[PATCH 19/19] Documentation: ACPI for ARM64

Mark Rutland
2014-07-29T14:04:12Z

On Tue, Jul 29, 2014 at 02:31:16PM +0100, Christoffer Dall wrote:
> On 29 July 2014 15:08, Mark Rutland <mark.rutland@arm.com> wrote:
> > On Tue, Jul 29, 2014 at 01:52:40PM +0100, Arnd Bergmann wrote:
> >> On Tuesday 29 July 2014 14:37:38 Christoffer Dall wrote:
> >> >
> >> > For reference, Red Hat's current arguing point for ACPI in VMs is
> >> > hotplug of things like CPUs and memory for very large VMs, but I
> >> > haven't thought too carefully about this just yet, as I don't have a
> >> > 100+ core ARM 64-bit hardware lying around...
> >>
> >> I thought you could run guests with more virtual CPUs that you have
> >> physical CPUs on the host.
> >>
> >> Regarding CPU and memory hotplug, don't we already have PSCI and
> >> xen-balloon/virtio-balloon for that?
> >
> > PSCI (0.1) was there for guests from the start, and ACPI doesn't do
> > anything different w.r.t. PSCI other than requiring PSCI 0.2 (which can
> > be used by guests supporting only PSCI 0.1). So there's no magic for
> > CPU hotplug provided by ACPI.
>
> With PSCI you can only provide your VM a bunch of CPUs and say that
> they're all turned off, and then turn some of them on later. I
> honestly don't know if you can do proper CPU hotplug with ACPI, but
> the RH guys seem to argue that you can. Again, I didn't think too
> carefully about this.

Ah, I see. That would make some sense.

> >
> > Do either of the balloon drivers allow for memory to be hot-added to a
> > system initially provisioned with less?
> >
> No, it's just about reclaiming memory. Same argument as above.

Ok. Thanks for the clarifications.

Cheers,
Mark.