Re: 8aeb879baf12 - significant system call latency regression, bisected

[Peter Zijlstra]

On Mon, Jun 15, 2026 at 11:30:11AM -0700, H. Peter Anvin wrote:

> Well, we could put it under an #ifdef by putting macro to detect when we use
> -fno-jump-tables. PeterZ and I have also been talking about making
> -fno-jump-tables unconditional, because at some point we found that the
> performance difference was negligible, at least when array_index_nospec() is
> necessary, and it makes it a lot easier to tune when you don't have to deal
> with code bases that compile. It is not just retpoline but also IBT
> (although the comment says "for now"); this of course means in practice that
> the kernels everyone uses are compiled without jump tables.

The IBT thing is because GCC (and I assume, but haven't checked, clang
too) generated NOTRACK prefixes for jump tables. And we have explicitly
disallowed NOTRACK for kernel IBT.

The "not yet" pertains to the compilers being changed to not use
NOTRACK; but I don't think this is anything anybody is actively chasing
up on.

So yeah, effectively jump-tables are disabled for everybody.