Re: [PATCH v4 6/8] arm/sysctl: Implement cpu hotplug ops

[Julien Grall <julien@xxxxxxx>]

Hi,

On 12/11/2025 10:51, Mykyta Poturai wrote:
> Implement XEN_SYSCTL_CPU_HOTPLUG_{ONLINE,OFFLINE} calls to allow for
> enabling/disabling CPU cores in runtime.
>
> For now this operations only support Arm64. For proper Arm32 support,
> there needs to be a mechanism to free per-cpu page tables, allocated in
> init_domheap_mappings.
> Also, hotplug is not supported if ITS, FFA, or TEE is enabled, as they
> use non-static IRQ actions.
>
> Create a Kconfig option RUNTIME_CPU_CONTROL that reflects this
> constraints.
>
> Signed-off-by: Mykyta Poturai <mykyta_poturai@xxxxxxxx>
>
> v3->v4:
> * don't reimplement cpu_up/down helpers
> * add Kconfig option
> * fixup formatting
>
> v2->v3:
> * no changes
>
> v1->v2:
> * remove SMT ops
> * remove cpu == 0 checks
> * add XSM hooks
> * only implement for 64bit Arm
> ---
>   xen/arch/arm/Kconfig  |  4 ++++
>   xen/arch/arm/sysctl.c | 32 ++++++++++++++++++++++++++++++++
>   2 files changed, 36 insertions(+)
>
> diff --git a/xen/arch/arm/Kconfig b/xen/arch/arm/Kconfig
> index cf6af68299..931ae51575 100644
> --- a/xen/arch/arm/Kconfig
> +++ b/xen/arch/arm/Kconfig
> @@ -274,6 +274,10 @@ config PCI_PASSTHROUGH
>         help
>           This option enables PCI device passthrough
>   
> +config RUNTIME_CPU_CONTROL
> +    def_bool y
> +    depends on ARM_64 && !TEE && !FFA && !HAS_ITS
> +
>   endmenu
>   
>   menu "ARM errata workaround via the alternative framework"
> diff --git a/xen/arch/arm/sysctl.c b/xen/arch/arm/sysctl.c
> index 32cab4feff..3c4e29d82c 100644
> --- a/xen/arch/arm/sysctl.c
> +++ b/xen/arch/arm/sysctl.c
> @@ -12,6 +12,7 @@
>   #include <xen/dt-overlay.h>
>   #include <xen/errno.h>
>   #include <xen/hypercall.h>
> +#include <xsm/xsm.h>
>   #include <asm/arm64/sve.h>
>   #include <public/sysctl.h>
>   
> @@ -23,6 +24,33 @@ void arch_do_physinfo(struct xen_sysctl_physinfo *pi)
>                                          XEN_SYSCTL_PHYSCAP_ARM_SVE_MASK);
>   }
>   
> +static long cpu_hotplug_sysctl(struct xen_sysctl_cpu_hotplug *hotplug)
As you moved the helper in common code. I was expecting the logic to 
handle CPU_HOTPLUG_ONLINE and CPU_HOTPLUG_OFFLINE to also move in 
common. Can you explain why this wasn't done?
> +{
> +#ifdef CONFIG_RUNTIME_CPU_CONTROL
> +    int ret;
> +
> +    switch ( hotplug->op )
> +    {
> +    case XEN_SYSCTL_CPU_HOTPLUG_ONLINE:
> +        ret = xsm_resource_plug_core(XSM_HOOK);
> +        if ( ret )
> +            return ret;
> +        return continue_hypercall_on_cpu(0, cpu_up_helper, _p(hotplug->cpu));
> +
> +    case XEN_SYSCTL_CPU_HOTPLUG_OFFLINE:
> +        ret = xsm_resource_unplug_core(XSM_HOOK);
> +        if ( ret )
> +            return ret;
> +        return continue_hypercall_on_cpu(0, cpu_down_helper, _p(hotplug->cpu));
> +
> +    default:
> +        return -EOPNOTSUPP;
> +    }
> +#else
> +    return -EOPNOTSUPP;
> +#endif
> +}
> +
>   long arch_do_sysctl(struct xen_sysctl *sysctl,
>                       XEN_GUEST_HANDLE_PARAM(xen_sysctl_t) u_sysctl)
>   {
> @@ -34,6 +62,10 @@ long arch_do_sysctl(struct xen_sysctl *sysctl,
>           ret = dt_overlay_sysctl(&sysctl->u.dt_overlay);
>           break;
>   
> +    case XEN_SYSCTL_cpu_hotplug:
> +        ret = cpu_hotplug_sysctl(&sysctl->u.cpu_hotplug);
> +        break;
> +
>       default:
>           ret = -ENOSYS;
>           break;
Cheers,

--
Julien Grall