Re: [Engine-devel] host cpu feature

From: "Yaniv Kaul" <ykaul(a)redhat.com&gt; To: "Doron Fediuck" <dfediuck(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 6:48:33 PM Subject: Re: [Engine-devel] host cpu feature On 12/05/2012 06:46 PM, Doron Fediuck wrote: > > ----- Original Message ----- >> From: "Laszlo Hornyak" <lhornyak(a)redhat.com&gt; >> To: "Doron Fediuck" <dfediuck(a)redhat.com&gt; >> Cc: "engine-devel" <engine-devel(a)ovirt.org&gt;, "Yaniv Kaul" >> <ykaul(a)redhat.com&gt; >> Sent: Wednesday, December 5, 2012 6:41:24 PM >> Subject: Re: [Engine-devel] host cpu feature >> >> >> >> ----- Original Message ----- >>> From: "Doron Fediuck" <dfediuck(a)redhat.com&gt; >>> To: "Laszlo Hornyak" <lhornyak(a)redhat.com&gt; >>> Cc: "engine-devel" <engine-devel(a)ovirt.org&gt;, "Yaniv Kaul" >>> <ykaul(a)redhat.com&gt; >>> Sent: Wednesday, December 5, 2012 5:33:35 PM >>> Subject: Re: [Engine-devel] host cpu feature >>> >>> >>> >>> ----- Original Message ----- >>>> 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. >>>> >>> Let's keep it simple. >>> 1. Please remove AllowMigrateCPUHost. No reason for us to do what >>> libvirt is asking to void. >> With pleasure :) >> >>> 2. host-passthrough should be available only for non migratable >>> VMs. >> Right. >> >> And no host-model support for now, right? >> > Right. > In can later be added if we have a good reasoning for it. Alternative idea, inspired by "Thus, if you hit any bugs, you are on your own" (http://libvirt.org/formatdomain.html#elementsCPU wrt 'host-passthrough'): A config option to determine if we use host-model or host-passthrough. Y.