Implement CPU hotplug on Arm

PATCH Superseded Mykyta Poturai 2 participants 10 messages May 15 RISC-V

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Mykyta Poturai

author

2026-05-15 11:55 UTC #0 lore raw

v8 cover Implement CPU hotplug on Arm

This series implements support for CPU hotplug/unplug on Arm. To achieve this, several things need to be done:

1. XEN_SYSCTL_CPU_HOTPLUG_* calls implemented on Arm64.
2. Enabled building of xen-hptool.
3. Migration of irqs from dying CPUs implemented.

Tested on QEMU and R-Car Gen5 HW.

v7->v8:
* see individual patches

v6->v7:
* new patch "Kconfig: Make cpu hotplug configurable

v5->v6:
* see individual patches

v4->v5:
* drop merged patches
* combine "smp: Move cpu_up/down helpers to common code" with "arm/sysctl: Implement cpu hotplug ops"
* see individual patches

v3->v4:
* add irq migration patches
* see individual patches

v2->v3:
* add docs

v1->v2:
* see individual patches

Mykyta Poturai (6):
arm/irq: Keep track of irq affinities
arm/irq: Migrate IRQs during CPU up/down operations
Kconfig: Make cpu hotplug configurable arm/sysctl: Implement cpu hotplug ops
tools: Allow building xen-hptool without CONFIG_MIGRATE
docs: Document CPU hotplug

 docs/misc/cpu-hotplug.txt         |  97 ++++++++++
 tools/misc/Makefile               |   8 +-
 tools/misc/xen-hptool-x86.c       | 277 ++++++++++++++++++++++++++++
 tools/misc/xen-hptool.c           | 293 ++----------------------------
 tools/misc/xen-hptool.h           |  14 ++
 xen/arch/arm/gic-vgic.c           |   2 +
 xen/arch/arm/include/asm/irq.h    |   6 +
 xen/arch/arm/irq.c                |  69 ++++++-
 xen/arch/arm/smp.c                |   9 +
 xen/arch/arm/smpboot.c            |   7 +
 xen/arch/arm/vgic.c               |  14 +-
 xen/arch/arm/vgic/vgic-mmio-v2.c  |  11 +-
 xen/arch/arm/vgic/vgic.c          |  21 ++-
 xen/arch/ppc/stubs.c              |   4 +
 xen/arch/riscv/stubs.c            |   5 +
 xen/arch/x86/include/asm/smp.h    |   3 -
 xen/arch/x86/platform_hypercall.c |  12 ++
 xen/arch/x86/smp.c                |  35 +---
 xen/arch/x86/sysctl.c             |  25 ++-
 xen/common/Kconfig                |   8 +
 xen/common/smp.c                  |  35 ++++
 xen/common/sysctl.c               |  42 +++++
 xen/include/xen/smp.h             |   4 +
 xen/xsm/flask/hooks.c             |   2 -
 24 files changed, 660 insertions(+), 343 deletions(-)
 create mode 100644 docs/misc/cpu-hotplug.txt
 create mode 100644 tools/misc/xen-hptool-x86.c
 create mode 100644 tools/misc/xen-hptool.h

Signature · 1 line

2.51.2

Mykyta Poturai

author

2026-05-15 11:55 UTC #1 lore raw

PATCH v8 1/6 arm/irq: Keep track of irq affinities

Currently on Arm the desc->affinity mask of an irq is never updated, which makes it hard to know the actual affinity of an interrupt. Fix this by updating the field in irq_set_affinity. Changing desc->affinity requires desc->lock to be held, so add an assertion to ensure that callers of irq_set_affinity are doing so correctly.

With desc->lock now being required for irq_set_affinity, add locking around calls to it where it was missing.

Signed-off-by Mykyta Poturai <[email protected]>

Reviewed-by Bertrand Marquis <[email protected]>

---
v6->v7:
* update commit message
* fix possible locking on null desc
* collect RBs

v5->v6:
* add missing locking around irq_set_affinity calls

v4->v5:
* add locking

v3->v4:
* patch introduced

---
 xen/arch/arm/gic-vgic.c          |  2 ++
 xen/arch/arm/irq.c               |  9 +++++++--
 xen/arch/arm/vgic.c              | 14 ++++++++++++--
 xen/arch/arm/vgic/vgic-mmio-v2.c | 11 +++++------
 xen/arch/arm/vgic/vgic.c         | 21 +++++++++++++--------
 5 files changed, 39 insertions(+), 18 deletions(-)

xen/arch/arm/gic-vgic.c +2 -0

index fae80e6cd2..28ad4903b4 100644
@@ -232,7 +232,9 @@ static void gic_update_one_lr(struct vcpu *v, int i)
             if ( test_bit(GIC_IRQ_GUEST_MIGRATING, &p->status) )
             {
                 struct vcpu *v_target = vgic_get_target_vcpu(v, irq);
+                spin_lock(&p->desc->lock);
                 irq_set_affinity(p->desc, cpumask_of(v_target->processor));
+                spin_unlock(&p->desc->lock);
                 clear_bit(GIC_IRQ_GUEST_MIGRATING, &p->status);
             }
         }

xen/arch/arm/irq.c +7 -2

index 73e58a5108..7204bc2b68 100644
@@ -216,10 +216,15 @@ static inline struct domain *irq_get_domain(struct irq_desc *desc)
     return irq_get_guest_info(desc)->d;
 }
 
