[libvirt] [PATCH] qemu conf: Use host-model for cpu mode by default

Kaneshige, Kenji kaneshige.kenji at jp.fujitsu.com
Thu Nov 15 02:55:00 UTC 2012

> -----Original Message-----
> From: Daniel P. Berrange [mailto:berrange at redhat.com]
> Sent: Wednesday, November 14, 2012 6:45 PM
> To: Ichikawa, Ken/市川 顕
> Cc: libvir-list at redhat.com; eblake at redhat.com; gaofeng at cn.fujitsu.com; Kaneshige, Kenji/金重 憲治
> Subject: Re: [PATCH] qemu conf: Use host-model for cpu mode by default
> On Wed, Nov 14, 2012 at 03:10:48AM +0000, Ichikawa, Ken wrote:
> > Now, qemu guest's default cpu model is qemu32/64 and it can be
> > configured per domain. In some case, host-model mode is suitable for
> > getting enough performance in the guest because of features from cpu
> > spec.
> >
> > This patch adds a config option to qemu.conf to use 'host-model' mode
> > as default and allow users to use host-model mode in domains on a host.
> > This is useful because
> >  - Guest owners don't need to touch their domain's config.
> >  - An administrator can reduce an item in their checklist for VM
> >    performance and guarantee all guests should run in the best performance.
> While I understand the benefits, I'm afraid I have to NACK this
> at this time because using host-model doesn't really work when
> trying to use nested-KVM. Until we have a solution for that, we
> neeed to stick with a CPU model that works everywhere, since
> users like OpenStack do actually use nested-KVM for testing work
> and already complained about this when I made OpenStack set the
> host-model by default.

Ken's patch is not just to change the default cpu mode to host-model.
Cpu mode is set to host-model only when user specifies
"make_default_cpu_host_model = 1" in qemu.conf. So I don't think it
cause the problem.

Kenji Kaneshige

> Daniel
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