barrier_nospec for POWER

Hello,

there is x86 barrier_nospec which implements the same barrier as the
earlier out-of-tree gmb() barrier but instead of sprinkling the barrier
randomly in kernel code it put it in copy_from_user. We have the
firmware support for barrier_nospec already reported from the hcall
which is used for RFI flushes. So copy-pasting this from x86 should not
be that difficult.

However, there is separate patch which protects the NR_syscalls check of
syscall entry which does not apply on powerpc because the check is done
in assembly. I did not manage to write a barrier_nospec that compiles
both in C and asm so you are spared a RFC patch.

Is there some work in progress on addressing this?

Thanks

Michal