+/* Must be called with desc->lock held */
 void irq_set_affinity(struct irq_desc *desc, const cpumask_t *mask)
 {
-    if ( desc != NULL )
-        desc->handler->set_affinity(desc, mask);
+    if ( desc == NULL )
+        return;
+
+    ASSERT(spin_is_locked(&desc->lock));
+    cpumask_copy(desc->affinity, mask);
+    desc->handler->set_affinity(desc, mask);
 }
 
 int request_irq(unsigned int irq, unsigned int irqflags,

xen/arch/arm/vgic.c +12 -2

index 6647071ad4..c59f6873db 100644
@@ -445,7 +445,9 @@ bool vgic_migrate_irq(struct vcpu *old, struct vcpu *new, unsigned int irq)
 
     if ( list_empty(&p->inflight) )
     {
+        spin_lock(&p->desc->lock);
         irq_set_affinity(p->desc, cpumask_of(new->processor));
+        spin_unlock(&p->desc->lock);
         spin_unlock_irqrestore(&old->arch.vgic.lock, flags);
         return true;
     }

@@ -453,7 +455,9 @@ bool vgic_migrate_irq(struct vcpu *old, struct vcpu *new, unsigned int irq)
     if ( !list_empty(&p->lr_queue) )
     {
         vgic_remove_irq_from_queues(old, p);
+        spin_lock(&p->desc->lock);
         irq_set_affinity(p->desc, cpumask_of(new->processor));
+        spin_unlock(&p->desc->lock);
         spin_unlock_irqrestore(&old->arch.vgic.lock, flags);
         vgic_inject_irq(new->domain, new, irq, true);
         return true;

@@ -473,6 +477,7 @@ void arch_move_irqs(struct vcpu *v)
     struct domain *d = v->domain;
     struct pending_irq *p;
     struct vcpu *v_target;
+    unsigned long flags;
     int i;
 
     /*

@@ -494,7 +499,13 @@ void arch_move_irqs(struct vcpu *v)
         p = irq_to_pending(v_target, virq);
 
         if ( v_target == v && !test_bit(GIC_IRQ_GUEST_MIGRATING, &p->status) )
+        {
+            if ( !p->desc )
+                continue;
+            spin_lock_irqsave(&p->desc->lock, flags);
             irq_set_affinity(p->desc, cpu_mask);
+            spin_unlock_irqrestore(&p->desc->lock, flags);
+        }
     }
 }

@@ -574,8 +585,8 @@ void vgic_enable_irqs(struct vcpu *v, uint32_t r, unsigned int n)
         spin_unlock_irqrestore(&v_target->arch.vgic.lock, flags);
         if ( p->desc != NULL )
         {
-            irq_set_affinity(p->desc, cpumask_of(v_target->processor));
             spin_lock_irqsave(&p->desc->lock, flags);
+            irq_set_affinity(p->desc, cpumask_of(v_target->processor));
             /*
              * The irq cannot be a PPI, we only support delivery of SPIs
              * to guests.

@@ -944,4 +955,3 @@ void vgic_check_inflight_irqs_pending(struct vcpu *v, unsigned int rank, uint32_
  * indent-tabs-mode: nil
  * End:
  */
-

xen/arch/arm/vgic/vgic-mmio-v2.c +5 -6

index b7c2d7ce99..fc04741ca1 100644
@@ -159,24 +159,23 @@ static void vgic_mmio_write_target(struct vcpu *vcpu,
     for ( i = 0; i < len; i++ )
     {
         struct vgic_irq *irq = vgic_get_irq(vcpu->domain, NULL, intid + i);
+        struct irq_desc *desc = irq_to_desc(irq->hwintid);
 
-        spin_lock_irqsave(&irq->irq_lock, flags);
+        spin_lock_irqsave(&desc->lock, flags);
+        spin_lock(&irq->irq_lock);
 
         irq->targets = (val >> (i * 8)) & cpu_mask;
         if ( irq->targets )
         {
             irq->target_vcpu = vcpu->domain->vcpu[ffs(irq->targets) - 1];
             if ( irq->hw )
-            {
-                struct irq_desc *desc = irq_to_desc(irq->hwintid);
-
                 irq_set_affinity(desc, cpumask_of(irq->target_vcpu->processor));
-            }
         }
         else
             irq->target_vcpu = NULL;
 
