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Diffstat (limited to 'Documentation/arch/arm64/cpu-hotplug.rst')
| -rw-r--r-- | Documentation/arch/arm64/cpu-hotplug.rst | 28 |
1 files changed, 16 insertions, 12 deletions
diff --git a/Documentation/arch/arm64/cpu-hotplug.rst b/Documentation/arch/arm64/cpu-hotplug.rst index 8fb438bf7781..7c3379b704aa 100644 --- a/Documentation/arch/arm64/cpu-hotplug.rst +++ b/Documentation/arch/arm64/cpu-hotplug.rst @@ -47,11 +47,12 @@ 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. +CPU Hotplug as all vCPU resources are statically described in the firmware +configuration tables (e.g. MADT), meaning their maximum possible count is +known at boot. However, vCPUs that are not enabled at boot are not marked +as ``present`` by the kernel until they are hotplugged. 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. @@ -60,16 +61,19 @@ 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. +that are hot-pluggable must have the ``online capable`` bit set and the +``enabled`` bit cleared in the MADT GICC structures to indicate they can be +enabled later. The boot CPU must be marked as ``enabled`` with its +``online capable`` bit cleared. 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. +must always set the ``ACPI_STA_DEVICE_PRESENT`` bit, while toggling the +``ACPI_STA_DEVICE_ENABLED`` bit to reflect its plug status. The kernel will +then dynamically mark the vCPU as ``present`` within the OS when the +``ACPI_STA_DEVICE_ENABLED`` bit becomes set during hot-add. 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 |