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