Re: [PATCH v4 2/2] cpu/hotplug: Fix NULL kobject warning in cpuhp_smt_enable()

[Catalin Marinas <catalin.marinas@arm.com>]

On Wed, Jun 10, 2026 at 03:52:02PM +0800, Jinjie Ruan wrote:
> 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.

The doc update looks fine as well, thanks for clarifying. My reviewed-by
stands.

-- 
Catalin