January 2018 was a sad time in the kernel community. The Meltdown and
Spectre vulnerabilities had finally been disclosed, and the required
workarounds hurt kernel performance in a number of ways. One of those
workarounds — retpolines —
continues to cause pain, with developers going
out of their way to avoid indirect calls, since they must now be implemented
with retpolines. In some cases, though, there may be a way to
avoid retpolines and regain much of the lost performance;
after a long gestation period, the “static calls” mechanism may finally be
nearing the point where it can be merged upstream.

Source: LWN.net – [$] Avoiding retpolines with static calls