Re: RFC: A new MIPS64 ABI

[David Daney <ddaney@caviumnetworks.com>]

On 02/15/2011 09:56 AM, Alexandre Oliva wrote:
> On Feb 14, 2011, David Daney<ddaney@caviumnetworks.com>  wrote:
>
> > 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.
> > 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.
> FTR, I don't really know why my Yeeloong is limited to 31-bit addresses,
> and I kind of hoped an n32 userland would improve that WRT o32, without
> wasting memory with longer pointers like n64 would.
>
> So, sorry if this is a dumb question, but wouldn't it be much easier to
> keep on using sign-extended addresses, and just make sure the kernel
> never allocates a virtual memory range that crosses a sign-bit change,
> or whatever other reason there is for addresses to be limited to the
> positive 2GB range in n32?
No, it is not possible.  The MIPS (and MIPS64) hardware architecture 
does not allow userspace access to addresses with the high bit (two bits 
for mips64) set.
Your complaint is a good summary of why I am thinking about n32-big.

David Daney