MPLS: Add limited GSO support

In the case where a non-MPLS packet is received and an MPLS stack is
added it may well be the case that the original skb is GSO but the
NIC used for transmit does not support GSO of MPLS packets.

The aim of this code is to provide GSO in software for MPLS packets
whose skbs are GSO.

SKB Usage:

When an implementation adds an MPLS stack to a non-MPLS packet it should do
the following to skb metadata:

* Set skb->inner_protocol to the old non-MPLS ethertype of the packet.
  skb->inner_protocol is added by this patch.

* Set skb->protocol to the new MPLS ethertype of the packet.

* Set skb->network_header to correspond to the
  end of the L3 header, including the MPLS label stack.

I have posted a patch, "[PATCH v3.29] datapath: Add basic MPLS support to
kernel" which adds MPLS support to the kernel datapath of Open vSwtich.
That patch sets the above requirements in datapath/actions.c:push_mpls()
and was used to exercise this code.  The datapath patch is against the Open
vSwtich tree but it is intended that it be added to the Open vSwtich code
present in the mainline Linux kernel at some point.

Features:

I believe that the approach that I have taken is at least partially
consistent with the handling of other protocols.  Jesse, I understand that
you have some ideas here.  I am more than happy to change my implementation.

This patch adds dev->mpls_features which may be used by devices
to advertise features supported for MPLS packets.

A new NETIF_F_MPLS_GSO feature is added for devices which support
hardware MPLS GSO offload.  Currently no devices support this
and MPLS GSO always falls back to software.

Alternate Implementation:

One possible alternate implementation is to teach netif_skb_features()
and skb_network_protocol() about MPLS, in a similar way to their
understanding of VLANs. I believe this would avoid the need
for net/mpls/mpls_gso.c and in particular the calls to
__skb_push() and __skb_push() in mpls_gso_segment().

I have decided on the implementation in this patch as it should
not introduce any overhead in the case where mpls_gso is not compiled
into the kernel or inserted as a module.

MPLS GSO suggested by Jesse Gross.
Based in part on "v4 GRE: Add TCP segmentation offload for GRE"
by Pravin B Shelar.

Cc: Jesse Gross <jesse@nicira.com>
Cc: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

3 files changed, 9 insertions, 1 deletions

diff --git a/include/linux/netdev_features.h b/include/linux/netdev_features.h
index 09906b7ca47d..a2a89a5c7be5 100644
--- a/include/linux/netdev_features.h
+++ b/include/linux/netdev_features.h
@@ -43,8 +43,9 @@ enum {
         NETIF_F_FSO_BIT,                /* ... FCoE segmentation */
         NETIF_F_GSO_GRE_BIT,            /* ... GRE with TSO */
         NETIF_F_GSO_UDP_TUNNEL_BIT,     /* ... UDP TUNNEL with TSO */
+        NETIF_F_GSO_MPLS_BIT,           /* ... MPLS segmentation */
         /**/NETIF_F_GSO_LAST =          /* last bit, see GSO_MASK */
-                NETIF_F_GSO_UDP_TUNNEL_BIT,
+                NETIF_F_GSO_MPLS_BIT,
 
         NETIF_F_FCOE_CRC_BIT,           /* FCoE CRC32 */
         NETIF_F_SCTP_CSUM_BIT,          /* SCTP checksum offload */
@@ -107,6 +108,7 @@ enum {
 #define NETIF_F_RXALL           __NETIF_F(RXALL)
 #define NETIF_F_GSO_GRE         __NETIF_F(GSO_GRE)
 #define NETIF_F_GSO_UDP_TUNNEL  __NETIF_F(GSO_UDP_TUNNEL)
+#define NETIF_F_GSO_MPLS        __NETIF_F(GSO_MPLS)
 #define NETIF_F_HW_VLAN_STAG_FILTER __NETIF_F(HW_VLAN_STAG_FILTER)
 #define NETIF_F_HW_VLAN_STAG_RX __NETIF_F(HW_VLAN_STAG_RX)
 #define NETIF_F_HW_VLAN_STAG_TX __NETIF_F(HW_VLAN_STAG_TX)
diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index ea7b6bce9ea0..6b2bb460d1d7 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -1088,6 +1088,8 @@ struct net_device {
          * need to set them appropriately.
          */
         netdev_features_t       hw_enc_features;
+        /* mask of fetures inheritable by MPLS */
+        netdev_features_t       mpls_features;
 
         /* Interface index. Unique device identifier    */
         int                     ifindex;
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 5663e3592784..8f2b830772a8 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -319,6 +319,8 @@ enum {
         SKB_GSO_GRE = 1 << 6,
 
         SKB_GSO_UDP_TUNNEL = 1 << 7,
+
+        SKB_GSO_MPLS = 1 << 8,
 };
 
 #if BITS_PER_LONG > 32
@@ -389,6 +391,7 @@ typedef unsigned char *sk_buff_data_t;
  *      @dropcount: total number of sk_receive_queue overflows
  *      @vlan_proto: vlan encapsulation protocol
  *      @vlan_tci: vlan tag control information
+ *      @inner_protocol: Protocol (encapsulation)
  *      @inner_transport_header: Inner transport layer header (encapsulation)
  *      @inner_network_header: Network layer header (encapsulation)
  *      @inner_mac_header: Link layer header (encapsulation)
@@ -509,6 +512,7 @@ struct sk_buff {
                 __u32           reserved_tailroom;
         };
 
+        __be16                  inner_protocol;
         __u16                   inner_transport_header;
         __u16                   inner_network_header;
         __u16                   inner_mac_header;