From owner-freebsd-questions@FreeBSD.ORG  Mon Oct  8 19:59:49 2007
Return-Path: <owner-freebsd-questions@FreeBSD.ORG>
Delivered-To: freebsd-questions@FreeBSD.org
Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34])
	by hub.freebsd.org (Postfix) with ESMTP id 852BE16A420
	for <freebsd-questions@FreeBSD.org>;
	Mon,  8 Oct 2007 19:59:49 +0000 (UTC)
	(envelope-from jkim@FreeBSD.org)
Received: from anuket.mj.niksun.com (gwnew.niksun.com [65.115.46.162])
	by mx1.freebsd.org (Postfix) with ESMTP id 446F313C468
	for <freebsd-questions@FreeBSD.org>;
	Mon,  8 Oct 2007 19:59:49 +0000 (UTC)
	(envelope-from jkim@FreeBSD.org)
Received: from niksun.com (anuket [10.70.0.5])
	by anuket.mj.niksun.com (8.13.6/8.13.6) with ESMTP id l98JbBKg001756;
	Mon, 8 Oct 2007 15:37:11 -0400 (EDT) (envelope-from jkim@FreeBSD.org)
From: Jung-uk Kim <jkim@FreeBSD.org>
To: freebsd-emulation@FreeBSD.org
Date: Mon, 8 Oct 2007 15:37:01 -0400
User-Agent: KMail/1.6.2
References: <200710082135.58099.mihai.dontu@gmail.com>
In-Reply-To: <200710082135.58099.mihai.dontu@gmail.com>
MIME-Version: 1.0
Content-Disposition: inline
Content-Type: text/plain;
  charset="utf-8"
Content-Transfer-Encoding: 8bit
Message-Id: <200710081537.03836.jkim@FreeBSD.org>
X-Virus-Scanned: ClamAV 0.90.2/4507/Mon Oct 8 14:42:59 2007 on
	anuket.mj.niksun.com
X-Virus-Status: Clean
Cc: freebsd-questions@FreeBSD.org,
	Mihai =?utf-8?q?Don=C8=9Bu?= <mihai.dontu@gmail.com>
Subject: Re: amd64_set_gsbase()
X-BeenThere: freebsd-questions@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: User questions <freebsd-questions.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-questions>, 
	<mailto:freebsd-questions-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-questions>
List-Post: <mailto:freebsd-questions@freebsd.org>
List-Help: <mailto:freebsd-questions-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-questions>, 
	<mailto:freebsd-questions-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Mon, 08 Oct 2007 19:59:49 -0000

On Monday 08 October 2007 02:35 pm, Mihai Donțu wrote:
> Hi,
>
> I have a "small" amd64 program that makes havy use of LDT (%GS to
> be more specific). The trouble is, in a multithreaded environment,
> the selector value gets lost (or reset?).
>
> The code *always* segfaults with this stack:
>   4 LWP 100126  0x0000000800dec07c in select () from /lib/libc.so.6
> * 3 Thread 0x517000 (runnable)  0x000000080055cfbc in ?? ()
>   2 Thread 0x517400 (LWP 100125)  0x0000000800c0d85c in
> pthread_testcancel () from /lib/libpthread.so.2 1 Thread 0x517800
> (runnable)  0x0000000800d5d000 in makecontext () from
> /lib/libc.so.6
>
> at this instruction:
>   0x000000080055cfbc:     mov    %gs:0x10,%r11
>
>   (gdb) p $gs
>   $1 = 0
>
> I've been reading on the net something about the kernel not
> preserving the GS across syscalls (or stmh). Is this true? and if
> so, is there a known workaround?
>
> I'm on a FreeBSD 6.2-STABLE-200706 (AMD64) machine.

Yes, you are correct.  A short version is "don't do that".  A long 
version goes like this.  %fs and %gs are not preserved while context 
switching on amd64.  In fact, you should not use amd64_set_gsbase() 
directly.  If you *really* have to mess up with base addresses, you 
have to use sysarch(2) syscall, i.e., sysarch(AMD64_SET_GSBASE, 
args).  However, it only changes the base address via MSR, i.e., %gs 
itself has no meaning.

Jung-uk Kim