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

[Bharata B Rao]

On Tue, Jul 31, 2018 at 3:57 PM, Igor Mammedov <address@hidden> wrote:

> On Wed, 25 Jul 2018 14:07:12 +0100
> Marc Zyngier <address@hidden> wrote:
>
> > On 25/07/18 13:28, Andrew Jones wrote:
> > > 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?
> >
> > I don't have any ARM-specific concern about that, and I think this is
> > the right approach. It has the good property of not requiring much
> > change in the kernel (other than actually supporting CPU hotplug).
> for x86 we allocate VCPUs dynamically (both QEMU and KVM)
> CCing ppc/s390 folks as I don't recall how it's implemented there.
>
> but we do not delete vcpus in KVM after they were created
> (as it deemed to be too complicated), we are just deleting QEMU part
> of it and keep kvm's vcpu for reuse with future hotplug.
>

Same with PPC, we too dynamically create vcpus and during unplug keep the
KVM's vcpus for reuse.

Regards,
Bharata.