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

[Christophe Leroy]

Le 09/12/2019 Ã 11:53, Michael Ellerman a ÃcritÂ:
> Segher Boessenkool <segher@xxxxxxxxxxxxxxxxxxx> writes:
> > On Sat, Dec 07, 2019 at 10:42:28AM +0100, Christophe Leroy wrote:
> > > Le 06/12/2019 Ã 21:59, Segher Boessenkool a ÃcritÂ:
> > > > 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 ?
> >
> > Kernel modules have their own TOC, I think?
>
> Yes.

Yes, this means that exported functions have to care about that, right ?
And that's the reason why exported assembly functions like copy_page() 
use _GLOBAL_TOC() and not _GLOBAL()

But main part of the kernel only has one TOC, so r2 can be assumed 
constant for non exported functions, can't it ?

> > > > 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 ?
> >
> > Hopefully, sure.  Would be nice if it was clearer that this works...  It
> > is too much like working by chance, the way it is :-(
>
> There's no way any of that code can end up in a module. Or at least if
> there is, that's a bug.

That's my conclusion as well. So I guess we can consider r2 as constant 
over those functions.

> > > 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 ?
>
> Yeah that might be cleaner.

I prepared a patch for that on PPC32, but it doesn't get rid of the IRQ 
stack switch completely because do_IRQ() is also called from other 
places like the timer interrupt.

And we will still have the switch for softirqs. We could move 
do_softirq_own_stack() to assembly and merge it with call_do_softirq(), 
but a find it cleaner to inline call_do_softirq() instead, now that we 
have demonstrated that r2 can't change.

Christophe