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 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.

-- 
Eduardo