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a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1698153220; h=from:from:sender:sender: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:list-id:list-help: list-unsubscribe:list-subscribe:list-post; bh=0wLme3tPLHrsytkaTz20AbsMQdVkg50xoNnDIj0EyAo=; b=K4M59n6gtnYDq7CxDa0YynnAyMdexh6sf88TrS3PnSjLP3PV4H9qSRLHwzMR10wgrdIIwf h9TGKmwgVMf4N3szs3Mnpv4gWj11cDcvyYGJSvaMeTUE0LrQuxFWh8H9oUyDlYKhwLQ3Sl bS0v7oJqsMMpnVT0mjcdHXIVYc8+oCI= X-MC-Unique: Jvt1ZnW1PRGjguVExr4xNg-1 X-Original-To: libvir-list@listman.corp.redhat.com From: Juan Quintela To: qemu-devel@nongnu.org Subject: [PULL 03/39] migration/doc: How to migrate when hosts have different features Date: Tue, 24 Oct 2023 15:12:29 +0200 Message-ID: <20231024131305.87468-4-quintela@redhat.com> In-Reply-To: <20231024131305.87468-1-quintela@redhat.com> References: <20231024131305.87468-1-quintela@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.7 X-BeenThere: libvir-list@redhat.com X-Mailman-Version: 2.1.29 Precedence: list List-Id: Development discussions about the libvirt library & tools List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Fam Zheng , Peter Maydell , Corey Minyard , libvir-list@redhat.com, Jason Wang , Mark Cave-Ayland , Li Zhijian , Peter Xu , Gerd Hoffmann , Eric Blake , Fabiano Rosas , qemu-block@nongnu.org, Juan Quintela , Daniel Henrique Barboza , Markus Armbruster , Halil Pasic , Marcel Apfelbaum , =?UTF-8?q?Marc-Andr=C3=A9=20Lureau?= , Christian Borntraeger , Thomas Huth , Corey Minyard , Harsh Prateek Bora , Stefan Weil , Richard Henderson , Nicholas Piggin , qemu-s390x@nongnu.org, qemu-arm@nongnu.org, =?UTF-8?q?C=C3=A9dric=20Le=20Goater?= , Stefan Hajnoczi , Samuel Thibault , David Hildenbrand , John Snow , David Gibson , Kevin Wolf , "Michael S. Tsirkin" , Ilya Leoshkevich , Hanna Reitz , Leonardo Bras , qemu-ppc@nongnu.org Errors-To: libvir-list-bounces@redhat.com Sender: "libvir-list" X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.8 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Transfer-Encoding: quoted-printable X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1698153223576100001 Content-Type: text/plain; charset="utf-8"; x-default="true" Sometimes devices have different features depending of things outside of qemu. For instance the kernel. Document how to handle that cases. Acked-by: Peter Xu Reviewed-by: Fabiano Rosas Signed-off-by: Juan Quintela Message-ID: <20231018112827.1325-4-quintela@redhat.com> --- 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=3Doff,stibp=3Doff + +Host B: + +$ qemu-system-x86_64 -cpu host,taa-no=3Doff + +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. --=20 2.41.0