RFC: A new MIPS64 ABI

[David Daney <ddaney at caviumnetworks dot com>]

Background:

Current MIPS 32-bit ABIs (both o32 and n32) are restricted to 2GB of
user virtual memory space.  This is due the way MIPS32 memory space is
segmented.  Only the range from 0..2^31-1 is available.  Pointer
values are always sign extended.

Because there are not already enough MIPS ABIs, I present the ...

Proposal: A new ABI to support 4GB of address space with 32-bit
pointers.

The proposed new ABI would only be available on MIPS64 platforms.  It
would be identical to the current MIPS n32 ABI *except* that pointers
would be zero-extended rather than sign-extended when resident in
registers.  In the remainder of this document I will call it
'n32-big'.  As a result, applications would have access to a full 4GB
of virtual address space.  The operating environment would be
configured such that the entire lower 4GB of the virtual address space
was available to the program.


At a low level here is how it would work:

1) Load a pointer to a register from memory:

n32:
	LW $reg, offset($reg)

n32-big:
	LWU $reg, offset($reg)

2) Load an address constant into a register:

n32:
	LUI $reg, high_part
	ORI $reg, low_part

n32-big:
	ORI $reg, high_part
	DSLL $reg, $reg, 16
	ORI $reg, low_part


Q: What would have to change to make this work?

o A new ELF header flag to denote the ABI.

o Linker support to use proper library search paths, and linker scrips
  to set the INTERP program header, etc.

o GCC has to emit code for the new ABI.

o Could all existing n32 relocation types be used?  I think so.

o Runtime libraries would have to be placed in a new location
  (/lib32big, /usr/lib32big ...)

o The C library's ld.so would have to use a distinct LD_LIBRARY_PATH
  for n32-big code.

o What would the Linux system call interface be?  I would propose
  using the existing Linux n32 system call interface.  Most system
  calls would just work.  Some, that pass pointers in in-memory
  structures, might require kernel modifications (sigaction() for
  example).