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

[Eduardo Habkost]

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.
> > 
> > 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?

I didn't take the time to count them, but I bet most of the commits I
listed on my previous e-mail message are not bug fixes, but new
features.

Also, it doesn't matter if the change is a bug fix or a new feature: if
it affects runnability of the VM, it has more consequences than a simple
guest-side ABI change, and libvirt can't tie it to the machine-type so
it needs another flag to enable it.

-- 
Eduardo