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

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Date:   Thu, 29 Nov 2018 18:15:39 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        David Laight <David.Laight@...lab.com>,
        Borislav Petkov <bp@...en8.de>, julia@...com, jeyu@...nel.org,
        "H. 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 08:59:31AM -0800, Andy Lutomirski wrote:

> If you make it conditional on CPL, do it for 32-bit as well, add
> comments, 

> and convince yourself that there isn’t a better solution
> (like pointing IP at a stub that retpolines to the target by reading
> the function pointer, a la the unoptimizable version), then okay, I
> guess, with only a small amount of grumbling.

Right; so we _could_ grow the trampoline with a retpoline indirect call
and ret. It just makes the trampoline a whole lot bigger, but it could
work.