Re: [Qemu-devel] [PATCH 0/2] target-i386: "custom" CPU model + script to dump existing CPU models

[Eduardo Habkost <ehabkost@redhat.com>]

On Tue, Jun 23, 2015 at 06:47:16PM +0200, Andreas Färber wrote:
> 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.

They don't need to use a new model with an old machine-type (although I
don't see a reason to prevent that). They need to be able to change the
machine-type to a new one without getting any changes that would make
the VM not runnable in the same host. Even if it is a bug fix. If it is
a change that can make the VM unrunnable, it needs to be controlled by a
separate flag, not by the machine-type.

-- 
Eduardo