linux-kernel - Re: The killing of ideal

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Date:   Wed, 10 Mar 2021 07:48:08 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>, bpf <bpf@...r.kernel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: The killing of ideal_nops[]

On Wed, Mar 10, 2021 at 6:29 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> 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 :-)

Ack for bpf bits.
I think the cleanup is good from the point of having one way to do things.
Though I won't be surprised if somebody comes along with a patch
to use different nops eventually.
When I first looked at it years ago I was wondering why segment selector
prefix is not used. afaik windows was using it, because having cs: ds: nop
makes intel use only one of the instruction decoders (the big and slow one)
which allegedly saves power, since the pipeline has bubbles.
Things could be completely different now in modern u-arches.