CPU Hotplug and ACPI

CPU hotplug in the arm64 world is commonly used to describe the kernel taking CPUs online/offline using PSCI. This document is about ACPI firmware allowing CPUs that were not available during boot to be added to the system later.

possible and present refer to the state of the CPU as seen by linux.

CPU Hotplug on physical systems - CPUs not present at boot

Physical systems need to mark a CPU that is possible but not present as being present. An example would be a dual socket machine, where the package in one of the sockets can be replaced while the system is running.

This is not supported.

In the arm64 world CPUs are not a single device but a slice of the system. There are no systems that support the physical addition (or removal) of CPUs while the system is running, and ACPI is not able to sufficiently describe them.

e.g. New CPUs come with new caches, but the platform’s cache topology is described in a static table, the PPTT. How caches are shared between CPUs is not discoverable, and must be described by firmware.

e.g. The GIC redistributor for each CPU must be accessed by the driver during boot to discover the system wide supported features. ACPI’s MADT GICC structures can describe a redistributor associated with a disabled CPU, but can’t describe whether the redistributor is accessible, only that it is not ‘always on’.

arm64’s ACPI tables assume that everything described is present.

CPU Hotplug on virtual systems - CPUs not enabled at boot

Virtual systems have the advantage that all the properties the system will ever have can be described at boot. There are no power-domain considerations as such devices are emulated.

CPU Hotplug on virtual systems is supported. It is distinct from physical CPU Hotplug as all resources are described as present, but CPUs may be marked as disabled by firmware. Only the CPU’s online/offline behaviour is influenced by firmware. An example is where a virtual machine boots with a single CPU, and additional CPUs are added once a cloud orchestrator deploys the workload.

For a virtual machine, the VMM (e.g. Qemu) plays the part of firmware.

Virtual hotplug is implemented as a firmware policy affecting which CPUs can be brought online. Firmware can enforce its policy via PSCI’s return codes. e.g. DENIED.

The ACPI tables must describe all the resources of the virtual machine. CPUs that firmware wishes to disable either from boot (or later) should not be enabled in the MADT GICC structures, but should have the online capable bit set, to indicate they can be enabled later. The boot CPU must be marked as enabled. The ‘always on’ GICR structure must be used to describe the redistributors.

CPUs described as online capable but not enabled can be set to enabled by the DSDT’s Processor object’s _STA method. On virtual systems the _STA method must always report the CPU as present. Changes to the firmware policy can be notified to the OS via device-check or eject-request.

CPUs described as enabled in the static table, should not have their _STA modified dynamically by firmware. Soft-restart features such as kexec will re-read the static properties of the system from these static tables, and may malfunction if these no longer describe the running system. Linux will re-discover the dynamic properties of the system from the _STA method later during boot.