Re: [PATCH v3 8/9] x86: use __uaccess_begin_nospec and ASM_IFENCE in get_user paths

[Linus Torvalds]

On Sat, Jan 13, 2018 at 11:05 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> I _know_ that lfence is expensive as hell on P4, for example.
>
> Yes, yes, "sbb" is often more expensive than most ALU instructions,
> and Agner Fog says it has a 10-cycle latency on Prescott (which is
> outrageous, but being one or two cycles more due to the flags
> generation is normal). So the sbb/and may certainly add a few cycles
> to the critical path, but on Prescott "lfence" is *50* cycles
> according to those same tables by Agner Fog.

Side note: I don't think P4 is really relevant for a performance
discussion, I was just giving it as an example where we do know actual
cycles.

I'm much more interested in modern Intel big-core CPU's, and just
wondering whether somebody could ask an architect.

Because I _suspect_ the answer from a CPU architect would be: "Christ,
the sbb/and sequence is much better because it doesn't have any extra
serialization", but maybe I'm wrong, and people feel that lfence is
particularly easy to do right without any real downside.

                Linus