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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 24 Mar 2020 09:33:21 -0700
Message-ID: <CAHk-=wim-2aaFi_GNs5KmX4ykkgQjnRo5D4B9ZK+1Oz=kpp_2A@mail.gmail.com>
Subject: Re: [RESEND][PATCH v3 14/17] static_call: Add static_cond_call()
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        "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>
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On Tue, Mar 24, 2020 at 9:22 AM Andy Lutomirski <luto@amacapital.net> wrote=
:
>
> I haven=E2=80=99t 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=E2=80=99t 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?

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.

                Linus