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

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Message-ID: <20130204050550.GA4503@ZenIV.linux.org.uk>
Date:	Mon, 4 Feb 2013 05:05:50 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	H?vard Skinnemoen <hskinnemoen@...il.com>
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 03, 2013 at 08:52:18PM -0800, H?vard Skinnemoen wrote:

> 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.

Umm...  In case of fallocate() the next argument is 64bit one, though;
sys_fallocate() will be looking for two 32bit words on stack, so no
matter how do we pass them to syscall, we'd better push two words in
the wrapper.

But yes, 32bit/32bit/64bit/32bit is another interesting case -
fanotify_mark is 32/32/64/32/32.  From what ABI says it would seem to
be r12/r11/r8:r9/r10/stack, but if I understand you correctly, we'll
end up wanting *two* arguments on stack...
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