From owner-freebsd-amd64@FreeBSD.ORG  Tue Jun  1 23:48:52 2004
Return-Path: <owner-freebsd-amd64@FreeBSD.ORG>
Delivered-To: freebsd-amd64@freebsd.org
Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125])
	by hub.freebsd.org (Postfix) with ESMTP id 3392216A4CE
	for <amd64@FreeBSD.ORG>; Tue,  1 Jun 2004 23:48:52 -0700 (PDT)
Received: from VARK.homeunix.com
	(ar59.lsanca2-4.27.98.47.lsanca2.dsl-verizon.net [4.27.98.47])
	by mx1.FreeBSD.org (Postfix) with ESMTP id E63BA43D1D
	for <amd64@FreeBSD.ORG>; Tue,  1 Jun 2004 23:48:51 -0700 (PDT)
	(envelope-from das@FreeBSD.ORG)
Received: from VARK.homeunix.com (localhost [127.0.0.1])
	by VARK.homeunix.com (8.12.10/8.12.10) with ESMTP id i526mlkj006232
	for <amd64@FreeBSD.ORG>; Tue, 1 Jun 2004 23:48:47 -0700 (PDT)
	(envelope-from das@FreeBSD.ORG)
Received: (from das@localhost)
	by VARK.homeunix.com (8.12.11/8.12.10/Submit) id i526ml8G006231
	for amd64@FreeBSD.ORG; Tue, 1 Jun 2004 23:48:47 -0700 (PDT)
	(envelope-from das@FreeBSD.ORG)
Date: Tue, 1 Jun 2004 23:48:46 -0700
From: David Schultz <das@FreeBSD.ORG>
To: amd64@FreeBSD.ORG
Message-ID: <20040602064846.GA6124@VARK.homeunix.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
Subject: Initial FP exception flags incorrect on amd64
X-BeenThere: freebsd-amd64@freebsd.org
X-Mailman-Version: 2.1.1
Precedence: list
List-Id: Porting FreeBSD to the AMD64 platform <freebsd-amd64.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-amd64>,
	<mailto:freebsd-amd64-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-amd64>
List-Post: <mailto:freebsd-amd64@freebsd.org>
List-Help: <mailto:freebsd-amd64-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-amd64>,
	<mailto:freebsd-amd64-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Wed, 02 Jun 2004 06:48:52 -0000

I discovered that new processes on amd64 have the inexact flag
raised by default, at least on sledge.  However, all the sticky
flags should be clear initially.  Here is a program that
demonstrates the problem:

#include <stdio.h>

int
main(int argc, char *argv[])
{
	int r;

	__asm("stmxcsr %0" : "=m" (r));
	printf("got 0x%02x, expecting 0x00\n", r & 0x3f);
}

I don't have any amd64 hardware of my own to test kernel patches
on, but if I were to make a wild guess as to how to solve the
problem, it would be the following patch.  I would appreciate it
if someone could address the problem, or at least let me know
whether my proposed fix works.

Index: sys/amd64/amd64/fpu.c
===================================================================
RCS file: /cvs/src/sys/amd64/amd64/fpu.c,v
retrieving revision 1.149
diff -u -r1.149 fpu.c
--- fpu.c	5 Apr 2004 21:25:51 -0000	1.149
+++ fpu.c	2 Jun 2004 06:08:34 -0000
@@ -73,6 +73,7 @@
 #define	fnstsw(addr)		__asm __volatile("fnstsw %0" : "=m" (*(addr)))
 #define	fxrstor(addr)		__asm("fxrstor %0" : : "m" (*(addr)))
 #define	fxsave(addr)		__asm __volatile("fxsave %0" : "=m" (*(addr)))
+#define	stmxcsr(addr)		__asm("stmxcsr %0" : "=m" (*(addr)))
 #define	start_emulating()	__asm("smsw %%ax; orb %0,%%al; lmsw %%ax" \
 				      : : "n" (CR0_TS) : "ax")
 #define	stop_emulating()	__asm("clts")
@@ -119,6 +120,8 @@
 	fninit();
 	control = __INITIAL_FPUCW__;
 	fldcw(&control);
+	control = __INITIAL_MXCSR__;
+	stmxcsr(&control);
 	fxsave(&fpu_cleanstate);
 	start_emulating();
 	fpu_cleanstate_ready = 1;