Re: [RFC PATCH v2 0/3] Support CPU hotplug for ARM64

[Maran Wilson]

On 7/10/2019 2:15 AM, Marc Zyngier wrote:
> On 09/07/2019 20:06, Maran Wilson wrote:
> > On 7/5/2019 3:12 AM, James Morse wrote:
> > > Hi guys,
> > >
> > > (CC: +kvmarm list)
> > >
> > > On 29/06/2019 03:42, Xiongfeng Wang wrote:
> > > > This patchset mark all the GICC node in MADT as possible CPUs even though it
> > > > is disabled. But only those enabled GICC node are marked as present CPUs.
> > > > So that kernel will initialize some CPU related data structure in advance before
> > > > the CPU is actually hot added into the system. This patchset also implement
> > > > 'acpi_(un)map_cpu()' and 'arch_(un)register_cpu()' for ARM64. These functions are
> > > > needed to enable CPU hotplug.
> > > >
> > > > To support CPU hotplug, we need to add all the possible GICC node in MADT
> > > > including those CPUs that are not present but may be hot added later. Those
> > > > CPUs are marked as disabled in GICC nodes.
> > > ... what do you need this for?
> > >
> > > (The term cpu-hotplug in the arm world almost never means hot-adding a new package/die to
> > > the platform, we usually mean taking CPUs online/offline for power management. e.g.
> > > cpuhp_offline_cpu_device())
> > >
> > > It looks like you're adding support for hot-adding a new package/die to the platform ...
> > > but only for virtualisation.
> > >
> > > I don't see why this is needed for virtualisation. The in-kernel irqchip needs to know
> > > these vcpu exist before you can enter the guest for the first time. You can't create them
> > > late. At best you're saving the host scheduling a vcpu that is offline. Is this really a
> > > problem?
> > >
> > > If we moved PSCI support to user-space, you could avoid creating host vcpu threads until
> > > the guest brings the vcpu online, which would solve that problem, and save the host
> > > resources for the thread too. (and its acpi/dt agnostic)
> > >
> > > I don't see the difference here between booting the guest with 'maxcpus=1', and bringing
> > > the vcpu online later. The only real difference seems to be moving the can-be-online
> > > policy into the hypervisor/VMM...
> > Isn't that an important distinction from a cloud service provider's
> > perspective?
> >
> > As far as I understand it, you also need CPU hotplug capabilities to
> > support things like Kata runtime under Kubernetes. i.e. when
> > implementing your containers in the form of light weight VMs for the
> > additional security ... and the orchestration layer cannot determine
> > ahead of time how much CPU/memory resources are going to be needed to
> > run the pod(s).
> Why would it be any different? You can pre-allocate your vcpus, leave
> them parked until some external agent decides to signal the container
> that it it can use another bunch of CPUs. At that point, the container
> must actively boot these vcpus (they aren't going to come up by magic).
>
> Given that you must have sized your virtual platform to deal with the
> maximum set of resources you anticipate (think of the GIC
> redistributors, for example), I really wonder what you gain here.

Maybe I'm not following the alternative proposal completely, but 
wouldn't a guest VM (who happens to be in control of its OS) be able to 
add/online vCPU resources without approval from the VMM this way?

Thanks,
-Maran

> > Thanks,
> > -Maran
> >
> > > I think physical package/die hotadd is a much bigger, uglier problem than doing the same
> > > under virtualisation. Its best to do this on real hardware first so we don't miss
> > > something. (cpu-topology, numa, memory, errata, timers?)
> > > I'm worried that doing virtualisation first means the firmware-requirements for physical
> > > hotadd stuff is "whatever Qemu does".
> For sure, I want to model the virtualization side after the actual HW,
> and not the other way around. Live reconfiguration of the interrupt
> topology (and thus the whole memory map) will certainly be challenging.
>
> Thanks,
>
> 	M.