[PATCH] riscv, bpf: add a speculation barrier for BPF_NOSPEC

[Lukas Gerlach]

Hi Paul,

thanks for the feedback.

I have not benchmarked fence.i in the eBPF context specifically, but
from my other work on Spectre mitigations on RISC-V I can confirm that
fence.i flushes the instruction cache on all cores I have tested, both
in-order and out-of-order, so there is a performance impact.

I agree that making this conditional similar to ARM64's proton-pack.c
is the right approach. Getting this infrastructure in place is a good
idea regardless, as the RISC-V hardware landscape is very diverse, and
we will likely need conditional mitigation support for other Spectre
defenses as well.

Thanks,
Lukas