linux-kernel - Re: 64-syscall args on 32-bit vs syscall()

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Message-ID: <1268684539.2335.22.camel@pasglop>
Date:	Tue, 16 Mar 2010 07:22:19 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	David Miller <davem@...emloft.net>
Cc:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	kernel@...savvy.com, drepper@...hat.com,
	torvalds@...ux-foundation.org, munroesj@...ux.vnet.ibm.com
Subject: Re: 64-syscall args on 32-bit vs syscall()

On Sun, 2010-03-14 at 22:54 -0700, David Miller wrote:
> From: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> Date: Mon, 15 Mar 2010 16:18:33 +1100
> 
> > Or is there any good reason -not- to do that in glibc ?
> 
> The whole point of syscall() is to handle cases where the C library
> doesn't know about the system call yet.
> 
> I think it's therefore very much "buyer beware".
> 
> On sparc it'll never work to use the workaround you're proposing since
> we pass everything in via registers.
> 
> So arch knowledge will always need to be present in these situations.

I'm not sure I follow. We also pass via register on powerpc, but the
offset introduced by the sysno argument breaks register pair alignment
which cannot be fixed up inside syscall().

However, if I change glibc's syscall to be something like

#define syscall(sysno, args...)	__syscall(0 /* dummy */, sysno, args)

And make __syscall then do something like:

	mr	r0, r4
	mr	r3, r5
	mr	r4, r6
	mr	r5, r7
	mr	r6, r8
	.../...
	sc
	blr

Then at least all that class of syscalls will be fixed. Of course this
has to be in glibc arch code. I was merely asking if that was something
our glibc folks would consider and whether somebody could think of a
better solution :-)

Cheers
,Ben.


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