virtio-net: send gratuitous packets when needed

As hypervior does not have the knowledge of guest network configuration, it's
better to ask guest to send gratuitous packets when needed.

This patch implements VIRTIO_NET_F_GUEST_ANNOUNCE feature: hypervisor would
notice the guest when it thinks it's time for guest to announce the link
presnece. Guest tests VIRTIO_NET_S_ANNOUNCE bit during config change interrupt
and woule send gratuitous packets through netif_notify_peers() and ack the
notification through ctrl vq.

We need to make sure the atomicy of read and ack in guest otherwise we may ack
more times than being notified. This is done through handling the whole config
change interrupt in an non-reentrant workqueue.

Signed-off-by: Jason Wang <[e-mail]>
Acked-by: Michael S. Tsirkin <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/virtio_net.c

···
 	/* Host will merge rx buffers for big packets (shake it! shake it!) */
 	bool mergeable_rx_bufs;
 
+	/* enable config space updates */
+	bool config_enable;
+
 	/* Active statistics */
 	struct virtnet_stats __percpu *stats;
 
 	/* Work struct for refilling if we run low on memory. */
 	struct delayed_work refill;
+
+	/* Work struct for config space updates */
+	struct work_struct config_work;
+
+	/* Lock for config space updates */
+	struct mutex config_lock;
 
 	/* Chain pages by the private ptr. */
 	struct page *pages;
···
 	return status == VIRTIO_NET_OK;
 }
 
+static void virtnet_ack_link_announce(struct virtnet_info *vi)
+{
+	rtnl_lock();
+	if (!virtnet_send_command(vi, VIRTIO_NET_CTRL_ANNOUNCE,
+				  VIRTIO_NET_CTRL_ANNOUNCE_ACK, NULL,
+				  0, 0))
+		dev_warn(&vi->dev->dev, "Failed to ack link announce.\n");
+	rtnl_unlock();
+}
+
 static int virtnet_close(struct net_device *dev)
 {
 	struct virtnet_info *vi = netdev_priv(dev);
···
 #endif
 };
 
-static void virtnet_update_status(struct virtnet_info *vi)
+static void virtnet_config_changed_work(struct work_struct *work)
 {
+	struct virtnet_info *vi =
+		container_of(work, struct virtnet_info, config_work);
 	u16 v;
+
+	mutex_lock(&vi->config_lock);
+	if (!vi->config_enable)
+		goto done;
 
 	if (virtio_config_val(vi->vdev, VIRTIO_NET_F_STATUS,
 			      offsetof(struct virtio_net_config, status),
 			      &v) < 0)
-		return;
+		goto done;
+
+	if (v & VIRTIO_NET_S_ANNOUNCE) {
+		netif_notify_peers(vi->dev);
+		virtnet_ack_link_announce(vi);
+	}
 
 	/* Ignore unknown (future) status bits */
 	v &= VIRTIO_NET_S_LINK_UP;
 
 	if (vi->status == v)
-		return;
+		goto done;
 
 	vi->status = v;
 
···
 		netif_carrier_off(vi->dev);
 		netif_stop_queue(vi->dev);
 	}
+done:
+	mutex_unlock(&vi->config_lock);
 }
 
 static void virtnet_config_changed(struct virtio_device *vdev)
 {
 	struct virtnet_info *vi = vdev->priv;
 
-	virtnet_update_status(vi);
+	queue_work(system_nrt_wq, &vi->config_work);
 }
 
 static int init_vqs(struct virtnet_info *vi)
···
 		goto free;
 
 	INIT_DELAYED_WORK(&vi->refill, refill_work);
+	mutex_init(&vi->config_lock);
+	vi->config_enable = true;
+	INIT_WORK(&vi->config_work, virtnet_config_changed_work);
 	sg_init_table(vi->rx_sg, ARRAY_SIZE(vi->rx_sg));
 	sg_init_table(vi->tx_sg, ARRAY_SIZE(vi->tx_sg));
 