-        spin_unlock_irqrestore(&irq->irq_lock, flags);
+        spin_unlock(&irq->irq_lock);
+        spin_unlock_irqrestore(&desc->lock, flags);
         vgic_put_irq(vcpu->domain, irq);
     }
 }

xen/arch/arm/vgic/vgic.c +13 -8

index b2c0e1873a..1c44236c4f 100644
@@ -812,22 +812,27 @@ void arch_move_irqs(struct vcpu *v)
     {
         struct vgic_irq *irq = vgic_get_irq(d, NULL, i + VGIC_NR_PRIVATE_IRQS);
         unsigned long flags;
+        irq_desc_t *desc;
 
         if ( !irq )
             continue;
 
-        spin_lock_irqsave(&irq->irq_lock, flags);
-
-        /* Only hardware mapped vIRQs that are targeting this vCPU. */
-        if ( irq->hw && irq->target_vcpu == v)
+        if ( !irq->hw )
         {
-            irq_desc_t *desc = irq_to_desc(irq->hwintid);
+            vgic_put_irq(d, irq);
+            continue;
+        }
 
+        desc = irq_to_desc(irq->hwintid);
+        spin_lock_irqsave(&desc->lock, flags);
+        spin_lock(&irq->irq_lock);
+
+        /* Only hardware mapped vIRQs that are targeting this vCPU. */
+        if ( irq->target_vcpu == v )
             irq_set_affinity(desc, cpumask_of(v->processor));
-        }
 
-        spin_unlock_irqrestore(&irq->irq_lock, flags);
-        vgic_put_irq(d, irq);
+        spin_unlock(&irq->irq_lock);
+        spin_unlock_irqrestore(&desc->lock, flags);
     }
 }

Signature · 1 line

2.51.2

Mykyta Poturai

author

2026-05-15 11:55 UTC #2 lore raw

PATCH v8 2/6 arm/irq: Migrate IRQs during CPU up/down operations

In reply to Mykyta Poturai

Move IRQs from dying CPU to the online ones when a CPU is getting offlined. When onlining, rebalance all IRQs in a round-robin fashion. Guest-bound IRQs are already handled by scheduler in the process of moving vCPUs to active pCPUs, so we only need to handle IRQs used by Xen itself.

Signed-off-by Mykyta Poturai <[email protected]>

---
v7->v8:
* check only existings ESPIs

v6->v7:
* replace ifdef with IS_ENABLED

v5->v6:
* don't do any balancing on boot
* only do balancing when cpu hotplug is enabled

v4->v5:
* handle CPU onlining as well
* more comments
* fix crash when ESPI is disabled
* don't assume CPU 0 is a boot CPU
* use insigned int for irq number
* remove assumption that all irqs a bound to CPU 0 by default from the commit message

v3->v4:
* patch introduced

---
 xen/arch/arm/include/asm/irq.h |  6 ++++
 xen/arch/arm/irq.c             | 60 ++++++++++++++++++++++++++++++++++
 xen/arch/arm/smpboot.c         |  7 ++++
 3 files changed, 73 insertions(+)

xen/arch/arm/include/asm/irq.h +6 -0

index 09788dbfeb..3ed55e02c3 100644
@@ -126,6 +126,12 @@ bool irq_type_set_by_domain(const struct domain *d);
 void irq_end_none(struct irq_desc *irq);
 #define irq_end_none irq_end_none
 
+#ifdef CONFIG_CPU_HOTPLUG
+void rebalance_irqs(unsigned int from, bool up);
+#else
+static inline void rebalance_irqs(unsigned int from, bool up) {}
+#endif
+
 #endif /* _ASM_HW_IRQ_H */
 /*
  * Local variables:

xen/arch/arm/irq.c +60 -0

72 lines not on this page · raw message

xen/arch/arm/smpboot.c +7 -0

index 7f3cfa812e..7d877179c0 100644
@@ -425,6 +425,13 @@ void __cpu_disable(void)
 
     smp_mb();
 
+    /*
+     * Now that the interrupts are cleared and the CPU marked as offline,
+     * move interrupts out of it
+     */
+    if ( IS_ENABLED(CONFIG_CPU_HOTPLUG) )
+        rebalance_irqs(cpu, false);
+
     /* Return to caller; eventually the IPI mechanism will unwind and the 
      * scheduler will drop to the idle loop, which will call stop_cpu(). */
 }

Signature · 1 line

2.51.2

Mykyta Poturai

author

2026-05-15 11:55 UTC #3 lore raw

PATCH v8 3/6 Kconfig: Make cpu hotplug configurable

In reply to Mykyta Poturai

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 <[email protected]>

---
v7->v8:
* fix style
* s/CPU_HOTPLUG/CPU_ONLINE_OFFLINE/

v6->v7:
* new patch

---
 xen/arch/x86/platform_hypercall.c | 12 ++++++++++++
 xen/arch/x86/smp.c                |  3 +++
 xen/arch/x86/sysctl.c             | 12 ++++++++++++
 xen/common/Kconfig                |  8 ++++++++
 4 files changed, 35 insertions(+)

xen/arch/x86/platform_hypercall.c +12 -0

index 0431f875af..3f6789c8b7 100644
@@ -735,6 +735,12 @@ ret_t do_platform_op(
     {
         int cpu = op->u.cpu_ol.cpuid;
 
