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Re: RFC: A new MIPS64 ABI

To: linux-mips <linux-mips@linux-mips.org>
Subject: Re: RFC: A new MIPS64 ABI
From: David Daney <ddaney@caviumnetworks.com>
Date: Thu, 17 Feb 2011 17:02:48 -0800
Cc: GCC <gcc@gcc.gnu.org>, binutils <binutils@sourceware.org>, Prasun Kapoor <prasun.kapoor@caviumnetworks.com>
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On 02/14/2011 12:29 PM, David Daney wrote:
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

That is not reality.  Really it is:

LUI $reg, R_MIPS_HI16
ADDIU $reg, R_MIPS_LO16



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


This one would really be:

ORI $reg, R_MIPS_HI16
DSLL $reg, $reg, 16
ADDIU $reg, R_MIPS_LO16



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).



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