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Date:   Wed, 25 Mar 2020 12:34:38 -0700
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Subject: Re: [RESEND][PATCH v3 14/17] static_call: Add static_cond_call()
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        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>,
        Andrew Lutomirski <luto@kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>
From:   hpa@zytor.com
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On March 24, 2020 9:33:21 AM PDT, Linus Torvalds <torvalds@linux-foundation=
=2Eorg> wrote:
>On Tue, Mar 24, 2020 at 9:22 AM Andy Lutomirski <luto@amacapital=2Enet>
>wrote:
>>
>> I haven=E2=80=99t checked if static calls currently support return valu=
es,
>but
>> the conditional case only makes sense for functions that return void=2E
>>
>> 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=
=2E
>
>Both good points=2E I take back my question=2E
>
>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)=2E
>
>I _think_ a "xorl %eax,%eax ; retq" is just three bytes and would fit
>in the tailcall slot too=2E
>
>                Linus

"movl $0,%eax" is five bytes, the same length as a call=2E Doesn't work fo=
r a tailcall, still, although if the sequence:

    jmp tailcall
    retq

=2E=2E=2E can be generated at the tailcall site then the jmp can get patch=
ed out=2E

This would be equivalent to disabling tailcalls except that the stack fram=
e is normally not unwound until between the call and the ret, so just disab=
ling tailcalls from the compiler pov doesn't work=2E
--=20
Sent from my Android device with K-9 Mail=2E Please excuse my brevity=2E