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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 29 Nov 2018 10:42:09 -0800
Message-ID: <CAHk-=whS-A2SmC_+qvzv2hxZZVqxpftzc4jxwJC9=yqzN+jX8g@mail.gmail.com>
Subject: Re: [PATCH v2 4/4] x86/static_call: Add inline static call
 implementation for x86-64
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrew Lutomirski <luto@kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, mhiramat@kernel.org,
        jbaron@akamai.com, Jiri Kosina <jkosina@suse.cz>,
        David.Laight@aculab.com, bp@alien8.de, julia@ni.com,
        jeyu@kernel.org, Peter Anvin <hpa@zytor.com>
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On Thu, Nov 29, 2018 at 10:00 AM Andy Lutomirski <luto@amacapital.net> wrote:
> > then it really sounds pretty safe to just say "ok, just make it
> > aligned and update the instruction with an atomic cmpxchg or
> > something".
>
> And how do we do that?  With a gcc plugin and some asm magic?

Asm magic.

You already have to mark the call sites with

    static_call(fn, arg1, arg2, ...);

and while it right now just magically depends on gcc outputting the
right code to call the trampoline. But it could do it as a jmp
instruction (tail-call), and maybe that works right, maybe it doesn't.
And maybe some gcc switch makes it output it as a indirect call due to
instrumentation or something. Doing it with asm magic would, I feel,
be safer anyway, so that we'd know *exactly* how that call gets done.

For example, if gcc does it as a jmp due to a tail-call, the
compiler/linker could in theory turn the jump into a short jump if it
sees that the trampoline is close enough. Does that happen? Probably
not. But I don't see why it *couldn't* happen in the current patch
series. The trampoline is just a regular function, even if it has been
defined by global asm.

Putting the trampoline in a different code section could fix things
like that (maybe there was a patch that did that and I missed it?) but
I do think that doing the call with an asm would *also* fix it.

But the "just always use a trampoline" is certainly the simpler model.

                  Linus