linux-kernel - Re: [PATCH v2 02/16] powerpc: override __ALIGN() and __ALIGN

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Message-Id: <CMJW5375AS4G.3VZ0XK6D3OZ9O@bobo>
Date:   Wed, 31 Aug 2022 13:24:05 +1000
From:   "Nicholas Piggin" <npiggin@...il.com>
To:     "Sathvika Vasireddy" <sv@...ux.ibm.com>,
        <linuxppc-dev@...ts.ozlabs.org>
Cc:     <jpoimboe@...hat.com>, <peterz@...radead.org>,
        <linux-kernel@...r.kernel.org>, <aik@...abs.ru>,
        <mpe@...erman.id.au>, <mingo@...hat.com>,
        <christophe.leroy@...roup.eu>, <rostedt@...dmis.org>,
        <mbenes@...e.cz>, <chenzhongjin@...wei.com>,
        <naveen.n.rao@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2 02/16] powerpc: override __ALIGN() and __ALIGN_STR()
 macros

On Mon Aug 29, 2022 at 3:52 PM AEST, Sathvika Vasireddy wrote:
> 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 __ALIGN() macro is:
> #define __ALIGN       .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.

Alignment can be desirable beyond the minimum requirement, for
example 16 byte alignment for functions could be helpful for
instruction fetch. So it should be explained why possible
benefits of the larger alignment are not worth it.

And if you have the patch in a series, it should be explained
why the patch is required for the series if it is not obvious.

Thanks,
Nick

>
> 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)
> +
>  #ifdef CONFIG_PPC64_ELF_ABI_V1
>  #define cond_syscall(x) \
>  	asm ("\t.weak " #x "\n\t.set " #x ", sys_ni_syscall\n"		\
> -- 
> 2.31.1