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 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?
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