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

[Igor Mammedov]

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.

> vcpu memory overhead is a generic concern though, and not only for ARM.
> We currently allow up to 512 vcpus per VM, which looks like a lot, but
> really isn't. If we're to allow this to be bumped up significantly, we
> should start accounting the vcpu-related memory against the user's
> allowance...
> 
> Thanks,
> 
>       M.