[PATCH] riscv, bpf: Emit fence.i for BPF_NOSPEC

[Lukas Gerlach]

Hi Luis,

thanks for the feedback and pointer to the powerpc implementation.

Regarding bpf_jit_bypass_spec_v1/v4(): currently this is per-architecture.
What we need is per-processor granularity, so we can disable mitigations
on in-order cores and keep them enabled on vulnerable out-of-order processors.

Regarding fence.i being an extension: all RISC-V processors supported by the
kernel that are vulnerable to these attacks support this instruction. Also,
if a dedicated speculation barrier becomes available in the future, it would
be easy to switch to that.

Thanks,
Lukas