[PATCH v3] cpu/hotplug: Fix NULL kobject warning in cpuhp_smt_enable()

[Will Deacon]

On Wed, May 20, 2026 at 10:20:23AM +0800, Jinjie Ruan wrote:
> On arm64, when booting with `maxcpus` greater than the number of present
> CPUs (e.g., QEMU -smp cpus=4,maxcpus=8), some CPUs are marked as 'present'
> but have not yet been registered via register_cpu(). Consequently,
> the per-cpu device objects for these CPUs are not yet initialized.
> 
> In cpuhp_smt_enable(), the code iterates over all present CPUs. Calling
> _cpu_up() for these unregistered CPUs eventually leads to
> sysfs_create_group() being called with a NULL kobject (or a kobject
> without a directory), triggering the following warning in
> fs/sysfs/group.c:
> 
> 	if (WARN_ON(!kobj || (!update && !kobj->sd)))
> 		return -EINVAL;
> 
> When booting with ACPI, arm64 smp_prepare_cpus() currently sets all
> enumerated CPUs as "present" regardless of their status in the MADT. This
> causes issues with SMT hotplug control. For instance, with QEMU's
> "-smp 4,maxcpus=8" configuration, the MADT GICC entries are populated as
> follows: the first four CPUs are marked Enabled while the remaining four
> are marked Online Capable to support potential hot-plugging.
> 
> Fix this by:
> 
> 1. When booting with ACPI, checking the ACPI_MADT_ENABLED flag in the GICC
>    entry before calling set_cpu_present() during SMP initialization.
> 
> 2. Properly managing the present mask in acpi_map_cpu() and
>    acpi_unmap_cpu() to support actual CPU hotplug events, This aligns with
>    other architectures like x86 and LoongArch.
> 
> 3. Update the arm64 CPU hotplug documentation to no longer state that all
>    online-capable vCPUs are marked as present by the kernel at boot time.
> 
> This ensures that only physically available or explicitly enabled CPUs
> are in the present mask, keeping the SMT control logic consistent with
> the actual hardware state.

Please can you check the Sashiko review comment?

https://sashiko.dev/#/patchset/20260520022023.126670-1-ruanjinjie@huawei.com

Cheers,

Will