Index: sys/x86/xen/xen_intr.c
===================================================================
--- sys/x86/xen/xen_intr.c
+++ sys/x86/xen/xen_intr.c
@@ -120,10 +120,6 @@
 
 DPCPU_DECLARE(struct vcpu_info *, vcpu_info);
 
-#define	XEN_INVALID_EVTCHN	0 /* Invalid event channel */
-
-#define	is_valid_evtchn(x)	((x) != XEN_INVALID_EVTCHN)
-
 struct xenisrc {
 	struct intsrc	xi_intsrc;
 	enum evtchn_type xi_type;
@@ -333,10 +329,10 @@
 		return (isrc);
 	}
 
-	if (xen_intr_auto_vector_count > NR_EVENT_CHANNELS) {
+	if (xen_intr_auto_vector_count >= NR_EVENT_CHANNELS) {
 		if (!warned) {
 			warned = 1;
-			printf("%s: Event channels exhausted.\n", __func__);
+			printf("%s: Xen interrupts exhausted.\n", __func__);
 		}
 		mtx_unlock(&xen_intr_x86_lock);
 		return (NULL);
@@ -372,28 +368,30 @@
 xen_intr_release_isrc(struct xenisrc *isrc)
 {
 
-	mtx_lock(&xen_intr_isrc_lock);
 	KASSERT(isrc->xi_intsrc.is_handlers == 0,
 	    ("Release called, but xenisrc still in use"));
-	evtchn_mask_port(isrc->xi_port);
-	evtchn_clear_port(isrc->xi_port);
+	mtx_lock(&xen_intr_isrc_lock);
+	if (isrc->xi_port < NR_EVENT_CHANNELS) {
+		evtchn_mask_port(isrc->xi_port);
+		evtchn_clear_port(isrc->xi_port);
+
+		/* Rebind port to CPU 0. */
+		evtchn_cpu_mask_port(isrc->xi_cpu, isrc->xi_port);
+		evtchn_cpu_unmask_port(0, isrc->xi_port);
 
-	/* Rebind port to CPU 0. */
-	evtchn_cpu_mask_port(isrc->xi_cpu, isrc->xi_port);
-	evtchn_cpu_unmask_port(0, isrc->xi_port);
+		if (isrc->xi_close != 0) {
+			struct evtchn_close close = { .port = isrc->xi_port };
+			if (HYPERVISOR_event_channel_op(EVTCHNOP_close, &close))
+				panic("EVTCHNOP_close failed");
+		}
 
-	if (isrc->xi_close != 0 && is_valid_evtchn(isrc->xi_port)) {
-		struct evtchn_close close = { .port = isrc->xi_port };
-		if (HYPERVISOR_event_channel_op(EVTCHNOP_close, &close))
-			panic("EVTCHNOP_close failed");
+		xen_intr_port_to_isrc[isrc->xi_port] = NULL;
 	}
-
-	xen_intr_port_to_isrc[isrc->xi_port] = NULL;
 	/* not reachable from xen_intr_port_to_isrc[], unlock */
 	mtx_unlock(&xen_intr_isrc_lock);
 
 	isrc->xi_cpu = 0;
-	isrc->xi_port = 0;
+	isrc->xi_port = ~0U;
 	isrc->xi_cookie = NULL;
 	/*
 	 * Only when ->xi_type == EVTCHN_TYPE_UNBOUND is the isrc under control
@@ -447,6 +445,10 @@
 	isrc->xi_port = local_port;
 	refcount_init(&isrc->xi_refcount, 1);
 	mtx_lock(&xen_intr_isrc_lock);
+	if (xen_intr_port_to_isrc[isrc->xi_port] != NULL) {
+		xen_intr_port_to_isrc[isrc->xi_port]->xi_port = ~0U;
+		isrc->xi_cpu = xen_intr_port_to_isrc[isrc->xi_port]->xi_cpu;
+	}
 	xen_intr_port_to_isrc[isrc->xi_port] = isrc;
 	mtx_unlock(&xen_intr_isrc_lock);
 
@@ -792,7 +794,7 @@
 		vector = first_evtchn_irq + isrc_idx;
 		isrc = (struct xenisrc *)intr_lookup_source(vector);
 		if (isrc != NULL) {
-			isrc->xi_port = 0;
+			isrc->xi_port = ~0U;
 			switch (isrc->xi_type) {
 			case EVTCHN_TYPE_IPI:
 				xen_rebind_ipi(isrc);
@@ -896,7 +898,7 @@
 
 	mtx_lock(&xen_intr_isrc_lock);
 	isrc = (struct xenisrc *)base_isrc;
-	if (!is_valid_evtchn(isrc->xi_port)) {
+	if (isrc->xi_port >= NR_EVENT_CHANNELS) {
 		mtx_unlock(&xen_intr_isrc_lock);
 		return (EINVAL);
 	}
@@ -1315,7 +1317,8 @@
 xen_intr_get_evtchn_from_port(evtchn_port_t port, xen_intr_handle_t *handlep)
 {
 
-	if (!is_valid_evtchn(port) || port >= NR_EVENT_CHANNELS)
+	/* event channel 0 is reserved, >= NR_EVENT_CHANNELS is invalid */
+	if (port == 0 || port >= NR_EVENT_CHANNELS)
 		return (EINVAL);
 
 	if (handlep == NULL) {