[v4] Implement CPU hotplug on Arm

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

Posted by Mykyta Poturai 6 days, 15 hours ago

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@epam.com>

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)
+{
+#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;
-- 
2.51.2

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

Posted by Julien Grall 2 days, 14 hours ago

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@epam.com>
> 
> 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

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

Posted by Grygorii Strashko 5 days, 10 hours ago


On 12.11.25 12:51, 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@epam.com>
> 
> 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
> +

Having it in arch code I think right as Arch can have own deps
(Linux does it this way), but name has to be fixed and documented (docs).

may be more common name HOTPLUG_CPU (like in linux)?
naming is up to you.

>   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)
> +{
> +#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;

Common code need to be placed under same config guards.

-- 
Best regards,
-grygorii

[PATCH v4 1/8] arm/time: Use static irqaction
[PATCH v4 2/8] arm/gic: Use static irqaction
[PATCH v4 3/8] arm/irq: Keep track of irq affinities
[PATCH v4 4/8] arm/irq: Migrate IRQs from dyings CPUs
[PATCH v4 5/8] smp: Move cpu_up/down helpers to common code
[PATCH v4 6/8] arm/sysctl: Implement cpu hotplug ops
[PATCH v4 7/8] tools: Allow building xen-hptool without CONFIG_MIGRATE
[PATCH v4 8/8] docs: Document CPU hotplug