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

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Date:   Fri, 29 Nov 2019 12:46:58 -0600
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        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()

Hi!

On Wed, Nov 27, 2019 at 04:15:15PM +0100, Christophe Leroy wrote:
> Le 27/11/2019 à 15:59, Segher Boessenkool a écrit :
> >On Wed, Nov 27, 2019 at 02:50:30PM +0100, Christophe Leroy wrote:
> >>So what do we do ? We just drop the "r2" clobber ?
> >
> >You have to make sure your asm code works for all ABIs.  This is quite
> >involved if you do a call to an external function.  The compiler does
> >*not* see this call, so you will have to make sure that all that the
> >compiler and linker do will work, or prevent some of those things (say,
> >inlining of the function containing the call).
> 
> But the whole purpose of the patch is to inline the call to __do_irq() 
> in order to avoid the trampoline function.

Yes, so you call __do_irq.  You have to make sure that what you tell the
compiler -- and what you *don't tell the compiler -- works with what the
ABIs require, and what the called function expects and provides.

> >That does not fix everything.  The called function requires a specific
> >value in r2 on entry.
> 
> Euh ... but there is nothing like that when using existing 
> call_do_irq().

> How does GCC know that call_do_irq() has same TOC as __do_irq() ?

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.

> >So all this needs verification.  Hopefully you can get away with just
> >not clobbering r2 (and not adding a nop after the bl), sure.  But this
> >needs to be checked.
> >
> >Changing control flow inside inline assembler always is problematic.
> >Another problem in this case (on all ABIs) is that the compiler does
> >not see you call __do_irq.  Again, you can probably get away with that
> >too, but :-)
> 
> Anyway it sees I reference it, as it is in input arguments. Isn't it 
> enough ?

It is enough for some things, sure.  But not all.


Segher