From patchwork Thu Feb 9 18:17:46 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: "Dr. David Alan Gilbert" X-Patchwork-Id: 9565327 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork.web.codeaurora.org (Postfix) with ESMTP id E222D60236 for ; Thu, 9 Feb 2017 18:18:26 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id D355528535 for ; Thu, 9 Feb 2017 18:18:26 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id C87AD28538; Thu, 9 Feb 2017 18:18:26 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.9 required=2.0 tests=BAYES_00, RCVD_IN_DNSWL_HI, T_HK_NAME_DR autolearn=ham version=3.3.1 Received: from lists.gnu.org (lists.gnu.org [208.118.235.17]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (No client certificate requested) by mail.wl.linuxfoundation.org (Postfix) with ESMTPS id 703D128535 for ; Thu, 9 Feb 2017 18:18:26 +0000 (UTC) Received: from localhost ([::1]:39609 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cbtIX-0003KX-Ge for patchwork-qemu-devel@patchwork.kernel.org; Thu, 09 Feb 2017 13:18:25 -0500 Received: from eggs.gnu.org ([2001:4830:134:3::10]:55062) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cbtI1-0003Db-FZ for qemu-devel@nongnu.org; Thu, 09 Feb 2017 13:17:54 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cbtHy-0003If-3E for qemu-devel@nongnu.org; Thu, 09 Feb 2017 13:17:53 -0500 Received: from mx1.redhat.com ([209.132.183.28]:56218) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1cbtHx-0003HO-Re for qemu-devel@nongnu.org; Thu, 09 Feb 2017 13:17:50 -0500 Received: from int-mx13.intmail.prod.int.phx2.redhat.com (int-mx13.intmail.prod.int.phx2.redhat.com [10.5.11.26]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id D17533CEF58; Thu, 9 Feb 2017 18:17:49 +0000 (UTC) Received: from dgilbert-t530.redhat.com (ovpn-116-251.ams2.redhat.com [10.36.116.251]) by int-mx13.intmail.prod.int.phx2.redhat.com (8.14.4/8.14.4) with ESMTP id v19IHmVm029756; Thu, 9 Feb 2017 13:17:48 -0500 From: "Dr. David Alan Gilbert (git)" To: qemu-devel@nongnu.org, quintela@redhat.com Date: Thu, 9 Feb 2017 18:17:46 +0000 Message-Id: <20170209181746.29896-1-dgilbert@redhat.com> X-Scanned-By: MIMEDefang 2.68 on 10.5.11.26 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.29]); Thu, 09 Feb 2017 18:17:49 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 209.132.183.28 Subject: [Qemu-devel] [PATCH] migration: Update docs to discourage version bumps X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: peter.maydell@linaro.org Errors-To: qemu-devel-bounces+patchwork-qemu-devel=patchwork.kernel.org@nongnu.org Sender: "Qemu-devel" X-Virus-Scanned: ClamAV using ClamSMTP From: "Dr. David Alan Gilbert" Version bumps break backwards migration; update the docs to explain to people that's bad and how to avoid it. Signed-off-by: Dr. David Alan Gilbert --- docs/migration.txt | 42 ++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 42 insertions(+) diff --git a/docs/migration.txt b/docs/migration.txt index b462ead..f375f4b 100644 --- a/docs/migration.txt +++ b/docs/migration.txt @@ -161,6 +161,11 @@ include/hw/hw.h. === More about versions === +Version numbers are intended for major incompatible changes to the +migration of a device, and using them breaks backwards-migration +compatibility; in general most changes can be made by adding Subsections +(see below) or _TEST macros (see below) which won't break compatibility. + You can see that there are several version fields: - version_id: the maximum version_id supported by VMState for that device. @@ -175,6 +180,9 @@ version_id. And the function load_state_old() (if present) is able to load state from minimum_version_id_old to minimum_version_id. This function is deprecated and will be removed when no more users are left. +The VMState with the 'version_id' value will always be generated and thus +can't be loaded by any older QEMU. + === Massaging functions === Sometimes, it is not enough to be able to save the state directly @@ -292,6 +300,40 @@ save/send this state when we are in the middle of a pio operation not enabled, the values on that fields are garbage and don't need to be sent. +Using a condition function that checks a 'property' to determine whether +to send a subsection allows backwards migration compatibility. +For example; + a) Add a new property using DEFINE_PROP_BOOL - e.g. support-foo and + default it to true. + b) Add an entry to the HW_COMPAT_ for the previous version + that sets the property to false. + c) Add a static bool support_foo function + d) Add a subsection with a .needed set to the support_foo function + e) (potentially) Add a pre_load that sets up a default value for 'foo' + to be used if the subsection isn't loaded. + +Now that subsection will not be generated when using an older +machine type and the migration stream will be accepted by older +QEMU versions. + += Not sending existing elements = + +Sometimes members of the VMState are no longer needed; + removing them will break migration compatibility + making them version dependent and bumping the version will break backwards + migration compatibility. + +The best way is to: + a) Add a new property/compatibility/function in the same way for subsections + above. + b) replace the VMSTATE macro with the _TEST version of the macro, e.g.: + VMSTATE_UINT32(foo, barstruct) + becomes + VMSTATE_UINT32_TEST(foo, barstruct, pre_version_baz) + + Sometime in the future when we no longer care about the ancient +versions these can be killed off. + = Return path = In most migration scenarios there is only a single data path that runs