linux-kernel - Re: uml and -regparm=3

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Message-ID: <20080110021404.GA11287@c2.user-mode-linux.org>
Date:	Wed, 9 Jan 2008 21:14:04 -0500
From:	Jeff Dike <jdike@...toit.com>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	andi@...stfloor.org, harvey.harrison@...il.com,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	user-mode-linux-devel@...ts.sourceforge.net
Subject: Re: uml and -regparm=3

On Wed, Jan 09, 2008 at 10:50:48PM +0100, Miklos Szeredi wrote:
> > FASTCALL is useless and should not make a difference. It enables
> > regparm on specific functions, but that should not make a difference
> > if it works or not.
> 
> __down_write() in include/asm-x86/rwsem.h seems to assume, that the
> semaphore pointer is passed in %eax down to rwsem_down_write_failed(),
> so regparm does make a difference there.

And rwsem_down_write_failed seems to think it's getting the pointer in
%eax:

Dump of assembler code for function rwsem_down_write_failed:
0x08193599 <rwsem_down_write_failed+0>: push   %ebp
0x0819359a <rwsem_down_write_failed+1>: mov    %esp,%ebp
0x0819359c <rwsem_down_write_failed+3>: push   %ebx
0x0819359d <rwsem_down_write_failed+4>: mov    %eax,%ebx
0x0819359f <rwsem_down_write_failed+6>: sub    $0x10,%esp
0x081935a2 <rwsem_down_write_failed+9>: push   $0xffffffff
0x081935a4 <rwsem_down_write_failed+11>:        lea    0xffffffec(%ebp),%eax
0x081935a7 <rwsem_down_write_failed+14>:        push   %eax
0x081935a8 <rwsem_down_write_failed+15>:        push   %ebx
0x081935a9 <rwsem_down_write_failed+16>:        movl   $0x2,0xfffffff8(%ebp)
0x081935b0 <rwsem_down_write_failed+23>:        call   0x8193423 <rwsem_down_failed_common>

This is clearly taking something from %eax and something on the stack
(and a -1) and passing it to rwsem_down_failed_common, corresponding
to this:
	rwsem_down_failed_common(sem, &waiter,
				RWSEM_WAITING_BIAS - RWSEM_ACTIVE_BIAS);

So, this does look right to me.

				Jeff

-- 
Work email - jdike at linux dot intel dot com
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