From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43)
	id 1LrwLE-0000tY-Nw
	for qemu-devel@nongnu.org; Thu, 09 Apr 2009 11:39:00 -0400
Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43)
	id 1LrwL9-0000nF-Uu
	for qemu-devel@nongnu.org; Thu, 09 Apr 2009 11:39:00 -0400
Received: from [199.232.76.173] (port=36653 helo=monty-python.gnu.org)
	by lists.gnu.org with esmtp (Exim 4.43) id 1LrwL9-0000mf-GF
	for qemu-devel@nongnu.org; Thu, 09 Apr 2009 11:38:55 -0400
Received: from mx20.gnu.org ([199.232.41.8]:14939)
	by monty-python.gnu.org with esmtps (TLS-1.0:RSA_AES_256_CBC_SHA1:32)
	(Exim 4.60) (envelope-from <paul@codesourcery.com>)
	id 1LrwL9-0001IG-5c
	for qemu-devel@nongnu.org; Thu, 09 Apr 2009 11:38:55 -0400
Received: from mail.codesourcery.com ([65.74.133.4])
	by mx20.gnu.org with esmtp (Exim 4.60)
	(envelope-from <paul@codesourcery.com>) id 1LrwL8-0005Pl-92
	for qemu-devel@nongnu.org; Thu, 09 Apr 2009 11:38:54 -0400
From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] buffer alignment for block backends
Date: Thu, 9 Apr 2009 16:38:51 +0100
References: <49DCA80A.1020808@eu.citrix.com> <49DDF3FF.8050706@codemonkey.ws>
	<20090409151944.GD5457@const.bordeaux.inria.fr>
In-Reply-To: <20090409151944.GD5457@const.bordeaux.inria.fr>
MIME-Version: 1.0
Content-Type: text/plain;
  charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable
Content-Disposition: inline
Message-Id: <200904091638.51737.paul@codesourcery.com>
Reply-To: qemu-devel@nongnu.org
List-Id: qemu-devel.nongnu.org
List-Unsubscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.gnu.org/pipermail/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: qemu-devel@nongnu.org
Cc: Samuel Thibault <samuel.thibault@ens-lyon.org>

On Thursday 09 April 2009, Samuel Thibault wrote:
> Anthony Liguori, le Thu 09 Apr 2009 08:11:27 -0500, a =E9crit :
> > >- ide.c:ide_init2
> >
> > This buffer is only used when not doing DMA.  When doing DMA, we are
> > able to do zero-copy IO so the alignment of the request depends on how
> > the guest aligned the request.  I suspect you'll find a lot of guests
> > that, in practice, do not align requests at 4k boundaries.
>
> Eeeeerrr, why shoudn't they?  Guests usually work on pages, which are
> already aligned on 4k boundaries.

Assuming that page =3D=3D 4k is going to get you into all sorts of trouble.

Paul