• Dave Martin's avatar
    ARM: 6501/1: Thumb-2: Correct data alignment for CONFIG_THUMB2_KERNEL in mm/proc-v7.S · 6323875d
    Dave Martin authored
    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.
    
    In this specific case, we can achieve the desired alignment by
    forcing a 32-bit branch instruction using the W() macro, since the
    assembler location counter is already 32-bit aligned in this case.
    Reviewed-by: default avatarWill Deacon <will.deacon@arm.com>
    Signed-off-by: default avatarDave Martin <dave.martin@linaro.org>
    Acked-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
    Signed-off-by: default avatarRussell King <rmk+kernel@arm.linux.org.uk>
    6323875d
proc-v7.S 11 KB