[PATCH v2 1/3] arm64: Mention 4K aligned load addresses in the PIE Kconfig help

[André Przywara]

On 04/09/2020 19:42, Stephen Warren wrote:
> On 9/4/20 3:07 AM, Edgar E. Iglesias wrote:
>> From: "Edgar E. Iglesias" <edgar.iglesias at xilinx.com>
>>
>> Mention the requirement of 4K aligned load addresses in the
>> help section for the POSITION_INDEPENDENT option.
>>
>> Suggested-by: Michal Simek <michal.simek at xilinx.com>
>> Signed-off-by: Edgar E. Iglesias <edgar.iglesias at xilinx.com>
>> ---
>>  arch/arm/Kconfig | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
>> index f30c2639ec..c144c08612 100644
>> --- a/arch/arm/Kconfig
>> +++ b/arch/arm/Kconfig
>> @@ -21,6 +21,9 @@ config POSITION_INDEPENDENT
>>  	  information that is embedded in the binary to support U-Boot
>>  	  relocating itself to the top-of-RAM later during execution.
>>  
>> +	  When this option is enabled, U-Boot needs to be loaded at a
>> +	  4K aligned address.
> 
> I don't believe this restriction should be documented as part of
> POSITION_INDEPENDENT; the restriction always exists at least for 64-bit
> ARM, since arch/arm/lib/relocate_64.S relocate_code uses the same
> assembly sequence that imposes this restriction, and IIUC that code is
> unconditionally used.

While this is true, the difference is that without POSITION_INDEPENDENT
the alignment is easily determined by the hardcoded load address. So we
should actually have a build time check on this.

With POSITION_INDEPENDENT, however, the load address is only known at
runtime (somewhat under the user's control, if you like). So a warning
or hint here might be useful. But maybe it should be noted as a general
restriction in the paragraph above:
" ... from almost any address" => "from almost any 4K aligned address"

Cheers,
Andre.