Re: [Engine-devel] host cpu feature

From: "Laszlo Hornyak" <lhornyak(a)redhat.com&gt; To: "Yaniv Kaul" <ykaul(a)redhat.com&gt; Cc: "engine-devel" <engine-devel(a)ovirt.org&gt; Sent: Wednesday, December 5, 2012 5:24:59 PM Subject: Re: [Engine-devel] host cpu feature ----- Original Message ----- > From: "Yaniv Kaul" <ykaul(a)redhat.com&gt; > To: "Laszlo Hornyak" <lhornyak(a)redhat.com&gt; > Cc: "Dan Kenigsberg" <danken(a)redhat.com&gt;, "engine-devel" > <engine-devel(a)ovirt.org&gt; > 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(a)redhat.com&gt; > >> To: "Yaniv Kaul" <ykaul(a)redhat.com&gt; > >> Cc: "Laszlo Hornyak" <lhornyak(a)redhat.com&gt;, "engine-devel" > >> <engine-devel(a)ovirt.org&gt; > >> 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.

> Y. > > > >