netdev-dpdk: Add coverage counter to count vhost IRQs.

When the dpdk vhost library executes an eventfd_write() call,
i.e. waking up the guest, a new callback will be called.

This patch adds the callback to count the number of
interrupts sent to the VM to track the number of times
interrupts where generated.

This might be of interest to find out system-calls were
called in the DPDK fast path.

The coverage counter is called "vhost_notification" and
can be read with:

  $ ovs-appctl coverage/read-counter vhost_notification
  13238319

Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

lib/netdev-dpdk.c

@@ -75,6 +75,7 @@ VLOG_DEFINE_THIS_MODULE(netdev_dpdk);
 static struct vlog_rate_limit rl = VLOG_RATE_LIMIT_INIT(5, 20);
 
 COVERAGE_DEFINE(vhost_tx_contention);
+COVERAGE_DEFINE(vhost_notification);
 
 #define DPDK_PORT_WATCHDOG_INTERVAL 5
 
@@ -169,6 +170,8 @@ static int new_device(int vid);
 static void destroy_device(int vid);
 static int vring_state_changed(int vid, uint16_t queue_id, int enable);
 static void destroy_connection(int vid);
+static void vhost_guest_notified(int vid);
+
 static const struct vhost_device_ops virtio_net_device_ops =
 {
     .new_device =  new_device,
@@ -177,6 +180,7 @@ static const struct vhost_device_ops virtio_net_device_ops =
     .features_changed = NULL,
     .new_connection = NULL,
     .destroy_connection = destroy_connection,
+    .guest_notified = vhost_guest_notified,
 };
 
 /* Custom software stats for dpdk ports */
@@ -3881,6 +3885,12 @@ destroy_connection(int vid)
     }
 }
 
+static
+void vhost_guest_notified(int vid OVS_UNUSED)
+{
+    COVERAGE_INC(vhost_notification);
+}
+
 /*
  * Retrieve the DPDK virtio device ID (vid) associated with a vhostuser
  * or vhostuserclient netdev.