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

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Message-ID: <20170907164651.3bmhdkbnxvj3pz7g@armageddon.cambridge.arm.com>
Date:   Thu, 7 Sep 2017 17:46:51 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Will Deacon <will.deacon@....com>,
        Rob Herring <robh@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Marc Zyngier <marc.zyngier@....com>,
        Russell King <linux@...linux.org.uk>,
        linux-kernel@...r.kernel.org, Olof Johansson <olof@...om.net>
Subject: Re: [PATCH] arm64: relax assembly code alignment from 16 byte to 4
 byte

On Thu, Aug 31, 2017 at 06:56:23PM +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.

Not sure why we ended up with 4. Possibly because we forgot the power of
2 difference for arm/arm64.

Queued for 4.14. Thanks.

-- 
Catalin