Implement XEN_SYSCTL_CPU_HOTPLUG_{ONLINE,OFFLINE} calls to allow for
enabling/disabling CPU cores in runtime.

Signed-off-by: Mykyta Poturai <mykyta_poturai@epam.com>

v2->v3:
* no changes

v1->v2:
* remove SMT ops
* remove cpu == 0 checks
* add XSM hooks
* only implement for 64bit 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
+
 long arch_do_sysctl(struct xen_sysctl *sysctl,
                     XEN_GUEST_HANDLE_PARAM(xen_sysctl_t) u_sysctl)
 {
@@ -34,6 +72,13 @@ long arch_do_sysctl(struct xen_sysctl *sysctl,
         ret = dt_overlay_sysctl(&sysctl->u.dt_overlay);
         break;
 
+/* CPU Hotplug only implemented for 64-bit Arm */
+#ifdef CONFIG_ARM_64
+    case XEN_SYSCTL_cpu_hotplug:
+        ret = cpu_hotplug_sysctl(&sysctl->u.cpu_hotplug);
+        break;
+#endif
+
     default:
         ret = -ENOSYS;
         break;
-- 
2.34.1

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@epam.com>
> 
> 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