+        if ( !IS_ENABLED(CONFIG_CPU_ONLINE_OFFLINE) )
+        {
+            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_ONLINE_OFFLINE) )
+        {
+            ret = -EOPNOTSUPP;
+            break;
+        }
+
         ret = xsm_resource_unplug_core(XSM_HOOK);
         if ( ret )
             break;

xen/arch/x86/smp.c +3 -0

index 7936294f5f..9046c826f8 100644
@@ -418,6 +418,7 @@ void cf_check call_function_interrupt(void)
     smp_call_function_interrupt();
 }
 
+#ifdef CONFIG_CPU_ONLINE_OFFLINE
 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

xen/arch/x86/sysctl.c +12 -0

index 0fbbdd8b28..1d0857b502 100644
@@ -53,6 +53,12 @@ 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_ONLINE_OFFLINE) )
+    {
+        ASSERT_UNREACHABLE();
+        return -EOPNOTSUPP;
+    }
+
     opt_smt = up;
 
     for_each_present_cpu ( cpu )

@@ -121,6 +127,12 @@ long arch_do_sysctl(
         long (*fn)(void *data);
         void *hcpu;
 
+        if ( !IS_ENABLED(CONFIG_CPU_ONLINE_OFFLINE) )
+        {
+            ret = -EOPNOTSUPP;
+            break;
+        }
+
         switch ( op )
         {
         case XEN_SYSCTL_CPU_HOTPLUG_ONLINE:

xen/common/Kconfig +8 -0

index 0a20aa0a12..9fc53fde48 100644
@@ -636,6 +636,14 @@ config SYSTEM_SUSPEND
 
 	  If unsure, say N.
 
+config CPU_ONLINE_OFFLINE
+	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

Signature · 1 line

2.51.2

Jan Beulich

Acked-by on v8 3/6

2026-05-15 12:21 UTC #4 lore raw

Re: [PATCH v8 3/6] Kconfig: Make cpu hotplug configurable

xen/common/Kconfig config SYSTEM_SUSPEND · line 640

Quoted 8 lines · Mykyta Poturai

xen/common/Kconfig +2 -0

@@ -636,6 +636,14 @@ config SYSTEM_SUSPEND
 
 	  If unsure, say N.
 
+config CPU_ONLINE_OFFLINE
+	bool "CPU online/offline support"

The "if EXPERT" that the next patch adds wants to move here. Then:

Acked-by Jan Beulich <[email protected]>

Jan

Mykyta Poturai

author

2026-05-15 11:55 UTC #5 lore raw

PATCH v8 4/6 arm/sysctl: Implement cpu hotplug ops

In reply to Mykyta Poturai

SMT-disable enforcement check is moved into a separate architecture-specific function.

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 enabled, and partially supported FFA, or TEE is enabled, as they use non-static IRQ actions.

Remove ifdef guards for x86 in flask, as cpu hotplug is now supported on more architectures.

Signed-off-by Mykyta Poturai <[email protected]>

---
v7->v8:
* simplify dependencies of config CPU_ONLINE_OFFLINE

v6->v7:
* use IS_ENABLED istead of ifdef in more places
* remove unneded variables
* more explicit fallthrough in do_sysctl

v5->v6:
* fix style issues
* rename arch_smt_cpu_disable -> arch_cpu_can_stay_online and invert the logic
* use IS_ENABLED istead of ifdef
* remove explicit list af arch-specific SYSCTL_CPU_HOTPLUG_* options from the common handler
* fix flask issue

v4->v5:
* move handling to common code
* rename config to CPU_HOTPUG
* merge with "smp: Move cpu_up/down helpers to common code"

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

Signed-off-by Mykyta Poturai <[email protected]>

---
 xen/arch/arm/smp.c             |  9 ++++++++
 xen/arch/ppc/stubs.c           |  4 ++++
 xen/arch/riscv/stubs.c         |  5 ++++
 xen/arch/x86/include/asm/smp.h |  3 ---
 xen/arch/x86/smp.c             | 34 +++------------------------
 xen/arch/x86/sysctl.c          | 13 ++++-------
 xen/common/Kconfig             |  6 ++---
 xen/common/smp.c               | 35 ++++++++++++++++++++++++++++
 xen/common/sysctl.c            | 42 ++++++++++++++++++++++++++++++++++
 xen/include/xen/smp.h          |  4 ++++
 xen/xsm/flask/hooks.c          |  2 --
 11 files changed, 109 insertions(+), 48 deletions(-)

xen/arch/arm/smp.c +9 -0

index b372472188..0ea64d2ee1 100644
@@ -44,6 +44,15 @@ void smp_send_call_function_mask(const cpumask_t *mask)
     }
 }
 
