arm/xen: Convert to hotplug state machine

Install the callbacks via the state machine and let the core invoke
the callbacks on the already online CPUs.

The get_cpu() in xen_starting_cpu() boils down to preempt_disable() since
we already know the CPU we run on. Disabling preemption shouldn't be required
here from what I see since it we don't switch CPUs while invoking the function.

Signed-off-by: Richard Cochran <rcochran@linutronix.de>
Signed-off-by: Anna-Maria Gleixner <anna-maria@linutronix.de>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-arm-kernel@lists.infradead.org
Cc: rt@linutronix.de
Cc: xen-devel@lists.xenproject.org
Link: http://lkml.kernel.org/r/20160713153336.971559670@linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/arm/xen/enlighten.c

@@ -161,12 +161,11 @@ static struct notifier_block xen_pvclock_gtod_notifier = {
 	.notifier_call = xen_pvclock_gtod_notify,
 };
 
-static void xen_percpu_init(void)
+static int xen_starting_cpu(unsigned int cpu)
 {
 	struct vcpu_register_vcpu_info info;
 	struct vcpu_info *vcpup;
 	int err;
-	int cpu = get_cpu();
 
 	/* 
 	 * VCPUOP_register_vcpu_info cannot be called twice for the same
@@ -190,7 +189,13 @@ static void xen_percpu_init(void)
 
 after_register_vcpu_info:
 	enable_percpu_irq(xen_events_irq, 0);
-	put_cpu();
+	return 0;
+}
+
+static int xen_dying_cpu(unsigned int cpu)
+{
+	disable_percpu_irq(xen_events_irq);
+	return 0;
 }
 
 static void xen_restart(enum reboot_mode reboot_mode, const char *cmd)
@@ -209,28 +214,6 @@ static void xen_power_off(void)
 	BUG_ON(rc);
 }
 
-static int xen_cpu_notification(struct notifier_block *self,
-				unsigned long action,
-				void *hcpu)
-{
-	switch (action) {
-	case CPU_STARTING:
-		xen_percpu_init();
-		break;
-	case CPU_DYING:
-		disable_percpu_irq(xen_events_irq);
-		break;
-	default:
-		break;
-	}
-
-	return NOTIFY_OK;
-}
-
-static struct notifier_block xen_cpu_notifier = {
-	.notifier_call = xen_cpu_notification,
-};
-
 static irqreturn_t xen_arm_callback(int irq, void *arg)
 {
 	xen_hvm_evtchn_do_upcall();
@@ -351,16 +334,14 @@ static int __init xen_guest_init(void)
 		return -EINVAL;
 	}
 
-	xen_percpu_init();
-
-	register_cpu_notifier(&xen_cpu_notifier);
-
 	pv_time_ops.steal_clock = xen_stolen_accounting;
 	static_key_slow_inc(&paravirt_steal_enabled);
 	if (xen_initial_domain())
 		pvclock_gtod_register_notifier(&xen_pvclock_gtod_notifier);
 
-	return 0;
+	return cpuhp_setup_state(CPUHP_AP_ARM_XEN_STARTING,
+				 "AP_ARM_XEN_STARTING", xen_starting_cpu,
+				 xen_dying_cpu);
 }
 early_initcall(xen_guest_init);
 

include/linux/cpuhotplug.h

@@ -47,6 +47,7 @@ enum cpuhp_state {
 	CPUHP_AP_KVM_STARTING,
 	CPUHP_AP_KVM_ARM_VGIC_STARTING,
 	CPUHP_AP_KVM_ARM_TIMER_STARTING,
+	CPUHP_AP_ARM_XEN_STARTING,
 	CPUHP_AP_LEDTRIG_STARTING,
 	CPUHP_AP_NOTIFY_STARTING,
 	CPUHP_AP_ONLINE,