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

[Andrew Jones]

On Wed, Jul 25, 2018 at 11:40:54AM +0100, Marc Zyngier wrote:
> On 24/07/18 19:35, Maran Wilson 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?
> 
> This is typically a project for someone who would have this particular
> itch to scratch, and who has a demonstrable need for this functionality.
> 
> Work wise, it would have to include adding physical CPU hotplug support
> to the arm64 kernel as a precondition, before worrying about doing it in
> KVM.
> 
> For KVM itself, particular area of interests would be:
> - Making GICv3 redistributors magically appear in the IPA space
> - Live resizing of GICv3 structures
> - Dynamic allocation of MPIDR, and mapping with vcpu_id

I have CPU topology description patches on the QEMU list now[*]. A next
step for me is to this MPIDR work. I probably won't get to it until the
end of August though.

[*] http://lists.nongnu.org/archive/html/qemu-devel/2018-07/msg01168.html

> 
> This should keep someone busy for a good couple of weeks (give or take a
> few months).

:-)

> 
> That being said, I'd rather see support in QEMU first, creating all the
> vcpu/redistributors upfront, and signalling the hotplug event via the
> virtual firmware. And then post some numbers to show that creating all
> the vcpus upfront is not acceptable.

I think the upfront allocation, allocating all possible cpus, but only
activating all present cpus, was the planned approach. What were the
concerns about that approach? Just vcpu memory overhead for too many
overly ambitious VM configs?

Thanks,
drew