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Re: [Qemu-devel] VCPU hotplug on KVM/ARM

From: Maran Wilson
Subject: Re: [Qemu-devel] VCPU hotplug on KVM/ARM
Date: Tue, 24 Jul 2018 11:35:31 -0700
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

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?


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, address@hidden 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, address@hidden 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
guest VM in KVM/ARM. I see that currently, it is not allowed to add a
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:

But I am looking at the KVM side for enabling adding a new VCPU. If you
point me to any relevant work/resources, which I can refer to then it
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

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

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