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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jun 23, 2015 at 04:51:00PM +0100, Daniel P. Berrange 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, 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.
> 
> Expanding on that, but tieing the CPU model to the machine type, QEMU
> has in turn effectively tied the machine type to the host hardware.
> eg, switching to a newer machine type, may then prevent the guest
> from being able to launch on the hardware that it was previously
> able to run on, due to some new requirement of the CPU model associated
> with the machine type.

So why not keep machine type stable?

> Libvirt wants the CPU models to be independant of the machine type,
> so in general only the CPU model is dependant on hardware capabilities
> and machine type is isolated from hardware.
> 
> Libvirt still intends to do versioning of the CPU models, but the
> versioning will be separate from the versioning of the machine types,
> and will be handled by libvirt itself.
> 
> This also allows us to get  further towards our goal which is to have a
> consistent representation of CPU models across all libvirt hypervisors.
> eg the same libvirt CPU model and versions can be made consistent across
> kvm, xen, vmware, etc, as they're not longer changing behind our back
> based on the qemu machine type.
> 
> Regards,
> Daniel
> -- 
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