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

[Igor Mammedov]

On Tue, 24 Jul 2018 11:35:31 -0700
Maran Wilson <address@hidden> wrote:

> 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?
I'm working on QEMU side of it as time allows.

I can guide or share task if are you interested in helping out with it?


> 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
> > _______________________________________________
> > kvmarm mailing list
> > address@hidden
> > https://lists.cs.columbia.edu/mailman/listinfo/kvmarm  
> 
>