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

[Jonathan Cameron <Jonathan.Cameron@Huawei.com>]

On Mon, 13 Mar 2023 15:50:52 +0000
James Morse <james.morse@arm.com> wrote:

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

From a quick glance that looks good to me.

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

We'll also need to kick off the spec change with a code-first proposal.
I think current standard way to do that is a bugzilla entry in EDK2 repo 
https://bugzilla.tianocore.org/buglist.cgi?component=Specification%20Update&product=EDK2%20Code%20First&resolution=---
and the get someone in ASWG to create equivalent tracking issue in mantis.

Great if you already have that in hand via relevant ARM folks.

Jonathan

> 
> 
> 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?  
>