From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([208.118.235.92]:42259)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <armbru@redhat.com>) id 1TukfZ-0007Ai-Q3
	for qemu-devel@nongnu.org; Mon, 14 Jan 2013 09:05:53 -0500
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <armbru@redhat.com>) id 1TukfR-0006Qd-R2
	for qemu-devel@nongnu.org; Mon, 14 Jan 2013 09:05:45 -0500
Received: from mx1.redhat.com ([209.132.183.28]:21531)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <armbru@redhat.com>) id 1TukfR-0006QY-HV
	for qemu-devel@nongnu.org; Mon, 14 Jan 2013 09:05:37 -0500
From: Markus Armbruster <armbru@redhat.com>
References: <1357923660-35202-1-git-send-email-elelueck@linux.vnet.ibm.com>
	<1357923660-35202-2-git-send-email-elelueck@linux.vnet.ibm.com>
Date: Mon, 14 Jan 2013 14:23:26 +0100
In-Reply-To: <1357923660-35202-2-git-send-email-elelueck@linux.vnet.ibm.com>
	(Einar Lueck's message of "Fri, 11 Jan 2013 18:01:00 +0100")
Message-ID: <878v7v290x.fsf@blackfin.pond.sub.org>
MIME-Version: 1.0
Content-Type: text/plain
Subject: Re: [Qemu-devel] [PATCH V2] hd-geometry.c: Integrate HDIO_GETGEO in
	guessing
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Einar Lueck <elelueck@linux.vnet.ibm.com>
Cc: kwolf@redhat.com, aliguori@us.ibm.com, stefanha@gmail.com, agraf@suse.de, qemu-devel@nongnu.org, Christian Borntraeger <borntraeger@de.ibm.com>, Jens Freimann <jfrei@linux.vnet.ibm.com>, cornelia.huck@de.ibm.com, pbonzini@redhat.com

Einar Lueck <elelueck@linux.vnet.ibm.com> writes:

> This patch extends the function hd_geometry_guess. If no geo could
> be guessed via guess_disk_lchs, a new function called guess_disk_pchs is
> called. The latter utilizes HDIO_GET_GEO ioctl to ask the underlying disk
> for geometry.
> If this is not successful (e.g. image files) geometry is derived
> from the size of the disk (as before).
> The new HDIO_GETGEO logic is required for two use cases:
> a) Support for geometries of Direct Attached Storage Disks (DASD)
> on s390x configured as backing of virtio block devices.
> b) Support for FCP attached SCSI disks that do not yet have a
> partition table. Without this patch, fdisk -l on the host would
> return different results then fdisk -l in the guest.

I'm afraid this could mess up existing, working disks.

Consider a disk where guess_disk_lchs() fails and HDIO_GETGEO succeeds.

The old code arbitrarily picks a "standard" physical geometry then.

Your code picks the one returned by HDIO_GETGEO.

Unless the two geometries happen to lead to a compatible address
translation, any guest that actually uses the translation now sees
different disk contents, with predictably bad results.

Can this happen?  If no, why not?