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Date:   Sat, 7 Dec 2019 11:40:57 -0600
From:   Segher Boessenkool <segher@kernel.crashing.org>
To:     Christophe Leroy <christophe.leroy@c-s.fr>
Cc:     Michael Ellerman <mpe@ellerman.id.au>,
        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()
Message-ID: <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>
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On Sat, Dec 07, 2019 at 10:42:28AM +0100, Christophe Leroy wrote:
> Le 06/12/2019 ? 21:59, Segher Boessenkool a ?crit?:
> >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?

> >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 :-(

> 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 ?

Maybe?  I'm out of my depth there.


Segher