Irq nunmber for per-vcpu event (virq/ipi) needs kept
accross vcpu plug/unplug, once allocated. We just 
reuse this irq number and bind it to a new event port.
Or else, /proc/interrupt exports messed statistics 
like:

ot@localhost ~]# cat /proc/interrupts
           CPU0       CPU1       CPU2       CPU3
256:     423066          0          0          0     Dynamic-irq  timer0
257:       3014          0          0          0     Dynamic-irq  resched0
258:         37          0          0          0     Dynamic-irq  callfunc0
259:        713          0          0          0     Dynamic-irq  xenbus
260:        343       1323         65         50     Dynamic-irq  xencons
261:        270         62          0          0     Dynamic-irq  blkif
262:          0    1053818       1220        855     Dynamic-irq  resched1
263:          0          0       2165         53     Dynamic-irq  callfunc1
264:          0       3218         69          0     Dynamic-irq  timer1
265:          0          0      42768        228     Dynamic-irq  resched2
266:          0         45          0       1867     Dynamic-irq  callfunc2
267:          0          0      39543        256     Dynamic-irq  timer2
268:          0       2244         46      33027     Dynamic-irq  resched3
269:       1585          0          0          0     Dynamic-irq  callfunc3
270:      11273          0       1885       1073     Dynamic-irq  timer3
NMI:          0          0          0          0
LOC:          0          0          0          0
ERR:          0
MIS:          0

This also applies to save/restore for same issue.

Signed-off-by Kevin Tian <kevin.tian@intel.com>

diff -r 99d1480afae8 -r 7988ed3b9cd9 linux-2.6-xen-sparse/drivers/xen/core/evtchn.c
--- a/linux-2.6-xen-sparse/drivers/xen/core/evtchn.c	Fri Feb 02 21:08:49 2007 +0800
+++ b/linux-2.6-xen-sparse/drivers/xen/core/evtchn.c	Sat Feb 03 12:30:46 2007 +0800
@@ -359,14 +359,21 @@ static int bind_virq_to_irq(unsigned int
 static int bind_virq_to_irq(unsigned int virq, unsigned int cpu)
 {
 	struct evtchn_bind_virq bind_virq;
-	int evtchn, irq;
+	int evtchn, irq, bind = 1;
 
 	spin_lock(&irq_mapping_update_lock);
 
-	if ((irq = per_cpu(virq_to_irq, cpu)[virq]) == -1) {
+	irq = per_cpu(virq_to_irq, cpu)[virq];
+	if (irq == -1) {
 		if ((irq = find_unbound_irq()) < 0)
 			goto out;
-
+	} else if (!evtchn_from_irq(irq)) {
+		/* Canonicalize the count */
+		irq_bindcount[irq]--;
+	} else
+		bind = 0;
+
+	if (bind) {
 		bind_virq.virq = virq;
 		bind_virq.vcpu = cpu;
 		if (HYPERVISOR_event_channel_op(EVTCHNOP_bind_virq,
@@ -392,14 +399,21 @@ static int bind_ipi_to_irq(unsigned int 
 static int bind_ipi_to_irq(unsigned int ipi, unsigned int cpu)
 {
 	struct evtchn_bind_ipi bind_ipi;
-	int evtchn, irq;
+	int evtchn, irq, bind = 1;
 
 	spin_lock(&irq_mapping_update_lock);
 
-	if ((irq = per_cpu(ipi_to_irq, cpu)[ipi]) == -1) {
+	irq = per_cpu(ipi_to_irq, cpu)[ipi];
+	if (irq == -1) {
 		if ((irq = find_unbound_irq()) < 0)
 			goto out;
-
+	} else if (!evtchn_from_irq(irq)) {
+		/* Canonicalize the count */
+		irq_bindcount[irq]--;
+	} else
+		bind = 0;
+
+	if (bind) {
 		bind_ipi.vcpu = cpu;
 		if (HYPERVISOR_event_channel_op(EVTCHNOP_bind_ipi,
 						&bind_ipi) != 0)
@@ -434,24 +448,24 @@ static void unbind_from_irq(unsigned int
 		    HYPERVISOR_event_channel_op(EVTCHNOP_close, &close))
 			BUG();
 
+		/* Closed ports are implicitly re-bound to VCPU0. */
+		bind_evtchn_to_cpu(evtchn, 0);
+
+		evtchn_to_irq[evtchn] = -1;
+
 		switch (type_from_irq(irq)) {
 		case IRQT_VIRQ:
-			per_cpu(virq_to_irq, cpu_from_evtchn(evtchn))
-				[index_from_irq(irq)] = -1;
-			break;
 		case IRQT_IPI:
-			per_cpu(ipi_to_irq, cpu_from_evtchn(evtchn))
-				[index_from_irq(irq)] = -1;
+			/* Keep irq number for per-vcpu type, and lock
+			 * it from used by others
+			 */
+			irq_info[irq] &= ~0xFFFF;
+			irq_bindcount[irq]++;
 			break;
 		default:
+			irq_info[irq] = IRQ_UNBOUND;
 			break;
 		}
-
-		/* Closed ports are implicitly re-bound to VCPU0. */
-		bind_evtchn_to_cpu(evtchn, 0);
-
-		evtchn_to_irq[evtchn] = -1;
-		irq_info[irq] = IRQ_UNBOUND;
 	}
 
 	spin_unlock(&irq_mapping_update_lock);
@@ -901,14 +915,18 @@ void irq_resume(void)
 	for (pirq = 0; pirq < NR_PIRQS; pirq++)
 		BUG_ON(irq_info[pirq_to_irq(pirq)] != IRQ_UNBOUND);
 
-	/* Secondary CPUs must have no VIRQ or IPI bindings. */
+	/* Secondary CPUs must have no VIRQ or IPI bindings to event
+	 * port, but irq number may be preserved
+	 */
 	for_each_possible_cpu(cpu) {
 		if (cpu == 0)
 			continue;
 		for (virq = 0; virq < NR_VIRQS; virq++)
-			BUG_ON(per_cpu(virq_to_irq, cpu)[virq] != -1);
+			BUG_ON(((irq = per_cpu(virq_to_irq, cpu)[virq]) != -1)
+					&& evtchn_from_irq(irq));
 		for (ipi = 0; ipi < NR_IPIS; ipi++)
-			BUG_ON(per_cpu(ipi_to_irq, cpu)[ipi] != -1);
+			BUG_ON(((irq = per_cpu(ipi_to_irq, cpu)[ipi]) != -1)
+					&& evtchn_from_irq(irq));
 	}
 
 	/* No IRQ <-> event-channel mappings. */