Index: sys/x86/xen/xen_intr.c
===================================================================
--- sys/x86/xen/xen_intr.c
+++ sys/x86/xen/xen_intr.c
@@ -113,9 +113,8 @@
 
 DPCPU_DECLARE(struct vcpu_info *, vcpu_info);
 
-#define	XEN_INVALID_EVTCHN	0 /* Invalid event channel */
-
-#define	is_valid_evtchn(x)	((x) != XEN_INVALID_EVTCHN)
+#define	INVALID_EVTCHN		(~(evtchn_port_t)0) /* Invalid event channel */
+#define	is_valid_evtchn(x)	((x) < NR_EVENT_CHANNELS)
 
 struct xenisrc {
 	struct intsrc	xi_intsrc;
@@ -310,10 +309,10 @@
 
 	KASSERT(mtx_owned(&xen_intr_isrc_lock), ("Evtchn alloc lock not held"));
 
-	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__);
 		}
 		return (NULL);
 	}
@@ -346,26 +345,29 @@
 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 (is_valid_evtchn(isrc->xi_port)) {
+		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 && 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");
-	}
+		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");
+		}
 
-	xen_intr_port_to_isrc[isrc->xi_port] = NULL;
+		xen_intr_port_to_isrc[isrc->xi_port] = NULL;
+	}
 	isrc->xi_cpu = 0;
 	isrc->xi_type = EVTCHN_TYPE_UNBOUND;
-	isrc->xi_port = 0;
+	isrc->xi_port = INVALID_EVTCHN;
 	isrc->xi_cookie = NULL;
 	mtx_unlock(&xen_intr_isrc_lock);
 	return (0);
@@ -605,6 +607,12 @@
 	if (!xen_domain())
 		return (0);
 
+	CTASSERT(is_valid_evtchn(0));
+	CTASSERT(is_valid_evtchn(NR_EVENT_CHANNELS - 1));
+	CTASSERT(!is_valid_evtchn(NR_EVENT_CHANNELS));
+	CTASSERT(!is_valid_evtchn(~(evtchn_port_t)0));
+	CTASSERT(!is_valid_evtchn(INVALID_EVTCHN));
+
 	mtx_init(&xen_intr_isrc_lock, "xen-irq-lock", NULL, MTX_DEF);
 
 	/*
@@ -761,7 +769,7 @@
 		vector = first_evtchn_irq + isrc_idx;
 		isrc = (struct xenisrc *)intr_lookup_source(vector);
 		if (isrc != NULL) {
-			isrc->xi_port = 0;
+			isrc->xi_port = INVALID_EVTCHN;
 			switch (isrc->xi_type) {
 			case EVTCHN_TYPE_IPI:
 				xen_rebind_ipi(isrc);
@@ -1284,7 +1292,7 @@
 xen_intr_get_evtchn_from_port(evtchn_port_t port, xen_intr_handle_t *handlep)
 {
 
-	if (!is_valid_evtchn(port) || port >= NR_EVENT_CHANNELS)
+	if (!is_valid_evtchn(port))
 		return (EINVAL);
 
 	if (handlep == NULL) {