linux-kernel - [PATCH 0/2] x86: Remove ideal

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Date:   Fri, 12 Mar 2021 12:32:53 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     x86@...nel.org, rostedt@...dmis.org, hpa@...or.com,
        torvalds@...uxfoundation.org
Cc:     linux-kernel@...r.kernel.org, linux-toolchains@...r.kernel.org,
        peterz@...radead.org, jpoimboe@...hat.com,
        alexei.starovoitov@...il.com, mhiramat@...nel.org
Subject: [PATCH 0/2] x86: Remove ideal_nops[]

Hi!

A while ago Steve complained about x86 being weird for having different NOPs [1]

Having cursed the same thing before, I figured it was time to look at the NOP
situation.

32bit simply isn't a performance target anymore, so all we need is a set of
NOPs that works on all.

x86_64 has two main NOP variants, NOPL and prefix NOP. NOPL was introduced by
P6 and is architecturally mandated for x86_64. However, some uarchs made the
choice to limit NOPL decoding to a single port, which obviously limits NOPL
throughput. Other uarchs have (severe) decoding penalties for excessive (>~3)
prefixes, hobbling prefix NOP throughput.

But the thing is, all the modern uarchs can handle both without issue; that is
AMD K10 (2007) and later and Intel Ivy Bridge (2012) and later. The only
exception is Atom, which has the prefix penalty.

Since ultimate performance of a 10 year old chip (Intel Sandy Bridge, 2011) is
simply irrelevant today, remove variable NOPs and use NOPL.

This gives us deterministic NOPs and restores sanity.

[1] https://lkml.kernel.org/r/20210302105827.3403656c@gandalf.local.home