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Subject: Re: [PATCH 1/2] powerpc/irq: don't use current_stack_pointer() in
 check_stack_overflow()
From:   Christophe Leroy <christophe.leroy@c-s.fr>
To:     Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>
Cc:     linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
References: <bae3e75a0c7f9037e4012ee547842c04cd527931.1575871613.git.christophe.leroy@c-s.fr>
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Message-ID: <74cb4227-1a24-6fe1-2df4-3d4b069453c4@c-s.fr>
Date:   Fri, 24 Jan 2020 07:03:36 +0000
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On 01/24/2020 06:19 AM, Christophe Leroy wrote:
> 
> 
> Le 24/01/2020 à 06:46, Michael Ellerman a écrit :
>>
>> If I do this it seems to work, but feels a little dicey:
>>
>>     asm ("" : "=r" (r1));
>>     sp = r1 & (THREAD_SIZE - 1);
> 
> 
> Or we could do add in asm/reg.h what we have in boot/reg.h:
> 
> register void *__stack_pointer asm("r1");
> #define get_sp()    (__stack_pointer)
> 
> And use get_sp()
> 

It works, and I guess doing it this way is acceptable as it's exactly 
what's done for current in asm/current.h with register r2.

Now I (still) get:

	sp = get_sp() & (THREAD_SIZE - 1);
  b9c:	54 24 04 fe 	clrlwi  r4,r1,19
	if (unlikely(sp < 2048)) {
  ba4:	2f 84 07 ff 	cmpwi   cr7,r4,2047


Allthough GCC 8.1 what doing exactly the same with the form CLANG don't 
like:

	register unsigned long r1 asm("r1");
	long sp = r1 & (THREAD_SIZE - 1);
  b84:	54 24 04 fe 	clrlwi  r4,r1,19
	if (unlikely(sp < 2048)) {
  b8c:	2f 84 07 ff 	cmpwi   cr7,r4,2047


Christophe