Re: [PATCH] static_call: fix function type mismatch

From: Ard Biesheuvel <ardb@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Sami Tolvanen <samitolvanen@google.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Arnd Bergmann <arnd@kernel.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Jason Baron <jbaron@akamai.com>, Ingo Molnar <mingo@redhat.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Frederic Weisbecker <frederic@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] static_call: fix function type mismatch
Date: Thu, 25 Mar 2021 08:45:56 +0100	[thread overview]
Message-ID: <CAMj1kXF9_Z5epEqyohCPWSfyYse+msq+Oyoekr9C_LXg+Pbx5w@mail.gmail.com> (raw)
In-Reply-To: <YFw+4Ba5ci/Bmg0k@hirez.programming.kicks-ass.net>

On Thu, 25 Mar 2021 at 08:43, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, Mar 25, 2021 at 01:42:41AM +0100, Rasmus Villemoes wrote:
> > > Actually, it looks like I can't select PREEMPT_DYNAMIC> and tweaking Kconfig
> >
> > Ah, there's no prompt on the "bool" line, so it doesn't show up. That
> > seems to be a mistake, since there's an elaborate help text which says
> >
> >           The runtime overhead is negligible with
> > HAVE_STATIC_CALL_INLINE enabled
> >           but if runtime patching is not available for the specific
> > architecture
> >           then the potential overhead should be considered.
> >
> > So it seems that it was meant to be "you can enable this if you really
> > want".
> >
> > to force enable it on arm64 results in a build error
>
> Right, PREEMPT_DYNAMIC really hard relies on HAVE_STATIC_CALL
>
> There's an implicit dependency in the select:
>
> config PREEMPT
>         ...
>         select PREEMPT_DYNAMIC if HAVE_PREEMPT_DYNAMIC
>
> > > ("implicit declaration of function 'static_call_mod'").
> >
> > Seems to be an omission in the last !HAVE_STATIC_CALL branch in
> > static_call_types.h, and there's also no
> > EXPORT_STATIC_CALL_TRAMP{,_GPL} in static_call.h for that case.
>
> That interface doesn't make sense for !HAVE_STATIC_CALL. It's impossible
> to not export the function pointer itself but still call it for
> !HAVE_STATIC_CALL.

I proposed an implementation for the indirect static call variety for
arm64 here [0] but we haven't yet decided whether it is needed, given
that indirect calls are mostly fine on arm64 (modulo CFI of course)

Maybe this helps?

[0] https://lore.kernel.org/linux-arm-kernel/20201120082103.4840-1-ardb@kernel.org/