[Engine-devel] host cpu feature

[Dan Kenigsberg]

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.

> 
> 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 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.

> 
> > 
> > - 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?