RFC 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.

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 under 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.

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 wrapper at GRO transport layer
udp: use indirect call wrapper for GRO socket lookup

 include/linux/indirect_call_wrapper.h | 77 +++++++++++++++++++++++++++
 include/net/inet_common.h             |  9 ++++
 net/core/dev.c                        | 10 +++-
 net/ipv4/af_inet.c                    | 15 +++++-
 net/ipv4/tcp_offload.c                |  5 ++
 net/ipv4/udp.c                        |  2 +
 net/ipv4/udp_offload.c                | 11 +++-
 net/ipv6/ip6_offload.c                | 14 ++++-
 net/ipv6/tcpv6_offload.c              |  5 ++
 net/ipv6/udp.c                        |  2 +
 net/ipv6/udp_offload.c                |  5 ++
 11 files changed, 147 insertions(+), 8 deletions(-)
 create mode 100644 include/linux/indirect_call_wrapper.h

2.19.2