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

[Michael S. Tsirkin]

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.
> 
> Regards,
> Daniel

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?

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