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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Segher Boessenkool <segher@kernel.crashing.org>,
        Christophe Leroy <christophe.leroy@c-s.fr>
Cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v4 2/2] powerpc/irq: inline call_do_irq() and call_do_softirq()
In-Reply-To: <20191207174057.GY3152@gate.crashing.org>
References: <20191121101552.GR16031@gate.crashing.org> <87y2w49rgo.fsf@mpe.ellerman.id.au> <20191125142556.GU9491@gate.crashing.org> <5fdb1c92-8bf4-01ca-f81c-214870c33be3@c-s.fr> <20191127145958.GG9491@gate.crashing.org> <2072e066-1ffb-867e-60ec-04a6bb9075c1@c-s.fr> <20191129184658.GR9491@gate.crashing.org> <ebc67964-e5a9-acd0-0011-61ba23692f7e@c-s.fr> <20191206205953.GQ3152@gate.crashing.org> <2a22feca-d6d6-6cb0-6c76-035234fa8742@c-s.fr> <20191207174057.GY3152@gate.crashing.org>
Date:   Mon, 09 Dec 2019 21:53:11 +1100
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Segher Boessenkool <segher@kernel.crashing.org> writes:
> On Sat, Dec 07, 2019 at 10:42:28AM +0100, Christophe Leroy wrote:
>> Le 06/12/2019 =C3=A0 21:59, Segher Boessenkool a =C3=A9crit=C2=A0:
>> >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.
>>=20
>> Do we have a way to make sure which TOC the functions are using ? Is=20
>> 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 kern=
el
>> >module?  Is there anything that prevents this / can this not happen for
>> >some fundamental reason I don't see?
>>=20
>> 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=
=20
>> kernel/softirq.c
>>=20
>> 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=
=20
>> where the real benefit is I guess.
>>=20
>> At the end, maybe the solution should be to switch to IRQ stack=20
>> immediately in the exception entry as x86_64 do ?

Yeah that might be cleaner.

cheers