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

[Maran Wilson]

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, 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
> > > > > 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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