From owner-freebsd-arch@FreeBSD.ORG  Tue Oct  7 08:41:21 2003
Return-Path: <owner-freebsd-arch@FreeBSD.ORG>
Delivered-To: freebsd-arch@freebsd.org
Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125])
	by hub.freebsd.org (Postfix) with ESMTP id BEF3016A4B3
	for <arch@freebsd.org>; Tue,  7 Oct 2003 08:41:21 -0700 (PDT)
Received: from Daffy.timing.com (ns1int.timing.com [206.168.13.218])
	by mx1.FreeBSD.org (Postfix) with ESMTP id D459C43F75
	for <arch@freebsd.org>; Tue,  7 Oct 2003 08:41:20 -0700 (PDT)
	(envelope-from ben@timing.com)
Received: from piglet.timing.com (oink@piglet.timing.com [206.168.13.178])
	by Daffy.timing.com (8.12.8p2/8.12.8) with ESMTP id h97FfI3R007432;
	Tue, 7 Oct 2003 09:41:18 -0600 (MDT)
	(envelope-from ben@timing.com)
Received: from piglet.timing.com (oink@localhost.timing.com [127.0.0.1])
	by piglet.timing.com (8.12.6p3/8.12.6) with ESMTP id h97FfII9034088;
	Tue, 7 Oct 2003 09:41:18 -0600 (MDT)
	(envelope-from ben@piglet.timing.com)
Received: (from ben@localhost)
	by piglet.timing.com (8.12.6p3/8.12.6/Submit) id h97FfHNF034085;
	Tue, 7 Oct 2003 09:41:17 -0600 (MDT)
From: Ben Mesander <ben@timing.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Message-ID: <16258.56989.685473.900678@piglet.timing.com>
Date: Tue, 7 Oct 2003 09:41:17 -0600
To: "Poul-Henning Kamp" <phk@phk.freebsd.dk>
In-Reply-To: <32324.1065508157@critter.freebsd.dk>
References: <200310061753.28562.sam@errno.com>
	<32324.1065508157@critter.freebsd.dk>
X-Mailer: VM 7.00 under Emacs 21.2.95.2
cc: Sam Leffler <sam@errno.com>
cc: arch@freebsd.org
Subject: Re: Alignment of disk-I/O from userland. 
X-BeenThere: freebsd-arch@freebsd.org
X-Mailman-Version: 2.1.1
Precedence: list
List-Id: Discussion related to FreeBSD architecture
	<freebsd-arch.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-arch>,
	<mailto:freebsd-arch-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-arch>
List-Post: <mailto:freebsd-arch@freebsd.org>
List-Help: <mailto:freebsd-arch-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-arch>,
	<mailto:freebsd-arch-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Tue, 07 Oct 2003 15:41:21 -0000

Poul-Henning Kamp writes:
 > 1:  Not "on all i/o operations going to a device", but rather "on i/o
 >     operations which take the physread/write fast-path to avoid a copyin/out
 >     overhead."  (disks and tapes mostly).  Ttys, /dev/null and all the
 >     "typical" devices are unaffected.

How about a flag for open(2), such as O_DIRECT, which would indicate
if the fd in question is such a device, and you did not sufficiently
align, etc. your buffers, then you get an error when you attempt the
I/O operation.

If you did not specify this flag when opening the device, then do the
bounce buffer dance for the I/O operation, and lose efficiency.

--Ben