Subject: [PATCH net-next v3 11/12] netdevsim: add loopback support

From: Jakub Kicinski <[email-masked]>
Date: Wed, 19 Nov 2025 18:10:23 -0800

Support device loopback. Apparently this mode has been historically
supported by the toeplitz test and I don't have any HW which lets
me test the conversion..

Reviewed-by: Petr Machata <[email-masked]>
Reviewed-by: Willem de Bruijn <[email-masked]>
Signed-off-by: Jakub Kicinski <[email-masked]>
---
 drivers/net/netdevsim/netdev.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/drivers/net/netdevsim/netdev.c b/drivers/net/netdevsim/netdev.c
index 2b713db16cd0..6927c1962277 100644
--- a/drivers/net/netdevsim/netdev.c
+++ b/drivers/net/netdevsim/netdev.c
@@ -133,15 +133,21 @@ static netdev_tx_t nsim_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	if (!nsim_ipsec_tx(ns, skb))
 		goto out_drop_any;
 
-	peer_ns = rcu_dereference(ns->peer);
-	if (!peer_ns)
-		goto out_drop_any;
+	/* Check if loopback mode is enabled */
+	if (dev->features & NETIF_F_LOOPBACK) {
+		peer_ns = ns;
+		peer_dev = dev;
+	} else {
+		peer_ns = rcu_dereference(ns->peer);
+		if (!peer_ns)
+			goto out_drop_any;
+		peer_dev = peer_ns->netdev;
+	}
 
 	dr = nsim_do_psp(skb, ns, peer_ns, &psp_ext);
 	if (dr)
 		goto out_drop_free;
 
-	peer_dev = peer_ns->netdev;
 	rxq = skb_get_queue_mapping(skb);
 	if (rxq >= peer_dev->num_rx_queues)
 		rxq = rxq % peer_dev->num_rx_queues;
@@ -976,7 +982,8 @@ static void nsim_setup(struct net_device *dev)
 			    NETIF_F_FRAGLIST |
 			    NETIF_F_HW_CSUM |
 			    NETIF_F_LRO |
-			    NETIF_F_TSO;
+			    NETIF_F_TSO |
+			    NETIF_F_LOOPBACK;
 	dev->pcpu_stat_type = NETDEV_PCPU_STAT_DSTATS;
 	dev->max_mtu = ETH_MAX_MTU;
 	dev->xdp_features = NETDEV_XDP_ACT_BASIC | NETDEV_XDP_ACT_HW_OFFLOAD;
-- 
2.51.1