[PATCH v2] powerpc/64: BE option to use ELFv2 ABI for big endian kernels

[Nicholas Piggin]

Excerpts from Segher Boessenkool's message of April 29, 2020 9:40 am:
> Hi!
> 
> On Tue, Apr 28, 2020 at 09:25:17PM +1000, Nicholas Piggin wrote:
>> +config BUILD_BIG_ENDIAN_ELF_V2
>> +	bool "Build big-endian kernel using ELFv2 ABI (EXPERIMENTAL)"
>> +	depends on PPC64 && CPU_BIG_ENDIAN && EXPERT
>> +	default n
>> +	select BUILD_ELF_V2
>> +	help
>> +	  This builds the kernel image using the ELFv2 ABI, which has a
>> +	  reduced stack overhead and faster function calls. This does not
>> +	  affect the userspace ABIs.
>> +
>> +	  ELFv2 is the standard ABI for little-endian, but for big-endian
>> +	  this is an experimental option that is less tested (kernel and
>> +	  toolchain). This requires gcc 4.9 or newer and binutils 2.24 or
>> +	  newer.
> 
> Is it clear that this is only for 64-bit?  Maybe this text should fit
> that in somewhere?

Don't know if it's necessary, the option only appears when 64-bit is
selected.

> It's not obvious to people who do not already know that ELFv2 is just
> the (nick-)name of a particular ABI, not a new kind of ELF (it is just
> version 1 ELF in fact), and that ABI is for 64-bit Power only.

I blame toolchain for -mabi=elfv2 ! And also some blame on ABI document 
which is called ELF V2 ABI rather than ELF ABI V2 which would have been 
unambiguous.

Kernel mostly gets it right (in the code I didn't write), and uses these

#if defined(_CALL_ELF) && _CALL_ELF == 2
#define PPC64_ELF_ABI_v2
#else
#define PPC64_ELF_ABI_v1
#endif

I can go through and change all my stuff and config options to ELF_ABI_v2.

Thanks,
Nick