[PATCH v6 1/3] evtchn: slightly defer lock acquire where possible

[Jan Beulich <jbeulich@suse.com>]

port_is_valid() and evtchn_from_port() are fine to use without holding
any locks. Accordingly acquire the per-domain lock slightly later in
evtchn_close() and evtchn_bind_vcpu().

Signed-off-by: Jan Beulich <jbeulich@suse.com>
---
v6: Re-base for re-ordering / shrinking of series.
v4: New.

--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -606,17 +606,14 @@ int evtchn_close(struct domain *d1, int
     int            port2;
     long           rc = 0;
 
- again:
-    spin_lock(&d1->event_lock);
-
     if ( !port_is_valid(d1, port1) )
-    {
-        rc = -EINVAL;
-        goto out;
-    }
+        return -EINVAL;
 
     chn1 = evtchn_from_port(d1, port1);
 
+ again:
+    spin_lock(&d1->event_lock);
+
     /* Guest cannot close a Xen-attached event channel. */
     if ( unlikely(consumer_is_xen(chn1)) && guest )
     {
@@ -1041,16 +1038,13 @@ long evtchn_bind_vcpu(unsigned int port,
     if ( (v = domain_vcpu(d, vcpu_id)) == NULL )
         return -ENOENT;
 
-    spin_lock(&d->event_lock);
-
     if ( !port_is_valid(d, port) )
-    {
-        rc = -EINVAL;
-        goto out;
-    }
+        return -EINVAL;
 
     chn = evtchn_from_port(d, port);
 
+    spin_lock(&d->event_lock);
+
     /* Guest cannot re-bind a Xen-attached event channel. */
     if ( unlikely(consumer_is_xen(chn)) )
     {