[RFC PATCH 00/32] ACPI/arm64: add support for virtual cpuhotplug

[James Morse]

Hi Jonathan,

On 07/03/2023 12:00, Jonathan Cameron wrote:
> On Fri,  3 Feb 2023 13:50:11 +0000
> James Morse <james.morse at arm.com> wrote:

>> On a system that supports cpuhotplug the MADT has to describe every possible
>> CPU at boot. Under KVM, the vGIC needs to know about every possible vCPU before
>> the guest is started.
>> With these constraints, virtual-cpuhotplug is really just a hypervisor/firmware
>> policy about which CPUs can be brought online.
>>
>> This series adds support for virtual-cpuhotplug as exactly that: firmware
>> policy. This may even work on a physical machine too; for a guest the part of
>> firmware is played by the VMM. (typically Qemu).
>>
>> PSCI support is modified to return 'DENIED' if the CPU can't be brought
>> online/enabled yet. The CPU object's _STA method's enabled bit is used to
>> indicate firmware's current disposition. If the CPU has its enabled bit clear,
>> it will not be registered with sysfs, and attempts to bring it online will
>> fail. The notifications that _STA has changed its value then work in the same
>> way as physical hotplug, and firmware can cause the CPU to be registered some
>> time later, allowing it to be brought online.

> As we discussed on an LOD call a while back, I think that we need some path to
> find out if the guest supports vCPU HP or not so that info can be queried by
> an orchestrator / libvirt etc.  In general the entity responsible for allocating
> extra vCPUs may not know what support the VM has for this feature.

I agree. For arm64 this is going to be important if/when there are machines that do
physical hotplug of CPUs too.


> There are various ways we could get this information into the VMM.
> My immediate thought is to use one of the ACPI interfaces that lets us write
> AML that can set an emulated register. A query to the VMM can check if this
> register is set.
> 
> So options.
> 
> _OSI() - Deprecated on ARM64 so lets not use that ;)

News to me, I've only just discovered it!


> _OSC() - Could add a bit to Table 6.13 Platform-Wide Capabilites in ACPI 6.5 spec.
>          Given x86 has a similar online capable bit perhaps this is the best option
>          though it is the one that requires a formal code first proposal to ASWG.

I've had a go at writing this one:
https://gitlab.arm.com/linux-arm/linux-jm/-/commit/220b0d8b0261d7467c8705e6f614d57325798859

It'll appear in the v1 of the series once the kernel and qemu bits are all lined up again.


Thanks,

James


> _OSC() - Could add a new UUID and put it under a suitable device - maybe all CPUs?
>          You could definitely argue this feature is an operating system property.
> _DSM() - Similar to OSC but always under a device.
>          Whilst can be used for this I'm not sure it really matches intended usecase.
> 
> Assuming everyone agrees this bit of introspection is useful,
> Rafael / other ACPI specialists: Any suggestions on how best to do this?