linux-kernel - Re: X32 psABI status

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Date:	Sun, 13 Feb 2011 16:21:49 +0100
From:	Florian Weimer <fw@...eb.enyo.de>
To:	"H.J. Lu" <hjl.tools@...il.com>
Cc:	x32-abi@...glegroups.com, GCC Development <gcc@....gnu.org>,
	GNU C Library <libc-alpha@...rceware.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: X32 psABI status

* H. J. Lu:

> On Sun, Feb 13, 2011 at 7:07 AM, Florian Weimer <fw@...eb.enyo.de> wrote:
>> * H. J. Lu:
>>
>>>> Actually, I'm wondering if you can do the translation in user space.
>>>> There already are 32-on-64 implementations in existence, without
>>>> kernel changes (recent Hotspot, LuaJIT, and probably some more).
>>>
>>> Please check out the x32 kernel source and provide feedback.
>>
>> I still don't understand why you need a separate syscall table.  You
>> should really be able to run on an unmodified amd64 kernel, in 64 bit
>
> That is done on purpose. x32 is designed for environments where the
> current ia32 API is sufficient. You can think it as ia32 with register
> extended to 64bit plus 8 more registers. Everything else is still 32bit.

I think of it as amd64 where all the process memory happens to reside
in the first 4 GB of address space, and pointers are stored as 32 bits
(and you'd also reduce the size of longs because sizeof(long) !=
sizeof(void *) will break too many programs).

As I said, both LuaJIT and Hotspot are already using this model, with
custom memory allocators and a user-space translation layers, so I
still don't see what you get by changing the kernel.  LuaJIT has even
implemented the amd64 ABI, so you can call C libraries from your
32-bit code.  (Note that LuaJIT uses 64-bit words to store 32-bit
pointers with several tag bits, but it does so even on pure 32-bit
platforms.)

If you want to make x32 closer to i386, I don't see the point.  Why
would it be problematic if it was as close to i386 as, say, armel?
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