Re: [PATCH 19/19] Documentation: ACPI for ARM64

[Hanjun Guo <hanjun.guo@linaro.org>]

On 2014-7-29 21:31, 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.  

Yes, agreed.

> 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.

Yes, we can do proper CPU hotplug with ACPI based physical hotplug (named
dynamic device configuration in ACPI spec), you can refer to section 6.3
"Device Insertion, Removal, and Status Objects" in ACPI 5.1.

There are mechanisms for handling dynamic insertion and removal of devices
and for determining device and notification processing status. When removing
a processor,

a) we will call PSCI or similar interface to offline a CPU from OS, then
   OS will not use it any more;

b) call ACPI API to trim the resources that allocated during device
   enumeration;

c) Call ACPI method _EJ0 and then will notify firmware or hypervisor device
   will be removed, jump to firmware or hypervisor to remove the device
   from that level;

CPU hotplug with ACPI is pretty mature on x86 and it works on VM too. I prepared
some patches to support CPU hotplug on ARM64 and simulated the hot add/remove
notify to test those patches, it worked as expected, I will send them out
when this patchset is ready for mainline.

Thanks
Hanjun