Re: [Engine-devel] host cpu feature

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'.

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).

Y.