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Re: RFC: A new MIPS64 ABI

On 02/14/2011 07:00 PM, Matt Thomas wrote:

On Feb 14, 2011, at 6:50 PM, David Daney wrote:

On 02/14/2011 06:33 PM, Matt Thomas wrote:

On Feb 14, 2011, at 6:22 PM, David Daney wrote:

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

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.

N32 doesn't require a LP64 kernel, just a 64-bit register aware kernel. Your N32-big does require a LP64 kernel.

But using 'official' kernel sources the only way to get a 64-bit register aware kernel is for it to also be LP64. So effectively, you do in fact need a 64-bit kernel to run n32 userspace code.

Not all the world is Linux. :) NetBSD supports N32 kernels.

Use of LP32 in the kernel is only really appropiate in systems with a small amount of memory. The proposed n32-big would run on such systems, but would probably *not* have an advantage over standard n32.

My proposed ABI would need trivial kernel changes:

o Fix a couple of places where pointers are sign extended instead of zero extended.

I think you'll find there are more of these than you'd expect.

You could be right, but to date in auditing the Linux kernel, sigaction() is the only place I have found.

o Change the stack address and address ranges returned by mmap().

My biggest concern is that many many mips opcodes expect properly sign-extended value for registers. Thusly N32-big will require using daddu/dadd/dsub/dsubu for addresses. So that's yet another departure from N32 which can use addu/add/sub/subu.

That's right. Which is why I said...

The main work would be in the compiler toolchain and runtime libraries.

You'd also need to update gas for la and dla expansion.

I am counting gas, ld and libc as part of the 'compiler toolchain'

David Daney

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