[Engine-devel] host cpu feature

[Andrew Cathrow]

----- Original Message -----
> From: "Itamar Heim" <iheim at redhat.com>
> To: snmishra at linux.vnet.ibm.com
> Cc: "engine-devel" <engine-devel at ovirt.org>
> Sent: Wednesday, December 5, 2012 3:56:02 PM
> Subject: Re: [Engine-devel] host cpu feature
> 
> On 12/05/2012 09:15 PM, snmishra at linux.vnet.ibm.com wrote:
> >
> > Quoting Doron Fediuck <dfediuck at redhat.com>:
> >
> >> ----- Original Message -----
> >>> From: "Laszlo Hornyak" <lhornyak at redhat.com>
> >>> To: "Doron Fediuck" <dfediuck at redhat.com>
> >>> Cc: "engine-devel" <engine-devel at ovirt.org>
> >>> Sent: Wednesday, December 5, 2012 7:14:46 PM
> >>> Subject: Re: [Engine-devel] host cpu feature
> >>>
> >>>
> >>>
> >>> ----- Original Message -----
> >>> > From: "Doron Fediuck" <dfediuck at redhat.com>
> >>> > To: "Yaniv Kaul" <ykaul at redhat.com>
> >>> > Cc: "Laszlo Hornyak" <lhornyak at redhat.com>, "engine-devel"
> >>> > <engine-devel at ovirt.org>
> >>> > Sent: Wednesday, December 5, 2012 6:10:55 PM
> >>> > Subject: Re: [Engine-devel] host cpu feature
> >>> >
> >>> > > Alternative idea, inspired by "Thus, if you hit any bugs, you
> >>> > > are
> >>> > > on
> >>> > > your own" (http://libvirt.org/formatdomain.html#elementsCPU
> >>> > > wrt
> >>> > > 'host-passthrough'):
> >>> > > A config option to determine if we use host-model or
> >>> > > host-passthrough.
> >>> > > Y.
> >>> > >
> >>> >
> >>> > I do not think the engine should go to this level.
> >>> > ie- it can ask for passthrough as a feature, and the
> >>> > actual implementation is handled by vdsm.
> >>> >
> >>>
> >>> If vdsm decides over host-passthrough or host-model, then how
> >>> will
> >>> the engine user know what exactly his VM gets. I think vdsm must
> >>> be
> >>> told exactly what to do.
> >>>
> >>
> >> VDSM maintains some level of independence. This is why it the
> >> engine
> >> should be able to ask for passthrough as a feature. Otherwize vdsm
> >> will
> >> handle it. So for now I suggest we stick to passthrough only, and
> >> if
> >> we get an RFE for advanced mode we'll support the host model.
> >
> > What are we gaining by using passthrough over host-model? Looking
> > at
> > libvirt documentation, it seems that both modes give host CPU
> > capabilities to guest VM. Whereas the downside of passthrough is
> > that it
> > limits migration. Whereas host-model will migrate to other hardware
> > and
> > if the destination hardware is better than source then the guest VM
> > performance can be improved by rebooting guest.
> >
> > As a stretch goal, ovirt can keep track of host capabilities and
> > inform
> > the user after migrating to a better host, that a reboot may
> > improve
> > guest performance.
> 
> pass-through may give better performance.

We need to be using -cpu host aka pass-through for performance. Selecting -cpu host on a Westmere cpu is different to -cpu Westmere on a Westmere cpu in terms of what the guest sees


> host-model would be relevant when we can support live migration
> inside
> the cluster for some of the nodes, which will be relevant when the
> scheduler is more pluggable/extendable than today.
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