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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 7 Jun 2019 10:48:06 -0700
Message-ID: <CAHk-=wg9dk6jXUcAE3FRkwHMqOra1MerS1Ehfgx-a2QQ22-Esg@mail.gmail.com>
Subject: Re: [PATCH 08/15] x86/alternatives: Teach text_poke_bp() to emulate instructions
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Masami Hiramatsu <mhiramat@kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Andy Lutomirski <luto@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        David Laight <David.Laight@aculab.com>,
        Borislav Petkov <bp@alien8.de>,
        Julia Cartwright <julia@ni.com>, Jessica Yu <jeyu@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Nadav Amit <namit@vmware.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Edward Cree <ecree@solarflare.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>
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On Fri, Jun 7, 2019 at 10:34 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> I was/am lazy and didn't want to deal with:
>
> arch/x86/include/asm/nops.h:#define GENERIC_NOP5_ATOMIC NOP_DS_PREFIX,GENERIC_NOP4
> arch/x86/include/asm/nops.h:#define K8_NOP5_ATOMIC 0x66,K8_NOP4
> arch/x86/include/asm/nops.h:#define K7_NOP5_ATOMIC NOP_DS_PREFIX,K7_NOP4
> arch/x86/include/asm/nops.h:#define P6_NOP5_ATOMIC P6_NOP5

Ugh. Maybe we could just pick one atomic sequence, and not have the
magic atomic nops be dynamic.

It's essentially what STATIC_KEY_INIT_NOP #define seems to do anyway.

NOP5_ATOMIC is already special, and not used for the normal nop
rewriting, only for kprobe/jump_label/ftrace.

So I suspect we could just replace all cases of

   ideal_nops[NOP_ATOMIC5]

with

   STATIC_KEY_INIT_NOP

and get rid of the whole "let's optimize the atomic 5-byte nop" entirely.

Hmm?

By definition, NOP_ATOMIC5 is just a single nop anyway, it's not used
for the potentially more complex alternative rewriting cases.

                Linus