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Subject: Re: [PATCH v4 2/2] powerpc/irq: inline call_do_irq() and
 call_do_softirq()
To:     Segher Boessenkool <segher@kernel.crashing.org>
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
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From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <ebc67964-e5a9-acd0-0011-61ba23692f7e@c-s.fr>
Date:   Wed, 4 Dec 2019 05:32:54 +0100
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Hi,

Le 29/11/2019 à 19:46, Segher Boessenkool a écrit :
> 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.

Indeed, there is only one caller for call_do_irq() which is do_IRQ().
And do_IRQ() is also calling __do_irq() directly (when the stack pointer 
is already set to IRQ stack). do_IRQ() and __do_irq() are both in 
arch/powerpc/kernel/irq.c

As far as I can see when replacing the call to call_do_irq() by a call 
to __do_irq(), the compiler doesn't do anything special with r2, and 
doesn't add any nop after the bl either, whereas for all calls outside 
irq.c, there is a nop added. So I guess that's ok ?

Now that call_do_irq() is inlined, we can even define __do_irq() as static.

And that's the same for do_softirq_own_stack(), it is only called from 
do_softirq() which is defined in the same file as __do_softirq() 
(kernel/softirq.c)

Christophe