Re: RFC: A new MIPS64 ABI

[Matt Thomas <jabbathespud at gmail dot com>]

On Feb 14, 2011, at 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.

I have to wonder if it's worth the effort.  The primary problem I see
is that this new ABI requires a 64bit kernel since faults through the
upper 2G will go through the XTLB miss exception vector.  

> 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)


That might be sufficient for userland, but the kernel will need
to do similar things (even if a 64bit kernel) when accessing 
structures supplied by 32-bit syscalls.  

It seems to be workable but if you need the additional address space
why not use N64?