xen/time: Add default value of -1 for IRQ and check for that.

If the timer interrupt has been de-init or is just now being
initialized, the default value of -1 should be preset as
interrupt line. Check for that and if something is odd
WARN us.
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/xen/time.c

@@ -377,7 +377,7 @@ static const struct clock_event_device xen_vcpuop_clockevent = {
 
 static const struct clock_event_device *xen_clockevent =
 	&xen_timerop_clockevent;
-static DEFINE_PER_CPU(struct clock_event_device, xen_clock_events);
+static DEFINE_PER_CPU(struct clock_event_device, xen_clock_events) = { .irq = -1 };
 
 static irqreturn_t xen_timer_interrupt(int irq, void *dev_id)
 {
@@ -401,6 +401,9 @@ void xen_setup_timer(int cpu)
 	struct clock_event_device *evt;
 	int irq;
 
+	evt = &per_cpu(xen_clock_events, cpu);
+	WARN(evt->irq >= 0, "IRQ%d for CPU%d is already allocated\n", evt->irq, cpu);
+
 	printk(KERN_INFO "installing Xen timer for CPU %d\n", cpu);
 
 	name = kasprintf(GFP_KERNEL, "timer%d", cpu);
@@ -413,7 +416,6 @@ void xen_setup_timer(int cpu)
 				      IRQF_FORCE_RESUME,
 				      name, NULL);
 
-	evt = &per_cpu(xen_clock_events, cpu);
 	memcpy(evt, xen_clockevent, sizeof(*evt));
 
 	evt->cpumask = cpumask_of(cpu);
@@ -426,6 +428,7 @@ void xen_teardown_timer(int cpu)
 	BUG_ON(cpu == 0);
 	evt = &per_cpu(xen_clock_events, cpu);
 	unbind_from_irqhandler(evt->irq, NULL);
+	evt->irq = -1;
 }
 
 void xen_setup_cpu_clockevents(void)