linux-kernel - Re: [patch 5/6] s390: .align 4096 statements in head.S

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Date:	Tue, 18 Jul 2006 08:43:21 -0400
From:	"linux-os \(Dick Johnson\)" <linux-os@...logic.com>
To:	"Martin Schwidefsky" <schwidefsky@...ibm.com>
Cc:	<linux-kernel@...r.kernel.org>, <heiko.carstens@...ibm.com>
Subject: Re: [patch 5/6] s390: .align 4096 statements in head.S

On Tue, 18 Jul 2006, Martin Schwidefsky wrote:

> From: Heiko Carstens <heiko.carstens@...ibm.com>
>
> [S390] .align 4096 statements in head.S
>
> SLES9 binutils don't like .align 4096 statements in head.S. Work around this
> by using .org statements.
>
> Signed-off-by: Heiko Carstens <heiko.carstens@...ibm.com>
> Signed-off-by: Martin Schwidefsky <schwidefsky@...ibm.com>
> ---
>
> arch/s390/kernel/head31.S |    4 ++--
> arch/s390/kernel/head64.S |    4 ++--
> 2 files changed, 4 insertions(+), 4 deletions(-)
>
> diff -urpN linux-2.6/arch/s390/kernel/head31.S linux-2.6-patched/arch/s390/kernel/head31.S
> --- linux-2.6/arch/s390/kernel/head31.S	2006-07-18 13:40:23.000000000 +0200
> +++ linux-2.6-patched/arch/s390/kernel/head31.S	2006-07-18 13:40:46.000000000 +0200
> @@ -273,7 +273,7 @@ startup_continue:
> .Lbss_end:  .long _end
> .Lparmaddr: .long PARMAREA
> .Lsccbaddr: .long .Lsccb
> -	.align	4096
> +	.org	0x12000
> .Lsccb:
> 	.hword	0x1000			# length, one page
> 	.byte	0x00,0x00,0x00
> @@ -290,7 +290,7 @@ startup_continue:
> .Lscpincr2:
> 	.quad	0x00
> 	.fill	3984,1,0
> -	.align	4096
> +	.org	0x13000
>
> #ifdef CONFIG_SHARED_KERNEL
> 	.org	0x100000
> diff -urpN linux-2.6/arch/s390/kernel/head64.S linux-2.6-patched/arch/s390/kernel/head64.S
> --- linux-2.6/arch/s390/kernel/head64.S	2006-07-18 13:40:23.000000000 +0200
> +++ linux-2.6-patched/arch/s390/kernel/head64.S	2006-07-18 13:40:46.000000000 +0200
> @@ -268,7 +268,7 @@ startup_continue:
> .Lparmaddr:
> 	.quad	PARMAREA
>
> -	.align 4096
> +	.org	0x12000
> .Lsccb:
> 	.hword 0x1000			# length, one page
> 	.byte 0x00,0x00,0x00
> @@ -285,7 +285,7 @@ startup_continue:
> .Lscpincr2:
> 	.quad 0x00
> 	.fill 3984,1,0
> -	.align 4096
> +	.org	0x13000
>
> #ifdef CONFIG_SHARED_KERNEL
> 	.org   0x100000
> -

Hardcoading like that can cause hard to find errors. It looks like
you wrote something in 'C' and tried to use its assembly code. You
should know that you don't need ".fill" if you have correctly allocated
data.

The following will align objects on a 0x1000 boundary:

.section	.data
foo:	.word	0
.org	(. + 0x1000) & -0x1000
bar:	.word	0
.org	(. + 0x1000) & -0x1000
xxx:	.word	0
.org	(. + 0x1000) & -0x1000
yyy:	.word	0
.end

The 'gas' assembler is very powerful and even allows macros:

.macro	ALIGN val
.org	(. + \val) & -\val
.endm

.section	.data
foo:	.word	0
ALIGN	0x1000
bar:	.word	0
ALIGN	0x1000
xxx:	.word	0
ALIGN	0x1000
yyy:	.word	0
ALIGN	0x1000
qqq:	.word	0
.end

This generates:
Disassembly of section .data:

00000000 <foo>:
 	...
00001000 <bar>:
 	...
00002000 <xxx>:
 	...
00003000 <yyy>:
 	...
00004000 <qqq>:
 	...

Note the alignment.

You might want to use this kind of construction because it will
eliminate alignment errors. If you add something that's out-of-range
'gas' will generate an error (like attempt of a negative origin).

Cheers,
Dick Johnson
Penguin : Linux version 2.6.16.24 on an i686 machine (5592.63 BogoMips).
New book: http://www.AbominableFirebug.com/
_
..

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