linux-kernel - Re: [PATCH] arm64: relax assembly code alignment from 16 byte to 4 byte

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Date:   Thu, 31 Aug 2017 11:38:50 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Lothar Waßmann <LW@...O-electronics.de>
Cc:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Rob Herring <robh@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Marc Zyngier <marc.zyngier@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Russell King <linux@...linux.org.uk>,
        linux-kernel@...r.kernel.org, Olof Johansson <olof@...om.net>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arm64: relax assembly code alignment from 16 byte to 4
 byte

On 31/08/17 11:23, Lothar Waßmann wrote:
> Hi,
> 
> On Thu, 31 Aug 2017 18:56:23 +0900 Masahiro Yamada wrote:
>> Aarch64 instructions must be word aligned.  The current 16 byte
>> alignment is more than enough.  Relax it into 4 byte alignment.
>>
>> Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
>> ---
>>
>> I do not know why arm64 Linux requires 16 byte alignment.
>>
>> I dug git-history of arch/arm64/include/asm/linkage.h
>> and the only commit I see is:
>>
>>   commit aeed41a9371ee02257b608eb06a9058507a7d0f4
>>   Author: Marc Zyngier <Marc.Zyngier@....com>
>>   Date:   Fri Oct 19 17:33:27 2012 +0100
>>
>>       arm64: fix alignment padding in assembly code
>>
>> It just opt out of the asm-generic variant to remove 0x90.
>> So, the amount of alignment might not be not optimized yet.
>>
>> Please correct me if I am missing something.
>>
>>
>>  arch/arm64/include/asm/linkage.h | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/linkage.h b/arch/arm64/include/asm/linkage.h
>> index 636c1bc..1b26629 100644
>> --- a/arch/arm64/include/asm/linkage.h
>> +++ b/arch/arm64/include/asm/linkage.h
>> @@ -1,7 +1,7 @@
>>  #ifndef __ASM_LINKAGE_H
>>  #define __ASM_LINKAGE_H
>>  
>> -#define __ALIGN		.align 4
>> -#define __ALIGN_STR	".align 4"
>> +#define __ALIGN		.align 2
>> +#define __ALIGN_STR	".align 2"
>>  
>>  #endif
>>
> My math tells me, that 2 is one half of 4 but 4 is one fourth of 16, so
> this change doesn't line up with your commit message, or am I missing
> something?

2^4 = 16
2^2 = 4

The ARM behaviour of the .align directive is a bit funky...

Robin.

> 
> 
> Lothar Waßmann
> 
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@...ts.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
> 

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