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

From: David Miller
Date: Sat Dec 15 2018 - 16:23:37 EST

From: Paolo Abeni <pabeni@xxxxxxxxxx>
Date: Fri, 14 Dec 2018 11:51:56 +0100

> 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.

Series applied, I'll push this out after a build check completes.