linux-kernel - Re: [PATCH v4 2/2] powerpc/irq: inline call_do_irq() and call_do

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Date:   Mon, 09 Dec 2019 21:53:11 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Segher Boessenkool <segher@...nel.crashing.org>,
        Christophe Leroy <christophe.leroy@....fr>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v4 2/2] powerpc/irq: inline call_do_irq() and call_do_softirq()

Segher Boessenkool <segher@...nel.crashing.org> writes:
> On Sat, Dec 07, 2019 at 10:42:28AM +0100, Christophe Leroy wrote:
>> Le 06/12/2019 à 21:59, Segher Boessenkool a écrit :
>> >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.

>> >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.

>> 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.

cheers