[PATCH net-next v3 0/4] net: mitigate retpoline overhead

From: Paolo Abeni
Date: Fri Dec 14 2018 - 05:52:23 EST

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.

v2 -> v3:
- fix build error with CONFIG_IPV6=m

v1 -> v2:
- list explicitly the builtin function names in INDIRECT_CALL_*(),
as suggested by Ed Cree
- expand the recipients list

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

[1] http://vger.kernel.org/netconf2018_files/PaoloAbeni_netconf2018.pdf
[2] https://linuxplumbersconf.org/event/2/contributions/99/attachments/98/117/lpc18_paper_af_xdp_perf-v2.pdf

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 | 35 +++++++++++++++---
net/ipv6/tcpv6_offload.c | 7 ++--
net/ipv6/udp_offload.c | 7 ++--
9 files changed, 136 insertions(+), 22 deletions(-)
create mode 100644 include/linux/indirect_call_wrapper.h