From owner-freebsd-arch@FreeBSD.ORG  Mon Oct  6 14:46:54 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 769DC16A4B3
	for <arch@FreeBSD.org>; Mon,  6 Oct 2003 14:46:54 -0700 (PDT)
Received: from khavrinen.lcs.mit.edu (khavrinen.lcs.mit.edu [18.24.4.193])
	by mx1.FreeBSD.org (Postfix) with ESMTP id 92A4B43FD7
	for <arch@FreeBSD.org>; Mon,  6 Oct 2003 14:46:53 -0700 (PDT)
	(envelope-from wollman@khavrinen.lcs.mit.edu)
Received: from khavrinen.lcs.mit.edu (localhost [IPv6:::1])
	by khavrinen.lcs.mit.edu (8.12.9/8.12.9) with ESMTP id h96Lkpgk093487
	(version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256
	verify=OK CN=khavrinen.lcs.mit.edu issuer=SSL+20Client+20CA);
	Mon, 6 Oct 2003 17:46:52 -0400 (EDT)
	(envelope-from wollman@khavrinen.lcs.mit.edu)
Received: (from wollman@localhost)
	by khavrinen.lcs.mit.edu (8.12.9/8.12.9/Submit) id h96Lkpx0093486;
	Mon, 6 Oct 2003 17:46:51 -0400 (EDT)
	(envelope-from wollman)
Date: Mon, 6 Oct 2003 17:46:51 -0400 (EDT)
From: Garrett Wollman <wollman@khavrinen.lcs.mit.edu>
Message-Id: <200310062146.h96Lkpx0093486@khavrinen.lcs.mit.edu>
To: phk@phk.freebsd.dk
X-Newsgroups: mit.lcs.mail.freebsd-arch
In-Reply-To: <21229.1065441969@critter.freebsd.dk>
Organization: MIT Laboratory for Computer Science
X-Spam-Score: -3.3 () IN_REP_TO
X-Scanned-By: MIMEDefang 2.37
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: Mon, 06 Oct 2003 21:46:54 -0000

PHK writes:

>Most code which does disk I/O from userland uses malloc'ed buffers for
>the sectors and the alignment comes for free, but we currently have
>no requirement that it be so.

I believe that the Standard allows practically arbitrary restrictions
on what may be done with devices (since most devices, aside from ttys,
are outside the scope of the Standard anyway).  It says that read()
may fail if:

[ENXIO]    A request was made of a nonexistent device, or the request
	   was outside the capabilities of the device.

(I'm assuming you're talking about reads from devices, as opposed to
reads from files.)

I think that gives us plenary authority to require appropriate
alignment of data buffers used to access disk devices directly.

-GAWollman