ice: allow toggling loopback mode via ndo_set_features callback

Add support for NETIF_F_LOOPBACK. This feature can be set via:
$ ethtool -K eth0 loopback <on|off>

Feature can be useful for local data path tests.
Acked-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Tested-by: George Kuruvinakunnel <george.kuruvinakunnel@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/ice/ice_main.c

@@ -3358,6 +3358,7 @@ static void ice_set_netdev_features(struct net_device *netdev)
 	netdev->features |= netdev->hw_features;
 
 	netdev->hw_features |= NETIF_F_HW_TC;
+	netdev->hw_features |= NETIF_F_LOOPBACK;
 
 	/* encap and VLAN devices inherit default, csumo and tso features */
 	netdev->hw_enc_features |= dflt_features | csumo_features |
@@ -5910,6 +5911,32 @@ ice_set_vlan_features(struct net_device *netdev, netdev_features_t features)
 	return 0;
 }
 
+/**
+ * ice_set_loopback - turn on/off loopback mode on underlying PF
+ * @vsi: ptr to VSI
+ * @ena: flag to indicate the on/off setting
+ */
+static int ice_set_loopback(struct ice_vsi *vsi, bool ena)
+{
+	bool if_running = netif_running(vsi->netdev);
+	int ret;
+
+	if (if_running && !test_and_set_bit(ICE_VSI_DOWN, vsi->state)) {
+		ret = ice_down(vsi);
+		if (ret) {
+			netdev_err(vsi->netdev, "Preparing device to toggle loopback failed\n");
+			return ret;
+		}
+	}
+	ret = ice_aq_set_mac_loopback(&vsi->back->hw, ena, NULL);
+	if (ret)
+		netdev_err(vsi->netdev, "Failed to toggle loopback state\n");
+	if (if_running)
+		ret = ice_up(vsi);
+
+	return ret;
+}
+
 /**
  * ice_set_features - set the netdev feature flags
  * @netdev: ptr to the netdev being adjusted
@@ -5968,7 +5995,10 @@ ice_set_features(struct net_device *netdev, netdev_features_t features)
 		      clear_bit(ICE_FLAG_CLS_FLOWER, pf->flags);
 	}
 
-	return 0;
+	if (changed & NETIF_F_LOOPBACK)
+		ret = ice_set_loopback(vsi, !!(features & NETIF_F_LOOPBACK));
+
+	return ret;
 }
 
 /**