On Tue, Jul 31, 2018 at 3:57 PM, Igor Mammedov <imamm...@redhat.com> wrote:
> On Wed, 25 Jul 2018 14:07:12 +0100 > Marc Zyngier <marc.zyng...@arm.com> 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.