+/*
+ * We currently don't support SMT on ARM so we don't need any special logic for
+ * CPU disabling
+ */
+inline bool arch_cpu_can_stay_online(unsigned int cpu)
+{
+    return true;
+}
+
 /*
  * Local variables:
  * mode: C

xen/arch/ppc/stubs.c +4 -0

index a333f06119..8f280ba080 100644
@@ -101,6 +101,10 @@ void smp_send_call_function_mask(const cpumask_t *mask)
     BUG_ON("unimplemented");
 }
 
+bool arch_cpu_can_stay_online(unsigned int cpu)
+{
+    BUG_ON("unimplemented");
+}
 /* irq.c */
 
 void irq_ack_none(struct irq_desc *desc)

xen/arch/riscv/stubs.c +5 -0

index acbb5b9123..e051f0fb3f 100644
@@ -65,6 +65,11 @@ void smp_send_call_function_mask(const cpumask_t *mask)
     BUG_ON("unimplemented");
 }
 
+bool arch_cpu_can_stay_online(unsigned int cpu)
+{
+    BUG_ON("unimplemented");
+}
+
 /* irq.c */
 
 void irq_ack_none(struct irq_desc *desc)

xen/arch/x86/include/asm/smp.h +0 -3

index 3f16e62696..cb3e0fed19 100644
@@ -50,9 +50,6 @@ int cpu_add(uint32_t apic_id, uint32_t acpi_id, uint32_t pxm);
 
 void __stop_this_cpu(void);
 
-long cf_check cpu_up_helper(void *data);
-long cf_check cpu_down_helper(void *data);
-
 long cf_check core_parking_helper(void *data);
 bool core_parking_remove(unsigned int cpu);
 uint32_t get_cur_idle_nums(void);

xen/arch/x86/smp.c +3 ~~-31~~

index 9046c826f8..caf4411705 100644
@@ -419,37 +419,9 @@ void cf_check call_function_interrupt(void)
 }
 
 #ifdef CONFIG_CPU_ONLINE_OFFLINE
-long cf_check cpu_up_helper(void *data)
+bool arch_cpu_can_stay_online(unsigned int cpu)
 {
-    unsigned int cpu = (unsigned long)data;
-    int ret = cpu_up(cpu);
-
-    /* Have one more go on EBUSY. */
-    if ( ret == -EBUSY )
-        ret = cpu_up(cpu);
-
-    if ( !ret && !opt_smt &&
-         cpu_data[cpu].compute_unit_id == INVALID_CUID &&
-         cpumask_weight(per_cpu(cpu_sibling_mask, cpu)) > 1 )
-    {
-        ret = cpu_down_helper(data);
-        if ( ret )
-            printk("Could not re-offline CPU%u (%d)\n", cpu, ret);
-        else
-            ret = -EPERM;
-    }
-
-    return ret;
-}
-
-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;
+    return opt_smt || cpu_data[cpu].compute_unit_id != INVALID_CUID ||
+           cpumask_weight(per_cpu(cpu_sibling_mask, cpu)) <= 1;
 }
 #endif

xen/arch/x86/sysctl.c +4 -9

index 1d0857b502..7a88ec7d7b 100644
@@ -121,7 +121,6 @@ long arch_do_sysctl(
 
     case XEN_SYSCTL_cpu_hotplug:
     {
-        unsigned int cpu = sysctl->u.cpu_hotplug.cpu;
         unsigned int op  = sysctl->u.cpu_hotplug.op;
         bool plug;
         long (*fn)(void *data);

@@ -129,6 +128,7 @@ long arch_do_sysctl(
 
         if ( !IS_ENABLED(CONFIG_CPU_ONLINE_OFFLINE) )
         {
+            ASSERT_UNREACHABLE();
             ret = -EOPNOTSUPP;
             break;
         }

@@ -136,15 +136,10 @@ long arch_do_sysctl(
         switch ( op )
         {
         case XEN_SYSCTL_CPU_HOTPLUG_ONLINE:
-            plug = true;
-            fn = cpu_up_helper;
-            hcpu = _p(cpu);
-            break;
-
         case XEN_SYSCTL_CPU_HOTPLUG_OFFLINE:
-            plug = false;
-            fn = cpu_down_helper;
-            hcpu = _p(cpu);
+            /* Handled by common code */
+            ASSERT_UNREACHABLE();
+            ret = -EOPNOTSUPP;
             break;
 
         case XEN_SYSCTL_CPU_HOTPLUG_SMT_ENABLE:

xen/common/Kconfig +3 -3

index 9fc53fde48..d0b8aa8d1c 100644
@@ -637,9 +637,9 @@ config SYSTEM_SUSPEND
 	  If unsure, say N.
 
 config CPU_ONLINE_OFFLINE
-	bool "CPU online/offline support"
-	depends on X86
-	default y
+	bool "CPU online/offline support" if EXPERT
+	depends on X86 || (ARM_64 && !HAS_ITS)
+	default X86
 	help
 	  Enable support for bringing CPUs online and offline at runtime. On
 	  X86 this is required for disabling SMT.

xen/common/smp.c +35 -0

index a011f541f1..16f4ed2d4e 100644
@@ -16,6 +16,7 @@
  * GNU General Public License for more details.
  */
 
