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

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Date:   Wed, 27 Nov 2019 16:15:15 +0100
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Segher Boessenkool <segher@...nel.crashing.org>
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()



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.

> 
>> Otherwise, to be on the safe side we can just save r2 in a local var
>> before the bl and restore it after. I guess it won't collapse CPU time
>> on a performant PPC64.
> 
> 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() ?

> 
> 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 ?

Christophe