linux-kernel - Re: [RESEND][PATCH v3 14/17] static_call: Add static_cond

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Message-ID: <20200324170306.GU20696@hirez.programming.kicks-ass.net>
Date:   Tue, 24 Mar 2020 18:03:06 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Jason Baron <jbaron@...mai.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Nadav Amit <namit@...are.com>,
        Peter Anvin <hpa@...or.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [RESEND][PATCH v3 14/17] static_call: Add static_cond_call()

On Tue, Mar 24, 2020 at 09:33:21AM -0700, Linus Torvalds wrote:
> On Tue, Mar 24, 2020 at 9:22 AM Andy Lutomirski <luto@...capital.net> wrote:
> >
> > I haven’t checked if static calls currently support return values, but
> > the conditional case only makes sense for functions that return void.
> >
> > Aside from that, it might be nice for passing NULL in to warn or bug
> > when the NULL pointer is stored instead of silently NOPping out the
> > call in cases where having a real implementation isn’t optional.
> 
> Both good points. I take back my question.
> 
> And it aside from warning about passing in NULL then it doesn't work,
> I wonder if we could warn - at build time - when then using the COND
> version with a function that doesn't return void?

I actually (abuse) do that in the last patch... the reason being that
DEFINE_STATIC_COND_CALL() ends up only needing a type expression for the
second argument, while DEFINE_STATIC_CALL() needs an actual function.

> Of course, one alternative is to just say "instead of using NOP, use
> 'xorl %eax,%eax'", and then we'd have the rule that a NULL conditional
> function returns zero (or NULL).
> 
> I _think_ a "xorl %eax,%eax ; retq" is just three bytes and would fit
> in the tailcall slot too.

Correct. The only problem is that our text patching machinery can't
replace multiple instructions :/