[Engine-devel] host cpu feature

Wed Dec 5 17:10:55 UTC 2012

----- Original Message -----
> From: "Yaniv Kaul" <ykaul at redhat.com>
> To: "Doron Fediuck" <dfediuck at redhat.com>
> Cc: "Laszlo Hornyak" <lhornyak at redhat.com>, "engine-devel" <engine-devel at ovirt.org>
> 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 at redhat.com>
> >> To: "Doron Fediuck" <dfediuck at redhat.com>
> >> Cc: "engine-devel" <engine-devel at ovirt.org>, "Yaniv Kaul"
> >> <ykaul at redhat.com>
> >> Sent: Wednesday, December 5, 2012 6:41:24 PM
> >> Subject: Re: [Engine-devel] host cpu feature
> >>
> >>
> >>
> >> ----- Original Message -----
> >>> From: "Doron Fediuck" <dfediuck at redhat.com>
> >>> To: "Laszlo Hornyak" <lhornyak at redhat.com>
> >>> Cc: "engine-devel" <engine-devel at ovirt.org>, "Yaniv Kaul"
> >>> <ykaul at redhat.com>
> >>> Sent: Wednesday, December 5, 2012 5:33:35 PM
> >>> Subject: Re: [Engine-devel] host cpu feature
> >>>
> >>>
> >>>
> >>> ----- 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.
> >> 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.
> 

I do not think the engine should go to this level.
ie- it can ask for passthrough as a feature, and the
actual implementation is handled by vdsm.