For the purposes of certification, we want as little code as possible to
be unconditionally compiled in. Make CPU hotplug and SMT operations
configurable to ease the process. This will also help with introducing
CPU hotplug on Arm, where it needs to be configurable.

Signed-off-by: Mykyta Poturai <mykyta_poturai@epam.com>
---
v6->v7:
* new patch
---
 xen/arch/x86/platform_hypercall.c | 12 ++++++++++++
 xen/arch/x86/smp.c                |  3 +++
 xen/arch/x86/sysctl.c             | 11 +++++++++++
 xen/common/Kconfig                |  8 ++++++++
 4 files changed, 34 insertions(+)

diff --git a/xen/arch/x86/platform_hypercall.c b/xen/arch/x86/platform_hypercall.c
index cd4f0ae5e5..e745151790 100644
--- a/xen/arch/x86/platform_hypercall.c
+++ b/xen/arch/x86/platform_hypercall.c
@@ -735,6 +735,12 @@ ret_t do_platform_op(
     {
         int cpu = op->u.cpu_ol.cpuid;
 
+        if ( !IS_ENABLED(CONFIG_CPU_HOTPLUG) )
+        {
+            ret = -EOPNOTSUPP;
+            break;
+        }
+
         ret = xsm_resource_plug_core(XSM_HOOK);
         if ( ret )
             break;
@@ -761,6 +767,12 @@ ret_t do_platform_op(
     {
         int cpu = op->u.cpu_ol.cpuid;
 
+        if ( !IS_ENABLED(CONFIG_CPU_HOTPLUG) )
+        {
+            ret = -EOPNOTSUPP;
+            break;
+        }
+
         ret = xsm_resource_unplug_core(XSM_HOOK);
         if ( ret )
             break;
diff --git a/xen/arch/x86/smp.c b/xen/arch/x86/smp.c
index 7936294f5f..a49505fb57 100644
--- a/xen/arch/x86/smp.c
+++ b/xen/arch/x86/smp.c
@@ -418,6 +418,7 @@ void cf_check call_function_interrupt(void)
     smp_call_function_interrupt();
 }
 
+#ifdef CONFIG_CPU_HOTPLUG
 long cf_check cpu_up_helper(void *data)
 {
     unsigned int cpu = (unsigned long)data;
@@ -445,8 +446,10 @@ long cf_check cpu_down_helper(void *data)
 {
     int cpu = (unsigned long)data;
     int ret = cpu_down(cpu);
+
     /* Have one more go on EBUSY. */
     if ( ret == -EBUSY )
         ret = cpu_down(cpu);
     return ret;
 }
+#endif
diff --git a/xen/arch/x86/sysctl.c b/xen/arch/x86/sysctl.c
index 1b04947516..bdad44fef1 100644
--- a/xen/arch/x86/sysctl.c
+++ b/xen/arch/x86/sysctl.c
@@ -53,6 +53,11 @@ static long cf_check smt_up_down_helper(void *data)
     unsigned int cpu, sibling_mask = boot_cpu_data.x86_num_siblings - 1;
     int ret = 0;
 
+    if ( !IS_ENABLED(CONFIG_CPU_HOTPLUG) )
+    {
+        ASSERT_UNREACHABLE();
+        return -EOPNOTSUPP;
+    }
     opt_smt = up;
 
     for_each_present_cpu ( cpu )
@@ -121,6 +126,12 @@ long arch_do_sysctl(
         long (*fn)(void *data);
         void *hcpu;
 
+        if ( !IS_ENABLED(CONFIG_CPU_HOTPLUG) )
+        {
+            ret = -EOPNOTSUPP;
+            break;
+        }
+
         switch ( op )
         {
         case XEN_SYSCTL_CPU_HOTPLUG_ONLINE:
diff --git a/xen/common/Kconfig b/xen/common/Kconfig
index d7e79e752a..0e5b4738a8 100644
--- a/xen/common/Kconfig
+++ b/xen/common/Kconfig
@@ -637,6 +637,14 @@ config SYSTEM_SUSPEND
 
 	  If unsure, say N.
 
+config CPU_HOTPLUG
+	bool "CPU online/offline support"
+	depends on X86
+	default y
+	help
+	  Enable support for bringing CPUs online and offline at runtime. On
+	  X86 this is required for disabling SMT.
+
 menu "Supported hypercall interfaces"
 	visible if EXPERT
 
-- 
2.51.2

On 30.03.2026 13:59, Mykyta Poturai wrote:
> For the purposes of certification, we want as little code as possible to
> be unconditionally compiled in. Make CPU hotplug and SMT operations
> configurable to ease the process. This will also help with introducing
> CPU hotplug on Arm, where it needs to be configurable.
> 
> Signed-off-by: Mykyta Poturai <mykyta_poturai@epam.com>

Looks largely okay from a technical pov; one nit and one (repeated) remark
below.

> --- a/xen/arch/x86/sysctl.c
> +++ b/xen/arch/x86/sysctl.c
> @@ -53,6 +53,11 @@ static long cf_check smt_up_down_helper(void *data)
>      unsigned int cpu, sibling_mask = boot_cpu_data.x86_num_siblings - 1;
>      int ret = 0;
>  
> +    if ( !IS_ENABLED(CONFIG_CPU_HOTPLUG) )
> +    {
> +        ASSERT_UNREACHABLE();
> +        return -EOPNOTSUPP;
> +    }
>      opt_smt = up;

Another blank line above this one perhaps?

> --- a/xen/common/Kconfig
> +++ b/xen/common/Kconfig
> @@ -637,6 +637,14 @@ config SYSTEM_SUSPEND
>  
>  	  If unsure, say N.
>  
> +config CPU_HOTPLUG
> +	bool "CPU online/offline support"
> +	depends on X86
> +	default y
> +	help
> +	  Enable support for bringing CPUs online and offline at runtime. On
> +	  X86 this is required for disabling SMT.

The name of this option may need input from others; I'm not quite convinced
that this is a good name, as there's no true "hot-plugging" involved here.
IOW I fear the present name is misleading.

Jan