netdevsim: forward skbs from one connected port to another

Forward skbs sent from one netdevsim port to its connected netdevsim
port using dev_forward_skb, in a spirit similar to veth.

Add a tx_dropped variable to struct netdevsim, tracking the number of
skbs that could not be forwarded using dev_forward_skb().

The xmit() function accessing the peer ptr is protected by an RCU read
critical section. The rcu_read_lock() is functionally redundant as since
v5.0 all softirqs are implicitly RCU read critical sections; but it is
useful for human readers.

If another CPU is concurrently in nsim_destroy(), then it will first set
the peer ptr to NULL. This does not affect any existing readers that
dereferenced a non-NULL peer. Then, in unregister_netdevice(), there is
a synchronize_rcu() before the netdev is actually unregistered and
freed. This ensures that any readers i.e. xmit() that got a non-NULL
peer will complete before the netdev is freed.

Any readers after the RCU_INIT_POINTER() but before synchronize_rcu()
will dereference NULL, making it safe.

The codepath to nsim_destroy() and nsim_create() takes both the newly
added nsim_dev_list_lock and rtnl_lock. This makes it safe with
concurrent calls to linking two netdevsims together.

Signed-off-by: David Wei <[e-mail]>
Reviewed-by: Maciek Machnikowski <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/netdevsim/netdev.c

···
 static netdev_tx_t nsim_start_xmit(struct sk_buff *skb, struct net_device *dev)
 {
 	struct netdevsim *ns = netdev_priv(dev);
+	unsigned int len = skb->len;
+	struct netdevsim *peer_ns;
 
+	rcu_read_lock();
 	if (!nsim_ipsec_tx(ns, skb))
-		goto out;
+		goto out_drop_free;
 
+	peer_ns = rcu_dereference(ns->peer);
+	if (!peer_ns)
+		goto out_drop_free;
+
+	skb_tx_timestamp(skb);
+	if (unlikely(dev_forward_skb(peer_ns->netdev, skb) == NET_RX_DROP))
+		goto out_drop_cnt;
+
+	rcu_read_unlock();
 	u64_stats_update_begin(&ns->syncp);
 	ns->tx_packets++;
-	ns->tx_bytes += skb->len;
+	ns->tx_bytes += len;
 	u64_stats_update_end(&ns->syncp);
+	return NETDEV_TX_OK;
 
-out:
+out_drop_free:
 	dev_kfree_skb(skb);
-
+out_drop_cnt:
+	rcu_read_unlock();
+	u64_stats_update_begin(&ns->syncp);
+	ns->tx_dropped++;
+	u64_stats_update_end(&ns->syncp);
 	return NETDEV_TX_OK;
 }
 
···
 		start = u64_stats_fetch_begin(&ns->syncp);
 		stats->tx_bytes = ns->tx_bytes;
 		stats->tx_packets = ns->tx_packets;
+		stats->tx_dropped = ns->tx_dropped;
 	} while (u64_stats_fetch_retry(&ns->syncp, start));
 }
 
···
 	eth_hw_addr_random(dev);
 
 	dev->tx_queue_len = 0;
-	dev->flags |= IFF_NOARP;
 	dev->flags &= ~IFF_MULTICAST;
 	dev->priv_flags |= IFF_LIVE_ADDR_CHANGE |
 			   IFF_NO_QUEUE;

drivers/net/netdevsim/netdevsim.h

···
 
 	u64 tx_packets;
 	u64 tx_bytes;
+	u64 tx_dropped;
 	struct u64_stats_sync syncp;
 
 	struct nsim_bus_dev *nsim_bus_dev;