linux-kernel - Re: [PATCH] arch: avr32: add dummy syscalls

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Message-ID: <CACiLriQbrKvo128hhH4Cd3U+qZwgXPGFEAs4b3=SsK59=4Kkkw@mail.gmail.com>
Date:	Sun, 3 Feb 2013 20:52:18 -0800
From:	Håvard Skinnemoen <hskinnemoen@...il.com>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Hans-Christian Egtvedt <egtvedt@...fundet.no>,
	Matthias Brugger <matthias.bgg@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	David Howells <dhowells@...hat.com>,
	Dave Jones <davej@...hat.com>,
	Will Deacon <will.deacon@....com>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arch: avr32: add dummy syscalls

On Sun, Feb 3, 2013 at 7:02 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Mon, Feb 04, 2013 at 01:31:11AM +0000, Al Viro wrote:
>
>> Unless I'm misreading ocavr32.pdf, that should be (R12, R10:R11, R9, R8) and
>> (R12, R10:R11, R9:R8, stack) resp., so fadvise64 doesn't need a wrapper, but
>> fadvise64_64 does.  And something like (s32, s32, s64, s64) would turn into
>> (R12, R11, R9:R8, stack, stack); AFAICS, we don't have anything that ugly...
>
> Oh, yes, we do - fallocate(2).  int fd, int mode, loff_t offset, loff_t len.
> On something like mips or sparc32 it packs nicely; on avr32 it doesn't.
> Could you confirm that I haven't misparsed the ABI?

You're right on -- in this case, the compiler will skip r10, and do
(r12, r11, r8:r9, stack). We pass the syscall number in r8, but we
also unconditionally move r7 to r8 in the syscall path, so it
shouldn't matter (libc does the opposite when necessary).

I remember some talk about having the compiler reuse r10 for the next
32-bit argument in cases like this, but I don't think it ever
happened.

Havard
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