Re: [Xen-devel] PVH CPU hotplug design document

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Jan 17, 2017 at 10:50:44AM -0500, Boris Ostrovsky wrote:
> On 01/17/2017 10:33 AM, Jan Beulich wrote:
> >>>> On 17.01.17 at 16:27, <boris.ostrovsky@xxxxxxxxxx> wrote:
> >> On 01/17/2017 09:44 AM, Jan Beulich wrote:
> >>>>>> On 17.01.17 at 15:13, <roger.pau@xxxxxxxxxx> wrote:
> >>>> There's only one kind of PVHv2 guest that doesn't require ACPI, and that 
> >>>> guest
> >>>> type also doesn't have emulated local APICs. We agreed that this model 
> >>>> was
> >>>> interesting from things like unikernels DomUs, but that's the only 
> >>>> reason why
> >>>> we are providing it. Not that full OSes couldn't use it, but it seems
> >>>> pointless.
> >>> You writing things this way makes me notice another possible design
> >>> issue here: Requiring ACPI is a bad thing imo, with even bare hardware
> >>> going different directions for at least some use cases (SFI being one
> >>> example). Hence I think ACPI should - like on bare hardware - remain
> >>> an optional thing. Which in turn require _all_ information obtained from
> >>> ACPI (if available) to also be available another way. And this other
> >>> way might by hypercalls in our case.
> >>
> >> At the risk of derailing this thread: why do we need vCPU hotplug for
> >> dom0 in the first place? What do we gain over "echo {1|0} >
> >> /sys/devices/system/cpu/cpuX/online" ?
> >>
> >> I can see why this may be needed for domUs where Xen can enforce number
> >> of vCPUs that are allowed to run (which we don't enforce now anyway) but
> >> why for dom0?
> > Good that you now ask this too - that's the PV hotplug mechanism,
> > and I've been saying all the time that this should be just fine for PVH
> > (Dom0 and DomU).
> 
> I think domU hotplug has some value in that we can change number VCPUs
> that the guest sees and ACPI-based hotplug allows us to do that in a
> "standard" manner.
> 
> For dom0 this doesn't seem to be necessary as it's a special domain
> available only to platform administrator.
> 
> Part of confusion I think is because PV hotplug is not hotplug, really,
> as far as Linux kernel is concerned.

Hm, I'm not really sure I'm following, but I think that we could translate this
Dom0 PV hotplug mechanism to PVH as:

 - Dom0 is provided with up to HVM_MAX_VCPUS local APIC entries in the MADT, and
   the entries > dom0_max_vcpus are marked as disabled.
 - Dom0 has HVM_MAX_VCPUS vCPUs ready to be started, either by using the local
   APIC or an hypercall.

Would that match what's done for classic PV Dom0?

Roger.

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