+#include <xen/cpu.h>
 #include <asm/hardirq.h>
 #include <asm/processor.h>
 #include <xen/spinlock.h>

@@ -104,6 +105,40 @@ void smp_call_function_interrupt(void)
     irq_exit();
 }
 
+#ifdef CONFIG_CPU_ONLINE_OFFLINE
+long cf_check cpu_up_helper(void *data)
+{
+    unsigned int cpu = (unsigned long)data;
+    int ret = cpu_up(cpu);
+
+    /* Have one more go on EBUSY. */
+    if ( ret == -EBUSY )
+        ret = cpu_up(cpu);
+
+    if ( !ret && !arch_cpu_can_stay_online(cpu) )
+    {
+        ret = cpu_down_helper(data);
+        if ( ret )
+            printk("Could not re-offline CPU%u (%d)\n", cpu, ret);
+        else
+            ret = -EPERM;
+    }
+
+    return ret;
+}
+
+long cf_check cpu_down_helper(void *data)
+{
+    unsigned 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 /* CONFIG_CPU_ONLINE_OFFLINE */
+
 /*
  * Local variables:
  * mode: C

xen/common/sysctl.c +42 -0

48 lines not on this page · raw message

xen/include/xen/smp.h +4 -0

index 2ca9ff1bfc..04530738c9 100644
@@ -76,4 +76,8 @@ extern void *stack_base[NR_CPUS];
 void initialize_cpu_data(unsigned int cpu);
 int setup_cpu_root_pgt(unsigned int cpu);
 
+bool arch_cpu_can_stay_online(unsigned int cpu);
+long cf_check cpu_up_helper(void *data);
+long cf_check cpu_down_helper(void *data);
+
 #endif /* __XEN_SMP_H__ */

xen/xsm/flask/hooks.c +0 -2

index 28522dcbd2..4ee68231ce 100644
@@ -835,9 +835,7 @@ static int cf_check flask_sysctl(int cmd)
     case XEN_SYSCTL_getdomaininfolist:
     case XEN_SYSCTL_page_offline_op:
     case XEN_SYSCTL_scheduler_op:
-#ifdef CONFIG_X86
     case XEN_SYSCTL_cpu_hotplug:
-#endif
         return 0;
 
     case XEN_SYSCTL_tbuf_op:

Signature · 1 line

2.51.2

Jan Beulich

2026-05-15 12:18 UTC #6 lore raw

Re: [PATCH v8 4/6] arm/sysctl: Implement cpu hotplug ops

Quoted 9 lines · Mykyta Poturai

SMT-disable enforcement check is moved into a separate architecture-specific function.
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 enabled, and partially supported FFA, or TEE is enabled, as they use non-static IRQ actions.

Quoted 8 lines

Remove ifdef guards for x86 in flask, as cpu hotplug is now supported on more architectures.
Signed-off-by Mykyta Poturai <[email protected]>
---
v7->v8:
* simplify dependencies of config CPU_ONLINE_OFFLINE

Which now raises the question of ...

xen/common/Kconfig config SYSTEM_SUSPEND · line 641

Quoted 11 lines

xen/common/Kconfig +2 -3

@@ -637,9 +637,9 @@ config SYSTEM_SUSPEND
 	  If unsure, say N.
 
 config CPU_ONLINE_OFFLINE
-	bool "CPU online/offline support"
-	depends on X86
-	default y
+	bool "CPU online/offline support" if EXPERT
+	depends on X86 || (ARM_64 && !HAS_ITS)

... this being overly strict towards other architectures. Typically we'd allow them to express such via select-ing a respective HAS_* control. In the absence of that, couldn't the above be further simplified to

depends on !HAS_ITS

Alternatively, to make exclusions more explicit, and to not preclude other ports to use HAS_ITS for something entirely different (just with the same acronym), maybe

	depends on !ARM || !HAS_ITS

? Then, if RISC-V and/or PPC really want to opt out of supporting this, they'd add themselves here. Yes as said, expressing such via HAS_* scales better.

xen/common/Kconfig config SYSTEM_SUSPEND · line 642

Quoted 1 line

xen/common/Kconfig +1 -0
+	default X86

This, I agree, wants to stay as is. At least for now.

Jan

Mykyta Poturai

author

2026-05-15 11:55 UTC #7 lore raw

PATCH v8 5/6 tools: Allow building xen-hptool without CONFIG_MIGRATE

In reply to Mykyta Poturai

With CPU hotplug sysctls implemented on Arm it becomes useful to have a tool for calling them.

According to the commit history it seems that putting hptool under config MIGRATE was a measure to fix IA64 build. As IA64 is no longer supported it can now be brought back. So build it unconditionally.

Operations specific to x86 architecture are moved into a separate file and only built on x86.

