[PATCH 19/19] Documentation: ACPI for ARM64

Christoffer Dall
2014-07-29T13:31:16Z

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.

>
> 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.

-Christoffer