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 <mykyta_poturai@epam.com>
---
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             | 59 ++++++++++++++++++++++++++++++++++
 xen/arch/arm/smpboot.c         |  7 ++++
 3 files changed, 72 insertions(+)

diff --git a/xen/arch/arm/include/asm/irq.h b/xen/arch/arm/include/asm/irq.h
index 09788dbfeb..3ed55e02c3 100644
--- a/xen/arch/arm/include/asm/irq.h
+++ b/xen/arch/arm/include/asm/irq.h
@@ -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:
diff --git a/xen/arch/arm/irq.c b/xen/arch/arm/irq.c
index 7204bc2b68..447bee428e 100644
--- a/xen/arch/arm/irq.c
+++ b/xen/arch/arm/irq.c
@@ -158,6 +158,60 @@ static int init_local_irq_data(unsigned int cpu)
     return 0;
 }
 
+#ifdef CONFIG_CPU_HOTPLUG
+static int cpu_next;
+
+static void balance_irq(int irq, unsigned int from, bool up)
+{
+    struct irq_desc *desc = irq_to_desc(irq);
+    unsigned long flags;
+
+    ASSERT(!cpumask_empty(&cpu_online_map));
+
+    spin_lock_irqsave(&desc->lock, flags);
+    if ( likely(!desc->action) )
+        goto out;
+
+    if ( likely(test_bit(_IRQ_GUEST, &desc->status) ||
+                test_bit(_IRQ_MOVE_PENDING, &desc->status)) )
+        goto out;
+
+    /*
+     * Setting affinity to a mask of multiple CPUs causes the GIC drivers to
+     * select one CPU from that mask. If the dying CPU was included in the IRQ's
+     * affinity mask, we cannot determine exactly which CPU the interrupt is
+     * currently routed to, as GIC drivers lack a concrete get_affinity API. So
+     * to be safe we must reroute it to a new, definitely online, CPU. In the
+     * case of CPU going down, we move only the interrupt that could reside on
+     * it. Otherwise, we rearrange all interrupts in a round-robin fashion.
+     */
+    if ( !up && !cpumask_test_cpu(from, desc->affinity) )
+        goto out;
+
+    cpu_next = cpumask_cycle(cpu_next, &cpu_online_map);
+    irq_set_affinity(desc, cpumask_of(cpu_next));
+
+out:
+    spin_unlock_irqrestore(&desc->lock, flags);
+}
+
+void rebalance_irqs(unsigned int from, bool up)
+{
+    int irq;
+
+    if ( cpumask_empty(&cpu_online_map) )
+        return;
+
+    for ( irq = NR_LOCAL_IRQS; irq < NR_IRQS; irq++ )
+        balance_irq(irq, from, up);
+
+#ifdef CONFIG_GICV3_ESPI
+    for ( irq = ESPI_BASE_INTID; irq < ESPI_MAX_INTID; irq++ )
+        balance_irq(irq, from, up);
+#endif
+}
+#endif /* CONFIG_CPU_HOTPLUG */
+
 static int cpu_callback(struct notifier_block *nfb, unsigned long action,
                         void *hcpu)
 {
@@ -172,6 +226,11 @@ static int cpu_callback(struct notifier_block *nfb, unsigned long action,
             printk(XENLOG_ERR "Unable to allocate local IRQ for CPU%u\n",
                    cpu);
         break;
+    case CPU_ONLINE:
+        if ( IS_ENABLED(CONFIG_CPU_HOTPLUG) &&
+             system_state >= SYS_STATE_active )
+            rebalance_irqs(cpu, true);
+        break;
     }
 
     return notifier_from_errno(rc);
diff --git a/xen/arch/arm/smpboot.c b/xen/arch/arm/smpboot.c
index 7f3cfa812e..7d877179c0 100644
--- a/xen/arch/arm/smpboot.c
+++ b/xen/arch/arm/smpboot.c
@@ -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(). */
 }
-- 
2.51.2