Signed-off-by Mykyta Poturai <[email protected]>

---
v7->v8:
* move x86 specific function into a separate file

v6->v7:
* no changes

v5->v6:
* don't change order in Makefile

v4->v5:
* make hptool always build

v3->v4:
* no changes

v2->v3:
* no changes

v1->v2:
* switch to configure from legacy config

---
 tools/misc/Makefile         |   8 +-
 tools/misc/xen-hptool-x86.c | 277 ++++++++++++++++++++++++++++++++++
 tools/misc/xen-hptool.c     | 293 ++----------------------------------
 tools/misc/xen-hptool.h     |  14 ++
 4 files changed, 311 insertions(+), 281 deletions(-)
 create mode 100644 tools/misc/xen-hptool-x86.c
 create mode 100644 tools/misc/xen-hptool.h

tools/misc/Makefile +5 -3

index 6ee783f43e..645819e455 100644
@@ -16,7 +16,7 @@ INSTALL_BIN                    += xencov_split
 INSTALL_BIN += $(INSTALL_BIN-y)
 
 # Everything to be installed in regular sbin/
-INSTALL_SBIN-$(CONFIG_MIGRATE) += xen-hptool
+INSTALL_SBIN                   += xen-hptool
 INSTALL_SBIN-$(CONFIG_X86)     += xen-hvmcrash
 INSTALL_SBIN-$(CONFIG_X86)     += xen-hvmctx
 INSTALL_SBIN-$(CONFIG_X86)     += xen-lowmemd

@@ -104,8 +104,10 @@ xenhypfs: xenhypfs.o
 xenlockprof: xenlockprof.o
 	$(CC) $(LDFLAGS) -o $@ $< $(LDLIBS_libxenctrl) $(APPEND_LDFLAGS)
 
-xen-hptool: xen-hptool.o
-	$(CC) $(LDFLAGS) -o $@ $< $(LDLIBS_libxenevtchn) $(LDLIBS_libxenctrl) $(LDLIBS_libxenguest) $(LDLIBS_libxenstore) $(APPEND_LDFLAGS)
+HPTOOL_OBJS-$(CONFIG_MIGRATE) += xen-hptool-x86.o
+
+xen-hptool: xen-hptool.o $(HPTOOL_OBJS-y)
+	$(CC) $(LDFLAGS) -o $@ $^ $(LDLIBS_libxenevtchn) $(LDLIBS_libxenctrl) $(LDLIBS_libxenguest) $(LDLIBS_libxenstore) $(APPEND_LDFLAGS)
 
 xenhypfs.o: CFLAGS += $(CFLAGS_libxenhypfs)

tools/misc/xen-hptool-x86.c new file +277 -0

277 lines not on this page · raw message

tools/misc/xen-hptool.c +15 ~~-278~~

373 lines not on this page · raw message

tools/misc/xen-hptool.h new file +14 -0

new file mode 100644
index 0000000000..2efa7c8177
@@ -0,0 +1,14 @@
+#ifndef __XEN_HPTOOL_H__
+#define __XEN_HPTOOL_H__
+
+#if defined(__i386__) || defined(__x86_64__)
+int hp_mem_online_func(int argc, char *argv[], xc_interface *xch);
+int hp_mem_query_func(int argc, char *argv[], xc_interface *xch);
+int hp_mem_offline_func(int argc, char *argv[], xc_interface *xch);
+int main_smt_enable(int argc, char *argv[], xc_interface *xch);
+int main_smt_disable(int argc, char *argv[], xc_interface *xch);
+#endif
+
+void show_help(void);
+
+#endif /* __XEN_HPTOOL_H__ */

Signature · 1 line

2.51.2

Jan Beulich

2026-05-15 12:29 UTC #8 lore raw

Re: [PATCH v8 5/6] tools: Allow building xen-hptool without CONFIG_MIGRATE

tools/misc/Makefile xenhypfs: xenhypfs.o · line 109

Quoted 9 lines · Mykyta Poturai

tools/misc/Makefile +3 -2

@@ -104,8 +104,10 @@ xenhypfs: xenhypfs.o
 xenlockprof: xenlockprof.o
 	$(CC) $(LDFLAGS) -o $@ $< $(LDLIBS_libxenctrl) $(APPEND_LDFLAGS)
 
-xen-hptool: xen-hptool.o
-	$(CC) $(LDFLAGS) -o $@ $< $(LDLIBS_libxenevtchn) $(LDLIBS_libxenctrl) $(LDLIBS_libxenguest) $(LDLIBS_libxenstore) $(APPEND_LDFLAGS)
+HPTOOL_OBJS-$(CONFIG_MIGRATE) += xen-hptool-x86.o
+
+xen-hptool: xen-hptool.o $(HPTOOL_OBJS-y)

I'm not a maintainer of this, but imo this wants to be

HPTOOL-OBJS-y := xen-hptool.o
HPTOOL-OBJS-$(CONFIG_MIGRATE) += xen-hptool-x86.o

xen-hptool: $(HPTOOL-OBJS-y)

Whether $(CONFIG_MIGRATE) is actually appropriate to use here (and not CONFIG_X86) is a separate question. What's moved into that new file is not only migration related stuff, after all.

