Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64

[Linus Torvalds]

On Thu, Nov 29, 2018 at 10:00 AM Andy Lutomirski <luto@xxxxxxxxxxxxxx> 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