[Engine-devel] host cpu feature

[Doron Fediuck]

----- Original Message -----
> From: "Laszlo Hornyak" <lhornyak at redhat.com>
> To: "Yaniv Kaul" <ykaul at redhat.com>
> Cc: "engine-devel" <engine-devel at ovirt.org>
> Sent: Wednesday, December 5, 2012 5:24:59 PM
> Subject: Re: [Engine-devel] host cpu feature
> 
> 
> ----- 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 4:10:13 PM
> > Subject: Re: [Engine-devel] host cpu feature
> > 
> > On 12/05/2012 04:59 PM, Laszlo Hornyak wrote:
> > >
> > > ----- Original Message -----
> > >> From: "Dan Kenigsberg" <danken 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 3:22:18 PM
> > >> Subject: Re: [Engine-devel] host cpu feature
> > >>
> > >> On Wed, Dec 05, 2012 at 04:05:00PM +0200, Yaniv Kaul wrote:
> > >>> On 12/05/2012 03:55 PM, Laszlo Hornyak wrote:
> > >>>>>>> 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 THAT simple, Engine could take this decision without
> > >> human
> > >> intervension.
> > > Is there a way engine can figure out if the cpu-models in all the
> > > hosts are the same?
> > > I mean even if some host flags are not handled by libvirt and
> > > therefore vdsm and engine...
> > > So I would really need that permission from the user.
> > >
> > >>>>>> 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?
> > >>> The link I followed from your feature page:
> > >>> http://libvirt.org/formatdomain.html#elementsCPU :
> > >>>
> > >>> host-model
> > >>> The host-model mode is essentially a shortcut to copying host
> > >>> CPU
> > >>> definition from capabilities XML into domain XML. Since the CPU
> > >>> definition is copied just before starting a domain, exactly the
> > >>> same
> > >>> XML can be used on different hosts while still providing the
> > >>> best
> > >>> guest CPU each host supports. Neither match attribute nor any
> > >>> feature elements can be used in this mode. Specifying CPU model
> > >>> is
> > >>> not supported either, but model's fallback attribute may still
> > >>> be
> > >>> used. Libvirt does not model every aspect of each CPU so the
> > >>> guest
> > >>> CPU will not match the host CPU exactly. On the other hand, the
> > >>> ABI
> > >>> provided to the guest is reproducible. During migration,
> > >>> complete
> > >>> CPU model definition is transferred to the destination host so
> > >>> the
> > >>> migrated guest will see exactly the same CPU model even if the
> > >>> destination host contains more capable CPUs for the running
> > >>> instance
> > >>> of the guest; but shutting down and restarting the guest may
> > >>> present
> > >>> different hardware to the guest according to the capabilities
> > >>> of
> > >>> the
> > >>> new host.
> > >>> host-passthrough
> > >>> With this mode, the CPU visible to the guest should be exactly
> > >>> the
> > >>> same as the host CPU even in the aspects that libvirt does not
> > >>> understand. Though the downside of this mode is that the guest
> > >>> environment cannot be reproduced on different hardware. Thus,
> > >>> if
> > >>> you
> > >>> hit any bugs, you are on your own.
> > >> That's exactly where AllowMigrateCPUHost fits well: when a user
> > >> ticks
> > >> this for a cluster he says "yeah, I like to be on my own."
> > >>
> > > cpu mode="host-passthrough" migration: I talked to the libvirt
> > > guys
> > > and they said it is OK if the hardware and the software are the
> > > same, and it will work, but they would not recommend.
> > >
> > > So if they do not recommend it, I would drop this from the
> > > feature
> > > spec. Anyone against it?
> > >
> > > Laszlo
> > 
> > I'm a bit against it. I don't see why it's that complicated:
> > Allow migration -> use 'host-model'
> > Do not allow -> use 'host-passthrough'.
> 
> So the actual cpu capabilities would depend not only on the 'host
> cpu' checkbox, but also on the 'pinned to host' checkbox. I think
> this logical trick would be both funny and confusing from the user's
> perspective.
> 
> > 
> > The reason of why we need host-passthrough is that otherwise I
> > suspect
> > we depend on libvirt for newer features to be somehow exposed to
> > the
> > guest (not sure about it).
> 
> Yes, with other words: this is a tuning feature.
> 

Let's keep it simple.
1. Please remove AllowMigrateCPUHost. No reason for us to do what libvirt is asking to void.
2. host-passthrough should be available only for non migratable VMs.


> > Y.
> > 
> > 
> > 
> >