Whether an almost-all-caps variable name is okay to use here is yet another (style) question. In the hypervisor we prefer lowercase forms, I think.

Jan

Mykyta Poturai

author

2026-05-15 11:55 UTC #9 lore raw

PATCH v8 6/6 docs: Document CPU hotplug

In reply to Mykyta Poturai

Signed-off-by Mykyta Poturai <[email protected]>

---
v7->v8:
* remove support status update
* update config option name

v6->v7:
* add testing and limitations

v5->v6:
* no changes

v4->v5:
* s/supported/implemented/
* update SUPPORT.md

v3->v4:
* update configuration section

v2->v3:
* patch introduced

---
 docs/misc/cpu-hotplug.txt | 97 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 97 insertions(+)
 create mode 100644 docs/misc/cpu-hotplug.txt

docs/misc/cpu-hotplug.txt new file +97 -0

new file mode 100644
index 0000000000..d2ed1d6b8b
@@ -0,0 +1,97 @@
+CPU Hotplug
+===========
+
+CPU hotplug is a feature that allows pCPU cores to be added to or removed from a
+running system without requiring a reboot. It is implemented on x86 and Arm64
+architectures.
+
+Implementation Details
+----------------------
+
+CPU hotplug is implemented through the `XEN_SYSCTL_CPU_HOTPLUG_*` sysctl calls.
+The specific calls are:
+
+- `XEN_SYSCTL_CPU_HOTPLUG_ONLINE`: Brings a pCPU online
+- `XEN_SYSCTL_CPU_HOTPLUG_OFFLINE`: Takes a pCPU offline
+- `XEN_SYSCTL_CPU_HOTPLUG_SMT_ENABLE`: Enables SMT threads (x86 only)
+- `XEN_SYSCTL_CPU_HOTPLUG_SMT_DISABLE`: Disables SMT threads (x86 only)
+
+All cores can be disabled, assuming hardware support, except for the boot core.
+Sysctl calls are routed to the boot core before doing any actual up/down
+operations on other cores.
+
+If there are Xen-bound interrupts pinned to the pCPU being offlined, they will
+be automatically migrated to other online pCPUs. Interrupts used by guest
+domains are handled by the scheduler when it reschedules the vCPUs to a new,
+online, pCPU. When a pCPU is being onlined, some Xen-bound interrupts will get
+redistributed to the newly onlined pCPU to prevent imbalance.
+
+If pCPU being offlined has some vCPUs pinned to it, they will be automatically
+unpinned and migrated to other online pCPUs.
+
+Limitations
+-----------
+
+On Arm64 cpu hotplug is currently not compatible with ITS, due to an issues with
+the redistributor assignment.
+
+On Arm64 there can be problems with FFA if secure FW support notification ABI.
+
+Configuration
+-------------
+
+The presence of the feature is controlled by CONFIG_CPU_ONLINE_OFFLINE option.
+It is enabled by default on x86 architecture. On Arm64, the option is disabled
+by default and marked as EXPERT. xen-hptool userspace tool is built
+unconditionally.
+
+Usage
+-----
+
+Disable core:
+
+$ xen-hptool cpu-offline 2
+Prepare to offline CPU 2
+(XEN) Removing cpu 2 from runqueue 0
+CPU 2 offlined successfully
+
+Enable core:
+
+$ xen-hptool cpu-online 2
+Prepare to online CPU 2
+(XEN) Bringing up CPU2
+(XEN) GICv3: CPU2: Found redistributor in region 0 @00000a004005c000
+(XEN) CPU2: Guest atomics will try 1 times before pausing the domain
+(XEN) CPU 2 booted.
+(XEN) Adding cpu 2 to runqueue 0
+CPU 2 onlined successfully
+
+Disabling a core with pinned vCPUs:
+
+$ xl vcpu-pin 0 3 3 3
+$ xl vcpu-pin 0 2 3 3
+$ xl vcpu-pin 0 1 3 3
+$ xl vcpu-pin 0 0 3 3
+$ xen-hptool cpu-offline 3
+Prepare to offline CPU 3
+(XEN) Breaking affinity for d0v0
+(XEN) Breaking affinity for d0v1
+(XEN) Breaking affinity for d0v2
+(XEN) Breaking affinity for d0v3
+(XEN) Removing cpu 3 from runqueue 0
+CPU 3 offlined successfully
+
+Testing
+-------
+
+The CPU hotplug feature has been tested on both x86 and Arm64 QEMU setups and on
+R-Car Gen5 (Arm64) hardware.
+
+The tests included:
+- Offlining and onlining cores with no pinned vCPUs
+- Offlining cores with pinned vCPUs
+- Offlining cores with Xen-bound interrupts
+- Offlining all cores except the boot core
+- Offlining the boot core (expected to fail)
+- Enabling and disabling SMT threads (x86 only)
+- Ofllining cores to which guests with passthrough devices are pinned

Signature · 1 line

2.51.2