Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 24 Mar 2020 18:03:06 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Andy Lutomirski <luto@amacapital.net>,
        the arch/x86 maintainers <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Jason Baron <jbaron@akamai.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, Nadav Amit <namit@vmware.com>,
        Peter Anvin <hpa@zytor.com>,
        Andrew Lutomirski <luto@kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: [RESEND][PATCH v3 14/17] static_call: Add static_cond_call()
Message-ID: <20200324170306.GU20696@hirez.programming.kicks-ass.net>
References: <CAHk-=wiumU4QxAkT+GqhBt5f-iUsoLNC0sqVfRKp0xypA6aNYg@mail.gmail.com>
 <86D80EA7-9087-4042-8119-12DD5FCEAA87@amacapital.net>
 <CAHk-=wim-2aaFi_GNs5KmX4ykkgQjnRo5D4B9ZK+1Oz=kpp_2A@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <CAHk-=wim-2aaFi_GNs5KmX4ykkgQjnRo5D4B9ZK+1Oz=kpp_2A@mail.gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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@amacapital.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 :/