Re: [RESEND][PATCH v3 14/17] static_call: Add static_cond_call()

[Peter Zijlstra <peterz@infradead.org>]

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 :/