xen/evtchn: use smp barriers for user event ring

The ring buffer for user events is local to the given kernel instance,
so smp barriers are fine for ensuring consistency.
Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Link: https://lore.kernel.org/r/20210219154030.10892-8-jgross@suse.comSigned-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/xen/evtchn.c

@@ -173,7 +173,7 @@ static irqreturn_t evtchn_interrupt(int irq, void *data)
 
 	if ((u->ring_prod - u->ring_cons) < u->ring_size) {
 		*evtchn_ring_entry(u, u->ring_prod) = evtchn->port;
-		wmb(); /* Ensure ring contents visible */
+		smp_wmb(); /* Ensure ring contents visible */
 		if (u->ring_cons == u->ring_prod++) {
 			wake_up_interruptible(&u->evtchn_wait);
 			kill_fasync(&u->evtchn_async_queue,
@@ -245,7 +245,7 @@ static ssize_t evtchn_read(struct file *file, char __user *buf,
 	}
 
 	rc = -EFAULT;
-	rmb(); /* Ensure that we see the port before we copy it. */
+	smp_rmb(); /* Ensure that we see the port before we copy it. */
 	if (copy_to_user(buf, evtchn_ring_entry(u, c), bytes1) ||
 	    ((bytes2 != 0) &&
 	     copy_to_user(&buf[bytes1], &u->ring[0], bytes2)))