[Engine-devel] host cpu feature

[Laszlo Hornyak]

----- Original Message -----
> From: "Yaniv Kaul" <ykaul at redhat.com>
> To: "Laszlo Hornyak" <lhornyak at redhat.com>
> Cc: "Dan Kenigsberg" <danken at redhat.com>, "engine-devel" <engine-devel at ovirt.org>
> Sent: Wednesday, December 5, 2012 2:45:46 PM
> Subject: Re: [Engine-devel] host cpu feature
> 
> On 12/05/2012 03:39 PM, Laszlo Hornyak wrote:
> >
> > ----- Original Message -----
> >> From: "Dan Kenigsberg" <danken at redhat.com>
> >> To: "Laszlo Hornyak" <lhornyak at redhat.com>
> >> Cc: "Yaniv Kaul" <ykaul at redhat.com>, "engine-devel"
> >> <engine-devel at ovirt.org>
> >> Sent: Wednesday, December 5, 2012 1:55:19 PM
> >> Subject: Re: [Engine-devel] host cpu feature
> >>
> >> On Wed, Dec 05, 2012 at 06:46:09AM -0500, Laszlo Hornyak wrote:
> >>>
> >>> ----- Original Message -----
> >>>> From: "Yaniv Kaul" <ykaul at redhat.com>
> >>>> To: "Laszlo Hornyak" <lhornyak at redhat.com>
> >>>> Cc: "engine-devel" <engine-devel at ovirt.org>
> >>>> Sent: Wednesday, December 5, 2012 12:23:47 PM
> >>>> Subject: Re: [Engine-devel] host cpu feature
> >>>>
> >>>> On 12/05/2012 12:32 PM, Laszlo Hornyak wrote:
> >>>>> Hi,
> >>>>>
> >>>>> CPU-Host support allows the virtual machines to see and utilize
> >>>>> the
> >>>>> host's CPU flags, this enables better performance in VM's, at
> >>>>> the
> >>>>> price of worse portablity.
> >>>>>
> >>>>> http://www.ovirt.org/Features/Cpu-host_Support
> >>>>>
> >>>>> Your feedback is welcome!
> >>>>>
> >>>>> Thank you,
> >>>>> Laszlo
> >>>>> _______________________________________________
> >>>>> Engine-devel mailing list
> >>>>> Engine-devel at ovirt.org
> >>>>> http://lists.ovirt.org/mailman/listinfo/engine-devel
> >>>> - I assume that when you allow migration, you'd use host-model?
> >>>> This
> >>>> is
> >>>> not clear from the design. It seems like we VDSM developers can
> >>>> choose
> >>>> to use either this or passthrough, while in practice we should
> >>>> support both.
> >> I join Kaul's question: it is an ovirt-level question whether
> >> hostPassthrough or hostModel or both should be supported. It
> >> should
> >> not
> >> be a unilateral vdsm decision.
> > Ah, possibly misunderstanding, I did not mean that VDSM should
> > decide whether to use host-passthrough or host-model. The engine
> > should decide.
> > I meant _you_ should decide which version of vdsm api modification
> > do you want :)
> >
> >>> If AllowMigrateCPUHost is set to true (in case you have the same
> >>> cpu model everywhere in your DC) migration of such hsots will be
> >>> enabled. Otherwise it will not be enabled.
> >> What is the breadth of AllowMigrateCPUHost? Engine wide? Per DC?
> >> Per
> >> cluster?
> > I thought of eninge-wide. The of course you can have different
> > models in two different DC, but they should be unique in one.
> > We can add this to DC or cluster level, imho it would be just
> > another checkbox on the UI that most users would not use.
> >
> >> I favor the latter; a user may have a cluster of exact-same hosts,
> >> where
> >> hostcpu migration is allowed, and other cluster where it is
> >> forbiden.
> >>
> >> The nice thing about hostModel (unlike hostPassthrough) is that
> >> once
> >> we
> >> created the VM we can migrate it to stronger hosts, and back to
> >> the
> >> original host. I suppose that it complicates the scheduler.
> > Yes with host-model you get the features that libvirt handles. In
> > such cases the engine could decide, if you want this
> > functionality. Well the scheduler architecture is just being
> > reinvented.
> >
> > For the host-passthrough, I think the AllowMigrateCPUHost
> > configuration option would be a simple decision for the
> > administrator: set it to true if all hosts are uniform. If it is
> > not set to true, then we will not allow migration of such VMs.
> 
> That's not what I understood from libvirt's documentation. I

You may be right, could you send an URL to that point of the documentation or copy-paste?

> understood
> that if you want host+migration, you need to use host-model.
> Otherwise -
> host-passthrough.
> Y.
> 
> >
> >>>> - I'm still convinced and commented on both relevat oVirt and
> >>>> libvirt
> >>>> BZs that we need to add x2apic support to the CPU, regardless of
> >>>> what
> >>>> the host CPU exposes.
> >>>> AFAIK, the KVM developers agree with me.
> >>> Not quite sure how is this related... could you send some URL's
> >>> for
> >>> the bugreports?
> 
>