Re: [PATCH v3 3/5] arm/sysctl: Implement cpu hotplug ops

[Julien Grall <julien@xxxxxxx>]

Hi,

On 10/10/2025 10:21, Mykyta Poturai wrote:
> Implement XEN_SYSCTL_CPU_HOTPLUG_{ONLINE,OFFLINE} calls to allow for
> enabling/disabling CPU cores in runtime.
>
> Signed-off-by: Mykyta Poturai <mykyta_poturai@xxxxxxxx>
>
> v2->v3:
> * no changes
>
> v1->v2:
> * remove SMT ops
> * remove cpu == 0 checks
> * add XSM hooks
> * only implement for 64bit Arm
Can you add some details in the commit message explaining why the 
feature is only enabled for 32-bit Arm?
> ---
>   xen/arch/arm/sysctl.c | 45 +++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 45 insertions(+)
>
> diff --git a/xen/arch/arm/sysctl.c b/xen/arch/arm/sysctl.c
> index 32cab4feff..fecd649db1 100644
> --- a/xen/arch/arm/sysctl.c
> +++ b/xen/arch/arm/sysctl.c
> @@ -12,6 +12,8 @@
>   #include <xen/dt-overlay.h>
>   #include <xen/errno.h>
>   #include <xen/hypercall.h>
> +#include <xen/cpu.h>
> +#include <xsm/xsm.h>
>   #include <asm/arm64/sve.h>
>   #include <public/sysctl.h>
>   
> @@ -23,6 +25,42 @@ void arch_do_physinfo(struct xen_sysctl_physinfo *pi)
>                                          XEN_SYSCTL_PHYSCAP_ARM_SVE_MASK);
>   }
>   
> +#ifdef CONFIG_ARM_64
> +static long cpu_up_helper(void *data)
> +{
> +    unsigned long cpu = (unsigned long) data;
> +    return cpu_up(cpu);
> +}
> +
> +static long cpu_down_helper(void *data)
> +{
> +    unsigned long cpu = (unsigned long) data;
> +    return cpu_down(cpu);
> +}
> +
> +static long cpu_hotplug_sysctl(struct xen_sysctl_cpu_hotplug *hotplug)
> +{
> +    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;
> +    }
> +}
> +#endif
The logic seems to be very similar to the x86 code. There are some 
slight differences in cpu_up_helper() and cpu_down_helper() but:
* For cpu_up_helper(), we could create an arch specific helper for the 
second if check.
* For cpu_down_helper(), it would be ok to call cpu_down() a second time 
on Arm.
Can you look at consolidating the code?

Cheers,

--
Julien Grall