From owner-freebsd-amd64@FreeBSD.ORG  Mon Feb 14 05:46:44 2005
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 6629316A4CE
	for <freebsd-amd64@freebsd.org>; Mon, 14 Feb 2005 05:46:44 +0000 (GMT)
Received: from pooker.samsco.org (pooker.samsco.org [168.103.85.57])
	by mx1.FreeBSD.org (Postfix) with ESMTP id 4689643D55
	for <freebsd-amd64@freebsd.org>; Mon, 14 Feb 2005 05:46:39 +0000 (GMT)
	(envelope-from scottl@samsco.org)
Received: from [192.168.254.19] (ibook-nai.samsco.home [192.168.254.19])
	(authenticated bits=0)
	by pooker.samsco.org (8.13.1/8.13.1) with ESMTP id j1E5krCx024704;
	Sun, 13 Feb 2005 22:46:54 -0700 (MST)
	(envelope-from scottl@samsco.org)
Message-ID: <42103B29.8070901@samsco.org>
Date: Sun, 13 Feb 2005 22:46:17 -0700
From: Scott Long <scottl@samsco.org>
User-Agent: Mozilla/5.0 (Macintosh; U; PPC Mac OS X Mach-O; en-US;
	rv:1.7.5) Gecko/20041217
X-Accept-Language: en-us, en
MIME-Version: 1.0
To: Thomas Krause -CI- <freebsd-amd64@chef-ingenieur.de>
References: <420F9CA1.4040207@chef-ingenieur.de>
In-Reply-To: <420F9CA1.4040207@chef-ingenieur.de>
Content-Type: text/plain; charset=us-ascii; format=flowed
Content-Transfer-Encoding: 7bit
X-Spam-Status: No, score=-2.8 required=3.8 tests=ALL_TRUSTED autolearn=failed 
	version=3.0.2
X-Spam-Checker-Version: SpamAssassin 3.0.2 (2004-11-16) on pooker.samsco.org
cc: freebsd-amd64@freebsd.org
Subject: Re: -mno-sse -mno-sse2 -mno-mmx -mno-3dnow when compiling kernel
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: Mon, 14 Feb 2005 05:46:44 -0000

Thomas Krause -CI- wrote:

> Hello,
> why are the Switches "-mno-sse -mno-sse2 -mno-mmx -mno-3dnow" are set
> when compiling an amd64 kernel? I think, the CPU supports all thease
> features.
> 
> Features=0x78bfbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,CLFLUSH,MMX,FXSR,SSE,SSE2> 
> 
> 
> I have no CPUTYPE specified in /etc/make.conf, as I could not
> find a matching option in /usr/src/share/examples/etc/make.conf.
> 
> Kind regards,
> Thomas


Floating point, MMX, SSE, SSE2, and 3dnow, registers are not usable in
the kernel.  By not using these registers, we save the expense of saving
and restoring them on every single syscall.  Using them without saving
them would run the risk of colliding with userland programs that do use
them.

Whether or not the kernel is compiled with instructions that use these
registers has absolutely no bearing on whether userland code can use
them.  It's purely about trading one optimization (using optimized
opcodes in the kernel) for another (not having to save and restore
dozens of registers to the stack on each syscall).  It might be
interesting to experiment with allowing a certain class of instructions
in the kernel and saving and restoring the associated registers, and
seeing if GCC will actually generate code that is better and overcomes
the save/restore expense.  Any takers?

Scott