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

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Message-ID: <20181210235809.GB22352@amd>
Date:   Tue, 11 Dec 2018 00:58:09 +0100
From:   Pavel Machek <pavel@....cz>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        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 2018-11-29 11:11:50, Linus Torvalds wrote:
> 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.

It might be easiest to use plain old NOP, no? :-).
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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