> VM that > > > > > >>> is running in a host and the VM machine-type changes, > > > > > >> > > > > > >> How does it change, and why? > > > > > > > > > > > > Sometimes we add features to a CPU model because they were not > > > > > > emulated by KVM > > > > > > and now they are. Sometimes we remove or add features or change > > > > > > other fields > > > > > > because we are fixing previous mistakes. Recently we we were going > > > > > > to remove > > > > > > features from models because of an Intel CPU errata, but then > > > > > > decided to create > > > > > > a new CPU model name instead. > > > > > > > > > > > > See some examples at the end of this message. > > > > > > > > > > > >> > > > > > >>> the VM should be > > > > > >>> still runnable in that host. QEMU doesn't provide that, our CPU > > > > > >>> models > > > > > >>> may change when we introduce new machine-types, so we are giving > > > > > >>> them a > > > > > >>> mechanism that allows libvirt to implement the policy they need. > > > > > >> > > > > > >> I don't mind wrt CPU specifically, but we absolutely do change > > > > > >> guest ABI > > > > > >> in many ways when we change machine types. > > > > > > > > > > > > All the other ABI changes we introduce in QEMU don't affect > > > > > > runnability of the > > > > > > VM in a given host, that's the problem we are trying to address > > > > > > here. ABI > > > > > > changes are expected when changing to a new machine, runnability > > > > > > changes > > > > > > aren't. > > > > > > > > > > > > > > > > > > Examples of commits changing CPU models: > > > > > [snip] > > > > > > > > > > I've always advocated remaining backwards-compatible and only making > > > > > CPU > > > > > model changes for new machines. You among others felt that was not > > > > > always necessary, and now you're using the lack thereof as an argument > > > > > to stop using QEMU's CPU models at all? That sounds convoluted... > > > > > > > > Whether QEMU changed the CPU for existing machines, or only for new > > > > machines is actually not the core problem. Even if we only changed > > > > the CPU in new machines that would still be an unsatisfactory situation > > > > because we want to be able to be able to access different versions of > > > > the CPU without the machine type changing, and access different versions > > > > of the machine type, without the CPU changing. IOW it is the fact that > > > > the > > > > changes in CPU are tied to changes in machine type that is the core > > > > problem. > > > > > > But that's because we are fixing bugs. If CPU X used to work on > > > hardware Y in machine type A and stopped in machine type B, this is > > > because we have determined that it's the right thing to do for the > > > guests and the users. We don't break stuff just for fun. > > > Why do you want to bring back the bugs we fixed? > > > > Huh, I never said we wanted to bring back bugs. This is about allowing > > libvirt to fix the CPU bugs in a way that is independant of the machine > > types and portable across hypervisors we deal with. We're absolutely > > still going to fix CPU model bugs and ensure stable guest ABI. > > > > Regards, > > Daniel > > So any single CPU flag now needs to be added in > - kvm > - qemu > - libvirt > > Next thing libvirt will decide it's a policy thing and so > needs to be pushed up to openstack.
I don't think that will happen, but if they really decide do do it, why should we try to stop them? libvirt and OpenStack know what their users do/need better than us, and if they believe moving data to OpenStack will provide what users need, they are free to do it. I trust libvirt developers to do the right thing, here. > > We should just figure out what you want to do and support it in QEMU. > > Are there many examples where a single flag used to work and then > stopped? I would say such a change is problematic anyway: > not everyone uses libvirt, you are breaking things for people > that run -M pc. People using -M pc have to live with the fact that the host-side requirements of -M pc may change in newer QEMU versions. (Again, this is not about ABI changes, but about adding new host-side hardware/kernel requirements to make a VM run) > > IMHO -M pc is not supposed to mean "can break at any time". It means "it may have new host-side requirements and may become runnable in your host (or require additional command-line flags to run) at any time". -- Eduardo