From mboxrd@z Thu Jan  1 00:00:00 1970
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Subject: Re: 64-syscall args on 32-bit vs syscall()
Date: Tue, 16 Mar 2010 07:27:09 +1100
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To: Ralf Baechle <ralf@linux-mips.org>
Cc: David Miller <davem@davemloft.net>, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, kernel@teksavvy.com, drepper@redhat.com, torvalds@linux-foundation.org, munroesj@linux.vnet.ibm.com

On Mon, 2010-03-15 at 14:44 +0100, Ralf Baechle wrote:
> Syscall is most often used for new syscalls that have no syscall stub in
> glibc yet, so the user of syscall() encodes this ABI knowledge.  If at a
> later stage syscall() is changed to have this sort of knowledge we break
> the API.  This is something only the kernel can get right. 

Well, no. The change I propose would not break the ABI on powerpc and
would auto-magically fix thoses cases :-) But again, you don't have to
do the same thing on MIPS or sparc, it's definitely arch specific.

IE. What you are saying is that a syscall defined in the kernel as:

	sys_foo(u64 arg);

To be called from userspace would require something like:

	u64 arg = 0x123456789abcdef01;

#if defined(__powerpc__) && WORDSIZE == 32
	syscall(SYS_foo, (u32)(arg >> 32), (u32)arg);
#ese
	syscall(SYS_foo, arg);

While with the trick of making syscall a macro wrapping an underlying
__syscall that has an added dummy argument, the register alignment is
"corrected" and thus -both- forms above suddenly work for me. That might
actually work for you too.

Cheers,
Ben.