From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43)
	id 1MWFGo-0002N4-27
	for qemu-devel@nongnu.org; Wed, 29 Jul 2009 15:57:02 -0400
Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43)
	id 1MWFGn-0002M4-DQ
	for qemu-devel@nongnu.org; Wed, 29 Jul 2009 15:57:01 -0400
Received: from [199.232.76.173] (port=49114 helo=monty-python.gnu.org)
	by lists.gnu.org with esmtp (Exim 4.43) id 1MWFGn-0002Ln-7a
	for qemu-devel@nongnu.org; Wed, 29 Jul 2009 15:57:01 -0400
Received: from fg-out-1718.google.com ([72.14.220.157]:6605)
	by monty-python.gnu.org with esmtp (Exim 4.60)
	(envelope-from <filip.navara@gmail.com>) id 1MWFGm-0004jB-UZ
	for qemu-devel@nongnu.org; Wed, 29 Jul 2009 15:57:01 -0400
Received: by fg-out-1718.google.com with SMTP id d23so389563fga.8
	for <qemu-devel@nongnu.org>; Wed, 29 Jul 2009 12:56:59 -0700 (PDT)
MIME-Version: 1.0
In-Reply-To: <200907292052.35755.paul@codesourcery.com>
References: <20090729120719.GN30449@redhat.com>
	<5b31733c0907291218k624f97bbx62d8d6d49b8f0759@mail.gmail.com>
	<5b31733c0907291224h340921f0j72195618b0b6cd2a@mail.gmail.com>
	<200907292052.35755.paul@codesourcery.com>
Date: Wed, 29 Jul 2009 21:56:59 +0200
Message-ID: <5b31733c0907291256r734416d2ifbe8b72105eba27c@mail.gmail.com>
Subject: Re: [Qemu-devel] [PATCH v2] make windows notice media change
From: Filip Navara <filip.navara@gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable
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: Paul Brook <paul@codesourcery.com>
Cc: qemu-devel@nongnu.org, Gleb Natapov <gleb@redhat.com>

On Wed, Jul 29, 2009 at 9:52 PM, Paul Brook<paul@codesourcery.com> wrote:
>> >>> Why shouldn't we support loading version 2 snapshots? Afterall that'=
s
>> >>> why we had the versioning in the first place.
>> >>
>> >> No it's not. Versioning was introduced to *prevent* loading old
>> >> snapshots and crashing or ending up with inconsistent guest state. I'=
m
>> >> still unconvinced that anything other than very short term backward
>> >> compatibility is worthwhile or even viable.
>> >
>> > I see it as a way to migrate a running guest to newer QEMU version,
>> > possibly even with live migration. In fact I used it quite often back =
in
>> > the day when snapshots were not part of qcow2 yet and when kqemu was
>> > still in its heydays.
>>
>> BTW, why would there be the version parameter in the first place if it
>> wasn't supposed to load older versions?!
>
> Like I already said: it's there to prevent an old version being loaded
> accidentally. Without this an incompatible change will =A0result in anyth=
ing
> from a crash to corrupt/inconsistent guest state. Versioning allows us to
> reject the snapshot and fail safely.

If that was the case then the if (version !=3D x) return -EINVAL check coul=
d
have been in the generic code and there would be no need for the version
parameter in the load function.

- F.