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

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Date:   Fri, 30 Nov 2018 12:18:33 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        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 Fri, Nov 30, 2018 at 11:51 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Fri, Nov 30, 2018 at 10:39 AM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> >
> > AFAICT, all the other proposed options seem to have major issues.
>
> I still absolutely detest this patch, and in fact it got worse from
> the test of the config variable.
>
> Honestly, the entry code being legible and simple is more important
> than the extra cycle from branching to a trampoline for static calls.
>
> Just don't do the inline case if it causes this much confusion.

With my entry maintainer hat on, I don't mind it so much, although the
implementation needs some work.  The #ifdef should just go away, and
there should be another sanity check in the sanity check section.

Or we could replace that IPI with x86's bona fide serialize-all-cpus
primitive and then we can just retry instead of emulating.  It's a
piece of cake -- we just trigger an SMI :)  /me runs away.

--Andy