Daniel, thanks for your clarification.

Another related question is, what will be the guest's real cpu model is the 
cpu_model is None? This is about a reported regression at 
https://bugs.launchpad.net/nova/+bug/1082414 . When the 
instance.vcpu_model.mode is None, we should compare the source/target cpu 
model, as the suggestion from Tony, am I right?

Thanks
--jyh

> -----Original Message-----
> From: Daniel P. Berrange [mailto:berra...@redhat.com]
> Sent: Wednesday, March 4, 2015 6:56 AM
> To: Jiang, Yunhong
> Cc: openstack-dev@lists.openstack.org; Xu, Hejie
> Subject: Re: [nova][libvirt] The None and 'none' for CONF.libvirt.cpu_mode
> 
> On Wed, Mar 04, 2015 at 02:52:06PM +0000, Jiang, Yunhong wrote:
> > Hi, Daniel
> >     I'm a bit confused of the None/'none' for CONF.libvirt.cpu_mode.
> Per my understanding, None means there is no configuration provided and
> libvirt will select the default value based on the virt_type, none means no
> cpu_mode information should be provided. For the guest, am I right?
> >
> >               In _get_guest_cpu_model_config() on virt/libvirt/driver.py,
> > if mode is 'none', kvm/qemu virt_type will return a
> > vconfig.LibvirtConfigGuestCPU() while other virt type will return None.
> > What's the difference of this return difference?
> 
> The LibvirtConfigGuestCPU  object is used for more than just configuring
> the CPU model. It is also used for expressing CPU topology (sockets, cores,
> threads) and NUMA topology. So even if cpu model is None, we still need
> that object in the kvm case.
> 
> Regards,
> Daniel
> --
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