Re: VCPU hotplug on KVM/ARM

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

It's been a few months since this email thread died off. Has anyone 
started working on a potential solution that would allow VCPU hotplug on 
KVM/ARM ? Or is this a project that is still waiting for an owner who 
has the time and inclination to get started?

Thanks,
-Maran

On 2/27/2018 5:21 AM, Andrew Jones wrote:
> On Tue, Feb 27, 2018 at 01:46:04PM +0100, Christoffer Dall wrote:
>> On Tue, Feb 27, 2018 at 05:34:28PM +0530, bthakur@codeaurora.org wrote:
>>> Hi Christoffer,
>>>
>>> Thanks for your reply.
>>>
>>> On 2018-02-27 16:17, Christoffer Dall wrote:
>>>> Hi Bhupinder,
>>>>
>>>> On Tue, Feb 27, 2018 at 03:01:17PM +0530, bthakur@codeaurora.org wrote:
>>>>> I hope it is the right forum to post my query.
>>>>>
>>>>>
>>>>>
>>>>> I am currently looking at the possibility of adding a new VCPU to a
>>>>> running
>>>>> guest VM in KVM/ARM. I see that currently, it is not allowed to add a
>>>>> new
>>>>> VCPU to a guest VM, if it is already initialized. The first check in
>>>>> kvm_arch_vcpu_create() returns failure if it is already initialized.
>>>>>
>>>> This would require a major rework of a lot of logic surrounding the GIC
>>>> and other parts of KVM initialization.
>>>>
>>>>>
>>>>> There was some work done in QEMU to add support for VCPU hotplug:
>>>>> https://lists.gnu.org/archive/html/qemu-arm/2017-05/msg00404.html
>>>>>
>>>>>
>>>>>
>>>>> But I am looking at the KVM side for enabling adding a new VCPU. If you
>>>>> can
>>>>> point me to any relevant work/resources, which I can refer to then it
>>>>> will
>>>>> help me.
>>>>>
>>>> I don't have any specific pointers, but I was always told that the way
>>>> we were going to do CPU hotplug would be to instantiate a large number
>>>> of VCPUs, and hotplug would be equivalent to turning on a VCPU which was
>>>> previously powered off.
>>>>
>>>> Is this not still a feasible solution?
>>> It should be a feasible solution provided the guest VM is not able to
>>> control the onlining/offlining of VCPUs. It should be controlled by the
>>> Host.
>>>
>> KVM could simply refuse to turn on some of the CPUs unless given
>> permission from host userspace.
>>
>>>> How does VCPU hotplug work on x86?
>>> On x86, you can add a vcpu through libvirt setvcpu command and it shows up
>>> in the guest VM as a new CPU if you do lscpu.
>>>
>> Sure, but what is the mechanism, does x86 qemu actually call
>> KVM_CREATE_VCPU, or is this also a question of turning on already
>> created vcpus ?
>>
> CC'ing Igor and qemu-devel
>
> drew
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