RFC: A new MIPS64 ABI

[David Daney]

On 02/14/2011 04:15 PM, Matt Thomas wrote:
>
> 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.
>

Yes, that is correct.  It is a 64-bit ABI, and like the existing n32 ABI 
requires a 64-bit kernel.


>> 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 is a userspace ABI.  The MIPS64 kernel already uses something similar 
(the -msym32 option).  There would be no change to the kernel.


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

In n64 pointers are 64-bits wide.  Programs that use many pointer laden 
data structures have a much larger cache/memory footprint than their n32 
versions.

Also the number of instructions required to load a 64-bit constant is 
much larger than that needed to load a 32-bit constant.

David Daney