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

[Daniel P. Berrange]

On Tue, Jun 23, 2015 at 07:41:50PM +0200, Andreas Färber wrote:
> Only if you use "-cpu ...,enforce", no?
> 
> The KVM feature filtering should take care of dropping features that are
> not available otherwise.
> 
> So we seem to be getting to the interesting case of the same machine
> (different from what was said previously!) but different hosts.
> 
> The QOM property gives you insights into which feature bits are set for
> the machine for the model (and for s390x I saw QMP extensions to the
> same effect, I thought). That way you could discover features to
> disable. However you'll only ever know which ones work once you've tried
> it once, right?

On this subject, a big problem in QOM in general is that it hasn't
included a distinction between object classes and object instances.
So there's no way for us to introspect what a machine type provides
without actually instantiating a guest with that machine type.

Again allowing libvirt to control the CPU model removes this problem
as libvirt will be able to determine what CPUs it can run on a given
host without having to probe all the different CPU <-> machine type
combinations for the QEMU on that host has.

This in turn simplifies life for apps using libvirt, as they will always
know that "CPUFoo-X.Y" will always mean the exact same thing on all the
hosts they have no matter what QEMU version/machine is in use.

Regards,
Daniel
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