netvsc: signal host if receive ring is emptied

Latency improvement related to NAPI conversion.
If all packets are processed from receive ring then need
to signal host.
Signed-off-by: NStephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/hyperv/netvsc.c

@@ -1195,10 +1195,15 @@ int netvsc_poll(struct napi_struct *napi, int budget)
 		nvchan->desc = hv_pkt_iter_next(channel, nvchan->desc);
 	}
 
-	/* If send of  pending receive completions suceeded
-	 *   and did not exhaust NAPI budget
+	/* if ring is empty, signal host */
+	if (!nvchan->desc)
+		hv_pkt_iter_close(channel);
+
+	/* If send of pending receive completions suceeded
+	 *   and did not exhaust NAPI budget this time
 	 *   and not doing busy poll
-	 * then reschedule if more data has arrived from host
+	 * then re-enable host interrupts
+	 *     and reschedule if ring is not empty.
 	 */
 	if (send_recv_completions(nvchan) == 0 &&
 	    work_done < budget &&