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

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>

1 files changed, 4 insertions, 0 deletions

diff --git a/arch/arm/kernel/head.S b/arch/arm/kernel/head.S
index dd6b369ac69c..591c097c89ab 100644
--- a/arch/arm/kernel/head.S
+++ b/arch/arm/kernel/head.S
@@ -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