linux-kernel - Re: [PATCH v2 0/4] Static calls

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Date:   Wed, 5 Dec 2018 09:04:22 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Steven Rostedt <rostedt@...dmis.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.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 Cartwright <julia@...com>, Jessica Yu <jeyu@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 0/4] Static calls

On Tue, Dec 04, 2018 at 03:41:01PM -0800, Andy Lutomirski wrote:
> 
> 
> > On Dec 4, 2018, at 3:08 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> > 
> > 
> > Where did this end up BTW?
> > 
> > I know that there's controversy about the
> > CONFIG_HAVE_STATIC_CALL_OPTIMIZED option, but I don't think the 
> > CONFIG_HAVE_STATIC_CALL_UNOPTIMIZED version was controversial. From the
> > v1 patch 0 description:
> > 
> > There are three separate implementations, depending on what the arch
> > supports:
> > 
> >  1) CONFIG_HAVE_STATIC_CALL_OPTIMIZED: patched call sites - requires
> >     objtool and a small amount of arch code
> > 
> >  2) CONFIG_HAVE_STATIC_CALL_UNOPTIMIZED: patched trampolines - requires
> >     a small amount of arch code
> > 
> >  3) If no arch support, fall back to regular function pointers
> > 
> > My benchmarks showed the best improvements with the
> > STATIC_CALL_OPTIMIZED, but it still showed improvement with the
> > UNOPTIMIZED version as well. Can we at least apply 2 and 3 from the
> > above (which happen to be the first part of the patch set. 1 comes in
> > at the end).
> 
> Sounds good to me.
> 
> > 
> > I would also just call it CONFIG_STATIC_CALL. If we every agree on the
> > optimized version, then we can call it CONFIG_STATIC_CALL_OPTIMIZED.
> > Have an option called UNOPTIMIZED just seems wrong.

(Poking my head up for a bit, soon to disappear again until next week)

Ard had already objected to "unoptimized", which was why for v2 I
renamed them to CONFIG_STATIC_CALL_OUTLINE and CONFIG_STATIC_CALL_INLINE.

I could rename it to CONFIG_STATIC_CALL and CONFIG_STATIC_CALL_INLINE if
you prefer.  I don't have much of an opinion either way.

I'll post a v3 next week or so, with the controversial bits more fully
separated from the non-controversial bits.  So at least the out-of-line
implementation can get merged.

> My objection to all the bike shed colors so far is that we *always*
> have static_call() — it’s just not always static.

Hm?  Do you mean you don't like that we have a generic function pointer
implementation?  or what?

> Anyway, I have a new objection to Josh’s create_gap proposal: what on
> Earth will kernel CET do to it?  Maybe my longjmp-like hack is
> actually better.

Does CET even care about iret?  I assumed it didn't.  If it does, your
proposal would have the same problem, no?

-- 
Josh