Merge branch 'udp-gso'

Willem de Bruijn says:

====================
udp gso

Segmentation offload reduces cycles/byte for large packets by
amortizing the cost of protocol stack traversal.

This patchset implements GSO for UDP. A process can concatenate and
submit multiple datagrams to the same destination in one send call
by setting socket option SOL_UDP/UDP_SEGMENT with the segment size,
or passing an analogous cmsg at send time.

The stack will send the entire large (up to network layer max size)
datagram through the protocol layer. At the GSO layer, it is broken
up in individual segments. All receive the same network layer header
and UDP src and dst port. All but the last segment have the same UDP
header, but the last may differ in length and checksum.

Initial results show a significant reduction in UDP cycles/byte.
See the main patch for more details and benchmark results.

        udp
          876 MB/s 14873 msg/s 624666 calls/s
            11,205,777,429      cycles

        udp gso
         2139 MB/s 36282 msg/s 36282 calls/s
            11,204,374,561      cycles

The patch set is broken down as follows:
- patch 1 is a prerequisite: code rearrangement, noop otherwise
- patch 2 implements the gso logic
- patch 3 adds protocol stack support for UDP_SEGMENT
- patch 4,5,7 are refinements
- patch 6 adds the cmsg interface
- patch 8..11 are tests

This idea was presented previously at netconf 2017-2
http://vger.kernel.org/netconf2017_files/rx_hardening_and_udp_gso.pdf

Changes v1 -> v2
  - Convert __udp_gso_segment to modify headers after skb_segment
  - Split main patch into two, one for gso logic, one for UDP_SEGMENT

Changes RFC -> v1
  - MSG_MORE:
      fixed, by allowing checksum offload with corking if gso
  - SKB_GSO_UDP_L4:
      made independent from SKB_GSO_UDP
      and removed skb_is_ufo() wrapper
  - NETIF_F_GSO_UDP_L4:
      add to netdev_features_string
      and to netdev-features.txt
      add BUILD_BUG_ON to match SKB_GSO_UDP_L4 value
  - UDP_MAX_SEGMENTS:
      introduce limit on number of segments per gso skb
      to avoid extreme cases like IP_MAX_MTU/IPV4_MIN_MTU
  - CHECKSUM_PARTIAL:
      test against missing feature after ndo_features_check
      if not supported return error, analogous to udp_send_check
  - MSG_ZEROCOPY: removed, deferred for now
====================

Signed-off-by: David S. Miller <davem@davemloft.net>