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ARM: 6500/1: Thumb-2: Correct data alignment for CONFIG_THUMB2_KERNEL in kernel/head.S

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Directives such as .long and .word do not magically cause the
assembler location counter to become aligned in gas.  As a result,
using these directives in code sections can result in misaligned
data words when building a Thumb-2 kernel (CONFIG_THUMB2_KERNEL).

This is a Bad Thing, since the ABI permits the compiler to assume
that fundamental types of word size or above are word- aligned when
accessing them from C.  If the data is not really word-aligned,
this can cause impaired performance and stray alignment faults in
some circumstances.

In general, the following rules should be applied when using data
word declaration directives inside code sections:

    * .quad and .double:
         .align 3

    * .long, .word, .single, .float:
         .align (or .align 2)

    * .short:
        No explicit alignment required, since Thumb-2
        instructions are always 2 or 4 bytes in size.
        immediately after an instruction.

Reviewed-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Dave Martin <dave.martin@linaro.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
This commit is contained in:
Dave Martin 2010-11-29 19:43:24 +01:00 committed by Russell King
parent 077248fcce
commit 4f79a5dd7c
1 changed files with 4 additions and 0 deletions
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4
arch/arm/kernel/head.S
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@ -262,6 +262,7 @@ __create_page_tables:
mov pc, lr
ENDPROC(__create_page_tables)
.ltorg
.align
__enable_mmu_loc:
.long .
.long __enable_mmu
@ -308,6 +309,8 @@ ENTRY(__secondary_switched)
b secondary_start_kernel
ENDPROC(__secondary_switched)
.align
.type __secondary_data, %object
__secondary_data:
.long .
@ -413,6 +416,7 @@ __fixup_smp_on_up:
mov pc, lr
ENDPROC(__fixup_smp)
.align
1: .word .
.word __smpalt_begin
.word __smpalt_end