Re: [PATCH v4 8/8] docs: Document CPU hotplug

[Julien Grall <julien@xxxxxxx>]

Hi,

On 12/11/2025 10:51, Mykyta Poturai wrote:
> Signed-off-by: Mykyta Poturai <mykyta_poturai@xxxxxxxx>
>
> v3->v4:
> * update configuration section
>
> v2->v3:
> * patch introduced
> ---
>   docs/misc/cpu-hotplug.txt | 51 +++++++++++++++++++++++++++++++++++++++
>   1 file changed, 51 insertions(+)
>   create mode 100644 docs/misc/cpu-hotplug.txt
>
> diff --git a/docs/misc/cpu-hotplug.txt b/docs/misc/cpu-hotplug.txt
> new file mode 100644
> index 0000000000..1fbad0ecf7
> --- /dev/null
> +++ b/docs/misc/cpu-hotplug.txt
> @@ -0,0 +1,51 @@
> +CPU Hotplug
> +===========
> +
> +CPU hotplug is a feature that allows pCPU cores to be added to or removed from 
> a
> +running system without requiring a reboot. It is supported on x86 and Arm64
> +architectures.
"supported" usually implies that we will backport fixes and issues XSA. 
Is this what you intended to say?
Asking because SUPPORT.md has the following:

### ACPI CPU Hotplug

    Status, x86: Experimental

There is also no mention for Arm in SUPPORT.md. As you enable the 
feature, we should clarify what's its state.
        > +
> +Implementation Details
> +----------------------
> +
> +CPU hotplug is implemented through the `XEN_SYSCTL_CPU_HOTPLUG_*` sysctl calls.
> +The specific calls are:
> +
> +- `XEN_SYSCTL_CPU_HOTPLUG_ONLINE`: Brings a pCPU online
> +- `XEN_SYSCTL_CPU_HOTPLUG_OFFLINE`: Takes a pCPU offline
> +- `XEN_SYSCTL_CPU_HOTPLUG_SMT_ENABLE`: Enables SMT threads (x86 only)
> +- `XEN_SYSCTL_CPU_HOTPLUG_SMT_DISABLE`: Disables SMT threads (x86 only)
> +
> +All cores can be disabled, assuming hardware support, except for core 0. Sysctl
> +calls are routed to core 0 before doing any actual up/down operations on other
> +cores.
> +
> +Configuration
> +-------------
> +
> +Sysctl handlers are enabled unconditionally on x86 architecture. On Arm64,
> +handlers are enabled by default when ITS, FFA, and TEE configs are disabled.
> +Building of the userspace tool "hptool" is controlled by the "hptool" flag in
> +the configure script. It is enabled by default and can be disabled with
> +--disable-hptool command line option.
> +
> +Usage
> +-----
> +
> +Disable core:
> +
> +$ xen-hptool cpu-offline 2
> +Prepare to offline CPU 2
> +(XEN) Removing cpu 2 from runqueue 0
> +CPU 2 offlined successfully
> +
> +Enable core:
> +
> +$ xen-hptool cpu-online 2
> +Prepare to online CPU 2
> +(XEN) Bringing up CPU2
> +(XEN) GICv3: CPU2: Found redistributor in region 0 @00000a004005c000
> +(XEN) CPU2: Guest atomics will try 1 times before pausing the domain
> +(XEN) CPU 2 booted.
> +(XEN) Adding cpu 2 to runqueue 0
> +CPU 2 onlined successfully
Cheers,

--
Julien Grall