linux-kernel - Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64

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Message-ID: <CAHk-=whxtKjkDbvA4EK0aU+FpqALyW+p79_nuPy99PjYFnkbvQ@mail.gmail.com>
Date:   Thu, 29 Nov 2018 11:11:50 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Lutomirski <luto@...nel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, mhiramat@...nel.org,
        jbaron@...mai.com, Jiri Kosina <jkosina@...e.cz>,
        David.Laight@...lab.com, bp@...en8.de, julia@...com,
        jeyu@...nel.org, Peter Anvin <hpa@...or.com>
Subject: Re: [PATCH v2 4/4] x86/static_call: Add inline static call
 implementation for x86-64

On Thu, Nov 29, 2018 at 11:08 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> What you can do then is basically add a single-byte prefix to the
> "call" instruction that does nothing (say, cs override), and then
> replace *that* with a 'int3' instruction.

Hmm. the segment prefixes are documented as being "reserved" for
branch instructions. I *think* that means just conditional branches
(Intel at one point used the prefixes for static prediction
information), not "call", but who knows..

It might be better to use an empty REX prefix on x86-64 or something like that.

               Linus