net: mitigate retpoline overhead

The spectre v2 counter-measures, aka retpolines, are a source of measurable
overhead[1]. We can partially address that when the function pointer refers to
a builtin symbol resorting to a list of tests vs well-known builtin function and
direct calls.

Experimental results show that replacing a single indirect call via
retpoline with several branches and a direct call gives performance gains
even when multiple branches are added - 5 or more, as reported in [2].

This may lead to some uglification around the indirect calls. In netconf 2018
Eric Dumazet described a technique to hide the most relevant part of the needed
boilerplate with some macro help.

This series is a [re-]implementation of such idea, exposing the introduced
helpers in a new header file. They are later leveraged to avoid the indirect
call overhead in the GRO path, when possible.

Overall this gives > 10% performance improvement for UDP GRO benchmark and
smaller but measurable for TCP syn flood.

The added infra can be used in follow-up patches to cope with retpoline overhead
in other points of the networking stack (e.g. at the qdisc layer) and possibly
even in other subsystems.

rfc -> v1:
 - use branch prediction hints, as suggested by Eric

v1  -> v2:
 - list explicitly the builtin function names in INDIRECT_CALL_*()
 - expand the recipients list

[1] http://vger.kernel.org/netconf2018_files/PaoloAbeni_netco...
[2] https://linuxplumbersconf.org/event/2/contributions/99/at...

Paolo Abeni (4):
  indirect call wrappers: helpers to speed-up indirect calls of builtin
  net: use indirect call wrappers at GRO network layer
  net: use indirect call wrappers at GRO transport layer
  udp: use indirect call wrappers for GRO socket lookup

 include/linux/indirect_call_wrapper.h | 51 +++++++++++++++++++++++++++
 include/net/inet_common.h             |  9 +++++
 net/core/dev.c                        | 15 ++++++--
 net/ipv4/af_inet.c                    | 13 +++++--
 net/ipv4/tcp_offload.c                |  6 ++--
 net/ipv4/udp_offload.c                | 15 +++++---
 net/ipv6/ip6_offload.c                | 18 +++++++---
 net/ipv6/tcpv6_offload.c              |  7 ++--
 net/ipv6/udp_offload.c                |  7 ++--
 9 files changed, 119 insertions(+), 22 deletions(-)
 create mode 100644 include/linux/indirect_call_wrapper.h

-- 
2.19.2