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