Am 23.06.2015 um 18:42 schrieb Daniel P. Berrange: > 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.
No, that's contradictory! Through the -x.y machines we leave bugs in the old models *exactly* to assure a stable guest ABI. Fixes are only be applied to new machines, thus I'm pointing out that you should not use a new CPU model with an old machine type. Andreas -- SUSE Linux GmbH, Maxfeldstr. 5, 90409 Nürnberg, Germany GF: Felix Imendörffer, Jane Smithard, Dilip Upmanyu, Graham Norton; HRB 21284 (AG Nürnberg)
