linux-kernel - Re: The killing of ideal

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Date:   Wed, 10 Mar 2021 15:24:47 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     hpa@...or.com, x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: The killing of ideal_nops[]

On Wed, Mar 10, 2021 at 09:13:24AM -0500, Steven Rostedt wrote:
> On Wed, 10 Mar 2021 11:22:48 +0100
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > After this FEATURE_NOPL is unused except for required-features for
> > x86_64. FEATURE_K8 is only used for PTI and FEATURE_K7 is unused.
> > 
> > AFAICT this negatively affects lots of 32bit (DONTCARE) and 32bit on
> > 64bit CPUs (CARELESS) and early AMD (K8) which is from 2003 and almost
> > 2 decades old by now (SHRUG).
> > 
> > Everything x86_64 since AMD K10 (2007) was using p6_nops.
> > 
> > And per FEATURE_NOPL being required for x86_64, all those CPUs can use
> > p6_nops. So stop caring about NOPs, simplify things and get on with life
> > :-)
> 
> Before ripping out all the ideal_nop logic, I wonder if we should just
> force the nops you want now (that is, don't change the selected
> ideal_nops, just "pretend" that the CPU wants p6_nops), and see if anyone
> complains. After a few releases, if there's no complaints, then we can
> rip out the ideal_nop logic.

Nah, just rip the entire thing out. You should be happy about
deterministic NOPs :-)

NOP encoding is not something CPUs should differentiate on, that's just
bollocks.