From patchwork Wed Oct 18 11:28:26 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Juan Quintela X-Patchwork-Id: 13426891 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org 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 smtp.lore.kernel.org (Postfix) with ESMTPS id 1BA9ECDB47E for ; Wed, 18 Oct 2023 11:29:16 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qt4jL-0003ji-He; Wed, 18 Oct 2023 07:28:51 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qt4jG-0003hN-V8 for qemu-devel@nongnu.org; Wed, 18 Oct 2023 07:28:46 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qt4jF-0005dO-0R for qemu-devel@nongnu.org; Wed, 18 Oct 2023 07:28:46 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1697628524; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=zlBi0uAz8yyAaofDaFbNNRcxIrbd/HojO8+WyRCrkSA=; b=PWEe/ph7Tsj5gzRjQyU4kQYrwPR4/pTXepM67FTHibcBfftIPiiip1O1I/V8DkQQNsiU00 dphD01HJSO5nOtHzeid+ua5UPaZXT2/ccLMY26IuAjIZirWPrGXgjosGoAcZ+PC7suDed5 s+nIyQr3DQnDRcjNXsDN1PrX/XdPJ1A= Received: from mimecast-mx02.redhat.com (mx-ext.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-76-bzJ_PMkMNoOYeAsZahFiHQ-1; Wed, 18 Oct 2023 07:28:33 -0400 X-MC-Unique: bzJ_PMkMNoOYeAsZahFiHQ-1 Received: from smtp.corp.redhat.com (int-mx09.intmail.prod.int.rdu2.redhat.com [10.11.54.9]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id DBB851C05AF5; Wed, 18 Oct 2023 11:28:32 +0000 (UTC) Received: from secure.mitica (unknown [10.39.194.127]) by smtp.corp.redhat.com (Postfix) with ESMTP id 00D62492BEE; Wed, 18 Oct 2023 11:28:31 +0000 (UTC) From: Juan Quintela To: qemu-devel@nongnu.org Cc: Juan Quintela , Peter Xu , Leonardo Bras , Fabiano Rosas Subject: [PATCH v4 3/4] migration/doc: How to migrate when hosts have different features Date: Wed, 18 Oct 2023 13:28:26 +0200 Message-ID: <20231018112827.1325-4-quintela@redhat.com> In-Reply-To: <20231018112827.1325-1-quintela@redhat.com> References: <20231018112827.1325-1-quintela@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.9 Received-SPF: pass client-ip=170.10.133.124; envelope-from=quintela@redhat.com; helo=us-smtp-delivery-124.mimecast.com 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_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sometimes devices have different features depending of things outside of qemu. For instance the kernel. Document how to handle that cases. Signed-off-by: Juan Quintela Acked-by: Peter Xu Reviewed-by: Fabiano Rosas --- If you have some example to put here, I am all ears. I guess that virtio-* with some features that are on qemu but not on all kernel would do the trick, but I am not a virtio guru myself. Patches welcome. --- docs/devel/migration.rst | 97 ++++++++++++++++++++++++++++++++++++++++ 1 file changed, 97 insertions(+) diff --git a/docs/devel/migration.rst b/docs/devel/migration.rst index 6fe275b1ec..974505e4a7 100644 --- a/docs/devel/migration.rst +++ b/docs/devel/migration.rst @@ -1138,3 +1138,100 @@ machine types to have the right value:: + { "virtio-blk-device", "num-queues", "1"}, ... }; + +A device with diferent features on both sides +--------------------------------------------- + +Let's assume that we are using the same QEMU binary on both sides, +just to make the things easier. But we have a device that has +different features on both sides of the migration. That can be +because the devices are different, because the kernel driver of both +devices have different features, whatever. + +How can we get this to work with migration. The way to do that is +"theoretically" easy. You have to get the features that the device +has in the source of the migration. The features that the device has +on the target of the migration, you get the intersection of the +features of both sides, and that is the way that you should launch +QEMU. + +Notice that this is not completely related to QEMU. The most +important thing here is that this should be handled by the managing +application that launches QEMU. If QEMU is configured correctly, the +migration will succeed. + +That said, actually doing it is complicated. Almost all devices are +bad at being able to be launched with only some features enabled. +With one big exception: cpus. + +You can read the documentation for QEMU x86 cpu models here: + +https://qemu-project.gitlab.io/qemu/system/qemu-cpu-models.html + +See when they talk about migration they recommend that one chooses the +newest cpu model that is supported for all cpus. + +Let's say that we have: + +Host A: + +Device X has the feature Y + +Host B: + +Device X has not the feature Y + +If we try to migrate without any care from host A to host B, it will +fail because when migration tries to load the feature Y on +destination, it will find that the hardware is not there. + +Doing this would be the equivalent of doing with cpus: + +Host A: + +$ qemu-system-x86_64 -cpu host + +Host B: + +$ qemu-system-x86_64 -cpu host + +When both hosts have different cpu features this is guaranteed to +fail. Especially if Host B has less features than host A. If host A +has less features than host B, sometimes it works. Important word of +last sentence is "sometimes". + +So, forgetting about cpu models and continuing with the -cpu host +example, let's see that the differences of the cpus is that Host A and +B have the following features: + +Features: 'pcid' 'stibp' 'taa-no' +Host A: X X +Host B: X + +And we want to migrate between them, the way configure both QEMU cpu +will be: + +Host A: + +$ qemu-system-x86_64 -cpu host,pcid=off,stibp=off + +Host B: + +$ qemu-system-x86_64 -cpu host,taa-no=off + +And you would be able to migrate between them. It is responsability +of the management application or of the user to make sure that the +configuration is correct. QEMU doesn't know how to look at this kind +of features in general. + +Notice that we don't recomend to use -cpu host for migration. It is +used in this example because it makes the example simpler. + +Other devices have worse control about individual features. If they +want to be able to migrate between hosts that show different features, +the device needs a way to configure which ones it is going to use. + +In this section we have considered that we are using the same QEMU +binary in both sides of the migration. If we use different QEMU +versions process, then we need to have into account all other +differences and the examples become even more complicated.