On Thu, 25 Mar 2021 at 08:43, Peter Zijlstra <pet...@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-a...@kernel.org/
