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

[Maran Wilson <maran.wilson@oracle.com>]

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.


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