[PATCH net-next] gro: introduce gro_mac_header_len

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Shlomo Pongratz reported GRO L2 header check was suited for Ethernet
only, and failed on IB/ipoib traffic.

He provided a patch faking a zeroed header to let GRO aggregates frames.

Roland Dreier, Herbert Xu, and others suggested we change GRO L2 header
check to be more generic.

This patch introduces a new netdevice field, gro_mac_header_len, giving
L2 header length, default to ETH_HLEN (14 bytes)

A device setup function can override this default value.

__napi_gro_receive() has special handling for the common case (Ethernet)
to avoid a memcmp() call and use an inline optimized function instead.

Signed-off-by: Eric Dumazet <eric.dumazet@xxxxxxxxx>
Reported-by: Shlomo Pongratz <shlomop@xxxxxxxxxxxx>
Cc: Roland Dreier <roland@xxxxxxxxxx>
Cc: Or Gerlitz <ogerlitz@xxxxxxxxxxxx>
Cc: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
 include/linux/netdevice.h |    1 +
 net/core/dev.c            |   11 +++++++++--
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 0eac07c..d17192b 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -1095,6 +1095,7 @@ struct net_device {
 	unsigned int		mtu;	/* interface MTU value		*/
 	unsigned short		type;	/* interface hardware type	*/
 	unsigned short		hard_header_len;	/* hardware hdr length	*/
+	unsigned int		gro_mac_header_len;
 	/* extra head- and tailroom the hardware may need, but not in all cases
 	 * can this be guaranteed, especially tailroom. Some cases also use
diff --git a/net/core/dev.c b/net/core/dev.c
index f124947..0b43939 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3491,14 +3491,20 @@ static inline gro_result_t
 __napi_gro_receive(struct napi_struct *napi, struct sk_buff *skb)
 	struct sk_buff *p;
+	unsigned int maclen = skb->dev->gro_mac_header_len;
 	for (p = napi->gro_list; p; p = p->next) {
 		unsigned long diffs;
 		diffs = (unsigned long)p->dev ^ (unsigned long)skb->dev;
 		diffs |= p->vlan_tci ^ skb->vlan_tci;
-		diffs |= compare_ether_header(skb_mac_header(p),
-					      skb_gro_mac_header(skb));
+		if (maclen == ETH_HLEN)
+			diffs |= compare_ether_header(skb_mac_header(p),
+						      skb_gro_mac_header(skb));
+		else if (!diffs)
+			diffs = memcmp(skb_mac_header(p),
+				       skb_gro_mac_header(skb),
+				       maclen);
 		NAPI_GRO_CB(p)->same_flow = !diffs;
 		NAPI_GRO_CB(p)->flush = 0;
@@ -5962,6 +5968,7 @@ struct net_device *alloc_netdev_mqs(int sizeof_priv, const char *name,
 	dev->priv_flags = IFF_XMIT_DST_RELEASE;
+	dev->gro_mac_header_len = ETH_HLEN;
 	dev->num_tx_queues = txqs;

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