[Engine-devel] host cpu feature

[Laszlo Hornyak]

----- 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.

> 
> > 
> > > 
> > > - 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?
>