From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.5 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE, SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 193F7C47247 for ; Tue, 5 May 2020 07:51:50 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id D4A6B20721 for ; Tue, 5 May 2020 07:51:49 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="bBcB9T+b" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D4A6B20721 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:48184 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jVsMn-0006eI-2C for qemu-devel@archiver.kernel.org; Tue, 05 May 2020 03:51:49 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:54684) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1jVsM0-00066C-Nh for qemu-devel@nongnu.org; Tue, 05 May 2020 03:51:00 -0400 Received: from us-smtp-1.mimecast.com ([207.211.31.81]:59383 helo=us-smtp-delivery-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1jVsLz-0007T4-BI for qemu-devel@nongnu.org; Tue, 05 May 2020 03:50:59 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1588665058; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=sYke/vIF0cYduZtP8zeR8AwYerfCyukkWfmBsg5UdEI=; b=bBcB9T+bkcqwVcUhSi4UDKm/9zzWw36yk3yNLg7ghtno9xfYyLsTAanTXLDZ/qOQ9V6R9A +btj78hgSlKQ7iwdzESlfz/UKQctvWW2bjRPHTZEFdrfPKQmOAFnPf42q8qZWHYwit1LNW DCRQsCxebGNqWPXigr0HSJHzrUKyMOg= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-301-rNlRLC6aPC26H1p129_SRQ-1; Tue, 05 May 2020 03:50:56 -0400 X-MC-Unique: rNlRLC6aPC26H1p129_SRQ-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id A34A439340; Tue, 5 May 2020 07:50:55 +0000 (UTC) Received: from linux.fritz.box (ovpn-112-11.ams2.redhat.com [10.36.112.11]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 7A49F60CD1; Tue, 5 May 2020 07:50:51 +0000 (UTC) Date: Tue, 5 May 2020 09:50:50 +0200 From: Kevin Wolf To: Eric Blake Subject: Re: [PATCH v5 4/7] qcow2: Deprecate use of qemu-img amend to change backing file Message-ID: <20200505075050.GB5759@linux.fritz.box> References: <20200403175859.863248-1-eblake@redhat.com> <20200403175859.863248-5-eblake@redhat.com> MIME-Version: 1.0 In-Reply-To: <20200403175859.863248-5-eblake@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: quoted-printable Content-Disposition: inline Received-SPF: pass client-ip=207.211.31.81; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/05/05 00:37:19 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_PASS=-0.001 autolearn=_AUTOLEARN X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: pkrempa@redhat.com, qemu-block@nongnu.org, kchamart@redhat.com, libvir-list@redhat.com, qemu-devel@nongnu.org, mreitz@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 03.04.2020 um 19:58 hat Eric Blake geschrieben: > The use of 'qemu-img amend' to change qcow2 backing files is not > tested very well. In particular, our implementation has a bug where > if a new backing file is provided without a format, then the prior > format is blindly reused, even if this results in data corruption, but > this is not caught by iotests. >=20 > There are also situations where amending other options needs access to > the original backing file (for example, on a downgrade to a v2 image, > knowing whether a v3 zero cluster must be allocated or may be left > unallocated depends on knowing whether the backing file already reads > as zero), but the command line does not have a nice way to tell us > both the backing file to use for opening the image as well as the > backing file to install after the operation is complete. >=20 > Even if we do allow changing the backing file, it is redundant with > the existing ability to change backing files via 'qemu-img rebase -u'. > It is time to deprecate this support (leaving the existing behavior > intact, even if it is buggy), and at a point in the future, require > the use of only 'qemu-img rebase' for adjusting backing chain > relations, saving 'qemu-img amend' for changes unrelated to the > backing chain. >=20 > Signed-off-by: Eric Blake I think the original idea was that in BlockDriver, special interfaces like the ones used by qemu-img rebase could eventually go away if we have a single consistent interface to change image options, which is amend. So ideally, bdrv_change_backing_file() should become a wrapper around amend. I'm not even sure if the behaviour you mention should be considered a bug because amend is a low-level interface. But even if that's the case, it would be a matter of simply adding a check. Kevin