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Message-ID: <4D5AC12D.3080108@caviumnetworks.com>
Date:   Tue, 15 Feb 2011 10:08:45 -0800
From:   David Daney <ddaney@caviumnetworks.com>
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To:     Alexandre Oliva <aoliva@redhat.com>
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Subject: Re: RFC: A new MIPS64 ABI
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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