From: Christophe Leroy <christophe.leroy@c-s.fr>
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
Subject: Re: [PATCH v4 2/2] powerpc/irq: inline call_do_irq() and call_do_softirq()
Date: Wed, 27 Nov 2019 16:15:15 +0100 [thread overview]
Message-ID: <2072e066-1ffb-867e-60ec-04a6bb9075c1@c-s.fr> (raw)
In-Reply-To: <20191127145958.GG9491@gate.crashing.org>
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
next prev parent reply other threads:[~2019-11-27 15:15 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-10 5:36 [PATCH v4 1/2] powerpc/irq: bring back ksp_limit management in C functions Christophe Leroy
2019-10-10 5:36 ` [PATCH v4 2/2] powerpc/irq: inline call_do_irq() and call_do_softirq() Christophe Leroy
2019-11-21 6:14 ` Michael Ellerman
2019-11-21 10:15 ` Segher Boessenkool
2019-11-25 10:32 ` Michael Ellerman
2019-11-25 14:25 ` Segher Boessenkool
2019-11-27 13:50 ` Christophe Leroy
2019-11-27 14:59 ` Segher Boessenkool
2019-11-27 15:15 ` Christophe Leroy [this message]
2019-11-29 18:46 ` Segher Boessenkool
2019-12-04 4:32 ` Christophe Leroy
2019-12-06 20:59 ` Segher Boessenkool
2019-12-07 9:42 ` Christophe Leroy
2019-12-07 17:40 ` Segher Boessenkool
2019-12-09 10:53 ` Michael Ellerman
2019-12-19 6:57 ` Christophe Leroy
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