linux-kernel - Re: [PATCH 02/16] powerpc: override __ALIGN() and __ALIGN

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Date:   Thu, 18 Aug 2022 16:16:47 +0530
From:   Sathvika Vasireddy <sv@...ux.vnet.ibm.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Cc:     "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "aik@...abs.ru" <aik@...abs.ru>,
        "mpe@...erman.id.au" <mpe@...erman.id.au>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "mbenes@...e.cz" <mbenes@...e.cz>,
        "npiggin@...il.com" <npiggin@...il.com>,
        "chenzhongjin@...wei.com" <chenzhongjin@...wei.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "naveen.n.rao@...ux.vnet.ibm.com" <naveen.n.rao@...ux.vnet.ibm.com>,
        Sathvika Vasireddy <sv@...ux.ibm.com>
Subject: Re: [PATCH 02/16] powerpc: override __ALIGN() and __ALIGN_STR()
 macros

Hi Christophe,

On 10/08/22 14:01, Christophe Leroy wrote:
>
> Le 08/08/2022 à 13:48, Sathvika Vasireddy a écrit :
>> Since we need an alignment of 4 bytes, override
>> __ALIGN() and __ALIGN_STR() accordingly.
> Why/When do we now need an alignment of 4 bytes ? Please explain the
> reason in the commit message.
Powerpc instructions must be word-aligned. Currently, there is an 
alignment of 16 bytes (by default),
and it is much more than what is required for powerpc (4 bytes).

The default expansion of these macros are:
#define __ALIGN        .align 4,0x90
#define __ALIGN_STR    ".align 4,0x90"

Since Powerpc Linux does not require a 16 byte alignment, override 
_ALIGN() and __ALIGN_STR() macros
to use required 4 byte alignment.

Sure, I'll explain the reason in commit message.
>
>> Signed-off-by: Sathvika Vasireddy <sv@...ux.ibm.com>
>> ---
>>    arch/powerpc/include/asm/linkage.h | 4 ++++
>>    1 file changed, 4 insertions(+)
>>
>> diff --git a/arch/powerpc/include/asm/linkage.h b/arch/powerpc/include/asm/linkage.h
>> index b71b9582e754..8df88fe61438 100644
>> --- a/arch/powerpc/include/asm/linkage.h
>> +++ b/arch/powerpc/include/asm/linkage.h
>> @@ -2,8 +2,12 @@
>>    #ifndef _ASM_POWERPC_LINKAGE_H
>>    #define _ASM_POWERPC_LINKAGE_H
>>    
>> +#include <linux/stringify.h>
>>    #include <asm/types.h>
>>    
>> +#define __ALIGN			.align 2
>> +#define __ALIGN_STR		__stringify(__ALIGN)
> Why do you need to use __stringify ?
>
> ARM64 does:
>
> #define __ALIGN		.align 2
> #define __ALIGN_STR	".align 2"
We could use either __stringify() or quotes, both do the same thing.
While arm64 does ".align 2",

x86 does:
#define __ALIGN         .p2align 4, 0x90
#define __ALIGN_STR     __stringify(__ALIGN)
>
>
>
>
>> +
>>    #ifdef CONFIG_PPC64_ELF_ABI_V1
>>    #define cond_syscall(x) \
>>    	asm ("\t.weak " #x "\n\t.set " #x ", sys_ni_syscall\n"		\

Thanks for reviewing!

- Sathvika