···
 	   otherwise get link status from config. */
 	if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_STATUS)) {
 		netif_carrier_off(dev);
-		virtnet_update_status(vi);
+		queue_work(system_nrt_wq, &vi->config_work);
 	} else {
 		vi->status = VIRTIO_NET_S_LINK_UP;
 		netif_carrier_on(dev);
···
 {
 	struct virtnet_info *vi = vdev->priv;
 
+	/* Prevent config work handler from accessing the device. */
+	mutex_lock(&vi->config_lock);
+	vi->config_enable = false;
+	mutex_unlock(&vi->config_lock);
+
 	unregister_netdev(vi->dev);
 
 	remove_vq_common(vi);
+
+	flush_work(&vi->config_work);
 
 	free_percpu(vi->stats);
 	free_netdev(vi->dev);
···
 static int virtnet_freeze(struct virtio_device *vdev)
 {
 	struct virtnet_info *vi = vdev->priv;
+
+	/* Prevent config work handler from accessing the device */
+	mutex_lock(&vi->config_lock);
+	vi->config_enable = false;
+	mutex_unlock(&vi->config_lock);
 
 	virtqueue_disable_cb(vi->rvq);
 	virtqueue_disable_cb(vi->svq);
···
 		napi_disable(&vi->napi);
 
 	remove_vq_common(vi);
+
+	flush_work(&vi->config_work);
 
 	return 0;
 }
···
 	if (!try_fill_recv(vi, GFP_KERNEL))
 		queue_delayed_work(system_nrt_wq, &vi->refill, 0);
 
+	mutex_lock(&vi->config_lock);
+	vi->config_enable = true;
+	mutex_unlock(&vi->config_lock);
+
 	return 0;
 }
 #endif
···
 	VIRTIO_NET_F_GUEST_ECN, VIRTIO_NET_F_GUEST_UFO,
 	VIRTIO_NET_F_MRG_RXBUF, VIRTIO_NET_F_STATUS, VIRTIO_NET_F_CTRL_VQ,
 	VIRTIO_NET_F_CTRL_RX, VIRTIO_NET_F_CTRL_VLAN,
+	VIRTIO_NET_F_GUEST_ANNOUNCE,
 };
 
 static struct virtio_driver virtio_net_driver = {

include/linux/virtio_net.h

···
 #define VIRTIO_NET_F_CTRL_RX	18	/* Control channel RX mode support */
 #define VIRTIO_NET_F_CTRL_VLAN	19	/* Control channel VLAN filtering */
 #define VIRTIO_NET_F_CTRL_RX_EXTRA 20	/* Extra RX mode control support */
+#define VIRTIO_NET_F_GUEST_ANNOUNCE 21	/* Guest can announce device on the
+					 * network */
 
 #define VIRTIO_NET_S_LINK_UP	1	/* Link is up */
+#define VIRTIO_NET_S_ANNOUNCE	2	/* Announcement is needed */
 
 struct virtio_net_config {
 	/* The config defining mac address (if VIRTIO_NET_F_MAC) */
···
 #define VIRTIO_NET_CTRL_VLAN       2
  #define VIRTIO_NET_CTRL_VLAN_ADD             0
  #define VIRTIO_NET_CTRL_VLAN_DEL             1
+
+/*
+ * Control link announce acknowledgement
+ *
+ * The command VIRTIO_NET_CTRL_ANNOUNCE_ACK is used to indicate that
+ * driver has recevied the notification; device would clear the
+ * VIRTIO_NET_S_ANNOUNCE bit in the status field after it receives
+ * this command.
+ */
+#define VIRTIO_NET_CTRL_ANNOUNCE       3
+ #define VIRTIO_NET_CTRL_ANNOUNCE_ACK         0
 
 #endif /* _LINUX_VIRTIO_NET_H */