xen/evtchn: Add an helper to reserve/allocate a port

In a follow-up patch we will want to either reserve or allocate a port
for various event channel helpers.

A new wrapper is introduced to either reserve a given port or allocate
a fresh one if zero.

Take the opportunity to replace the open-coded version in
evtchn_bind_virq().

Signed-off-by: Stanislav Kinsburskii <staskins@amazon.com>
Signed-off-by: Julien Grall <jgrall@amazon.com>
Signed-off-by: Rahul Singh <rahul.singh@arm.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/common/event_channel.c

@@ -305,6 +305,18 @@ void evtchn_free(struct domain *d, struct evtchn *chn)
     xsm_evtchn_close_post(chn);
 }
 
+static int evtchn_get_port(struct domain *d, evtchn_port_t port)
+{
+    int rc;
+
+    if ( port != 0 )
+        rc = evtchn_allocate_port(d, port);
+    else
+        rc = get_free_port(d);
+
+    return rc ?: port;
+}
+
 int evtchn_alloc_unbound(evtchn_alloc_unbound_t *alloc)
 {
     struct evtchn *chn;
@@ -462,19 +474,10 @@ int evtchn_bind_virq(evtchn_bind_virq_t *bind, evtchn_port_t port)
     if ( read_atomic(&v->virq_to_evtchn[virq]) )
         ERROR_EXIT(-EEXIST);
 
-    if ( port != 0 )
-    {
-        if ( (rc = evtchn_allocate_port(d, port)) != 0 )
-            ERROR_EXIT(rc);
-    }
-    else
-    {
-        int alloc_port = get_free_port(d);
-
-        if ( alloc_port < 0 )
-            ERROR_EXIT(alloc_port);
-        port = alloc_port;
-    }
+    port = rc = evtchn_get_port(d, port);
+    if ( rc < 0 )
+        ERROR_EXIT(rc);
+    rc = 0;
 
     chn = evtchn_from_port(d, port);