RFC udp: implement GRO support

This series implements GRO support for UDP sockets, as the RX counterpart of commit bec1f6f69736 ("udp: generate gso with UDP_SEGMENT").
The core functionality is implemented by the second patch, introducing a new sockopt to enable UDP_GRO, while patch 3 implements support for passing the segment size to the user space via a new cmsg.
UDP GRO performs a socket lookup for each ingress packets and aggregate datagram directed to UDP GRO enabled sockets with constant l4 tuple.

UDP GRO packets can land on non GRO-enabled sockets, e.g. due to iptables NAT rules, and that could potentially confuse existing applications.

The solution adopted here is to de-segment the GRO packet before enqueuing as needed. Since we must cope with packet reinsertion after de-segmentation, the relevant code is factored-out in ipv4 and ipv6 specific helpers and exposed to UDP usage.

While the current code can probably be improved, this safeguard ,implemented in the patches 4-7, allows future enachements to enable UDP GSO offload on more virtual devices eventually even on forwarded packets.

The last 4 for patches implement some performance and functional self-tests, re-using the existing udpgso infrastructure. The problematic scenario described above is explicitly tested.

This revision of the series try to address the feedback provided by Willem, Steffen and Subash fixing several bugs all along

rfc v2 - rfc v3:
- cope better with exceptional conditions
- test cases cleanup

rfc v1 - rfc v2:
- use a new option to enable UDP GRO
- use static keys to protect the UDP GRO socket lookup
- cope with UDP GRO misdirection
- add self-tests

Paolo Abeni (10):
udp: implement complete book-keeping for encap_needed
udp: implement GRO for plain UDP sockets.
udp: add support for UDP_GRO cmsg
ip: factor out protocol delivery helper
ipv6: factor out protocol delivery helper
udp: cope with UDP GRO packet misdirection
selftests: add GRO support to udp bench rx program
selftests: conditionally enable XDP support in udpgso_bench_rx
selftests: add some benchmark for UDP GRO
selftests: add functionals test for UDP GRO

 include/linux/udp.h                           |  25 ++-
 include/net/udp.h                             |  51 ++++-
 include/net/udp_tunnel.h                      |   6 +
 include/uapi/linux/udp.h                      |   1 +
 net/ipv4/ip_input.c                           |  73 ++++---
 net/ipv4/udp.c                                |  54 ++++-
 net/ipv4/udp_offload.c                        | 109 ++++++++--
 net/ipv6/ip6_input.c                          |  28 +--
 net/ipv6/udp.c                                |  44 +++-
 net/ipv6/udp_offload.c                        |   6 +-
 tools/testing/selftests/net/Makefile          |  70 +++++++
 tools/testing/selftests/net/udpgro.sh         | 147 +++++++++++++
 tools/testing/selftests/net/udpgro_bench.sh   |  94 +++++++++
 tools/testing/selftests/net/udpgso_bench.sh   |   2 +-
 tools/testing/selftests/net/udpgso_bench_rx.c | 193 ++++++++++++++++--
 tools/testing/selftests/net/udpgso_bench_tx.c |  22 +-
 tools/testing/selftests/net/xdp_dummy.c       |  13 ++
 17 files changed, 816 insertions(+), 122 deletions(-)
 create mode 100755 tools/testing/selftests/net/udpgro.sh
 create mode 100755 tools/testing/selftests/net/udpgro_bench.sh
 create mode 100644 tools/testing/selftests/net/xdp_dummy.c

2.17.2