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Subject: Re: [PATCH v5] powerpc/irq: inline call_do_irq() and
 call_do_softirq() on PPC32
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
References: <72a6cd86137b2a7ab835213cf5c74df6ed2f6ea7.1575739197.git.christophe.leroy@c-s.fr>
 <20191212125222.GB3381@infradead.org>
From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <878e4ac8-9bfa-394d-8bca-f09a78f54904@c-s.fr>
Date:   Thu, 12 Dec 2019 17:34:46 +0100
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Le 12/12/2019 à 13:52, Christoph Hellwig a écrit :
> On Sat, Dec 07, 2019 at 05:20:04PM +0000, Christophe Leroy wrote:
>> call_do_irq() and call_do_softirq() are simple enough to be
>> worth inlining.
>>
>> Inlining them avoids an mflr/mtlr pair plus a save/reload on stack.
>> It also allows GCC to keep the saved ksp_limit in an nonvolatile reg.
>>
>> This is inspired from S390 arch. Several other arches do more or
>> less the same. The way sparc arch does seems odd thought.
> 
> Any reason you only do this for 32-bit and not 64-bit as well?
> 

Yes ... There has been a long discussion on this in v4, see 
https://patchwork.ozlabs.org/patch/1174288/

The problem is that on PPC64, r2 register is used as TOC pointer and it 
is apparently not straithforward to make sure the caller and the callee 
are using the same TOC.

On PPC32 it's more simple, r2 is current task_struct at all time, it 
never changes.

Christophe