[PATCH v2 net-next repost] MPLS: Use mpls_features to activate software MPLS GSO segmentation

[Simon Horman <horms-/R6kz+dDXgpPR4JQBCEnsQ@public.gmane.org>]

If an MPLS packet requires segmentation then use mpls_features
to determine if the software implementation should be used.

As no driver advertises MPLS GSO segmentation this will always be
the case.

I had not noticed that this was necessary before as software MPLS GSO
segmentation was already being used in my test environment. I believe that
the reason for that is the skbs in question always had fragments and the
driver I used does not advertise NETIF_F_FRAGLIST (which seems to be the
case for most drivers). Thus software segmentation was activated by
skb_gso_ok().

Thanks to Jesse Gross for prompting me to investigate this.

Acked-by: Jesse Gross <jesse-l0M0P4e3n4LQT0dZR+AlfA@public.gmane.org>
Signed-off-by: Simon Horman <horms-/R6kz+dDXgpPR4JQBCEnsQ@public.gmane.org>

---
v2
* Added Ack from Jesse Gross
* Removed duplicate 'Thus' from changelog
---
 net/core/dev.c | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/net/core/dev.c b/net/core/dev.c
index 0355ca5..15f3e6f 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -2540,6 +2540,40 @@ netdev_features_t netif_skb_features(struct sk_buff *skb)
 }
 EXPORT_SYMBOL(netif_skb_features);
 
+/* If MPLS offload request, verify we are testing hardware MPLS features
+ * instead of standard features for the netdev.
+ */
+#ifdef CONFIG_NET_MPLS_GSO
+static inline netdev_features_t net_mpls_features(struct sk_buff *skb,
+	struct net_device *dev, netdev_features_t features)
+{
+	/* There is no support for MPLS LRO. So the only way that
+	 * an MPLS skb could require GSO segmentation is if it
+	 * was received as a non-MPLS skb and then became an MPLS skb.
+	 * This may be effected by Open vSwitch in which case the
+	 * mac_len will non-zero and not equal to skb_network_offset
+	 * as the former indicates the end of L2 the latter indicates
+	 * the beginning of L3 and there is a gap between them occupied
+	 * by the MPLS label stack.
+	 *
+	 * Thus it is possible to avoid traversing any VLAN tags that are
+	 * present to determine if the ethtype is MPLS. Instead the
+	 * inequality of mac_llen and skb_network_offset are used to
+	 * determine if a packet is MPLS for the purpose of determining
+	 * offload features.
+	 */
+	if (skb->mac_len && skb->mac_len != skb_network_offset(skb))
+		features &= dev->mpls_features;
+	return features;
+}
+#else
+static inline netdev_features_t net_mpls_features(struct sk_buff *skb,
+	struct net_device *dev, netdev_features_t features)
+{
+	return features;
+}
+#endif
+
 int dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev,
 			struct netdev_queue *txq)
 {
@@ -2576,6 +2610,8 @@ int dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev,
 		if (skb->encapsulation)
 			features &= dev->hw_enc_features;
 
+		features = net_mpls_features(skb, dev, features);
+
 		if (netif_needs_gso(skb, features)) {
 			if (unlikely(dev_gso_segment(skb, features)))
 				goto out_kfree_skb;
-- 
1.8.5.2