Re: [PATCH v4 2/2] powerpc/irq: inline call_do_irq() and call_do_softirq()

[Christophe Leroy]

Le 06/12/2019 Ã 21:59, Segher Boessenkool a ÃcritÂ:
> On Wed, Dec 04, 2019 at 05:32:54AM +0100, Christophe Leroy wrote:
> > Le 29/11/2019 Ã 19:46, Segher Boessenkool a ÃcritÂ:
> > > The existing call_do_irq isn't C code.  It doesn't do anything with r2,
> > > as far as I can see; __do_irq just gets whatever the caller of call_do_irq
> > > has.
> > >
> > > So I guess all the callers of call_do_irq have the correct r2 value always
> > > already?  In that case everything Just Works.
> >
> > Indeed, there is only one caller for call_do_irq() which is do_IRQ().
> > And do_IRQ() is also calling __do_irq() directly (when the stack pointer
> > is already set to IRQ stack). do_IRQ() and __do_irq() are both in
> > arch/powerpc/kernel/irq.c
> >
> > As far as I can see when replacing the call to call_do_irq() by a call
> > to __do_irq(), the compiler doesn't do anything special with r2, and
> > doesn't add any nop after the bl either, whereas for all calls outside
> > irq.c, there is a nop added. So I guess that's ok ?
>
> If the compiler can see the callee wants the same TOC as the caller has,
> it does not arrange to set (and restore) it, no.  If it sees it may be
> different, it does arrange for that (and the linker then will check if
> it actually needs to do anything, and do that if needed).
>
> In this case, the compiler cannot know the callee wants the same TOC,
> which complicates thing a lot -- but it all works out.

Do we have a way to make sure which TOC the functions are using ? Is 
there several TOC at all in kernel code ?

> > Now that call_do_irq() is inlined, we can even define __do_irq() as static.
> >
> > And that's the same for do_softirq_own_stack(), it is only called from
> > do_softirq() which is defined in the same file as __do_softirq()
> > (kernel/softirq.c)
>
> I think things can still go wrong if any of this is inlined into a kernel
> module?  Is there anything that prevents this / can this not happen for
> some fundamental reason I don't see?

This can't happen can it ?
do_softirq_own_stack() is an outline function, defined in powerpc irq.c
Its only caller is do_softirq() which is an outline function defined in 
kernel/softirq.c

That prevents inlining, doesn't it ?


Anyway, until we clarify all this I'll limit my patch to PPC32 which is 
where the real benefit is I guess.

At the end, maybe the solution should be to switch to IRQ stack 
immediately in the exception entry as x86_64 do ?

And do_softirq_own_stack() could be entirely written in assembly like 
x86_64 as well ?

Christophe