[PATCH v2 2/4] arm64: move early boot code to the .init segment

[Ard Biesheuvel]

On 14 April 2016 at 18:32, Will Deacon <will.deacon at arm.com> wrote:
> Hi Ard,
>
> On Wed, Mar 30, 2016 at 05:43:07PM +0200, Ard Biesheuvel wrote:
>> Apart from the arm64/linux and EFI header data structures, there is nothing
>> in the .head.text section that must reside at the beginning of the Image.
>> So let's move it to the .init section where it belongs.
>>
>> Note that this involves some minor tweaking of the EFI header, primarily
>> because the address of 'stext' no longer coincides with the start of the
>> .text section. It also requires a couple of relocated symbol references
>> to be slightly rewritten or their definition moved to the linker script.
>
> [...]
>
>>  ENTRY(stext)
>>       bl      preserve_boot_args
>>       bl      el2_setup                       // Drop to EL1, w20=cpu_boot_mode
>> @@ -223,12 +224,12 @@ ENTRY(stext)
>>        * the TCR will have been set.
>>        */
>>       ldr     x27, 0f                         // address to jump to after
>> -                                             // MMU has been enabled
>> +     neg     x27, x27                        // MMU has been enabled
>>       adr_l   lr, __enable_mmu                // return (PIC) address
>>       b       __cpu_setup                     // initialise processor
>>  ENDPROC(stext)
>>       .align  3
>> -0:   .quad   __mmap_switched - (_head - TEXT_OFFSET) + KIMAGE_VADDR
>> +0:   .quad   (_text - TEXT_OFFSET) - __mmap_switched - KIMAGE_VADDR
>
> I'm struggling to understand why you need to change this. Furthermore,
> it looks like the gas docs for expressions require that addition/subtraction
> can only be performed on arguments that are in the same section, so
> this feels pretty rickety.
>
> What's the problem you're solving here?
>

The problem is that this function is no longer in the same section as
_head aka _text, so the latter can only appear in the reloc expression
as a positive term, and the remaining ones will all be combined into
the addend.