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 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 6B835C77B7A for ; Wed, 17 May 2023 12:22:58 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1pzGAU-0002eq-Pf; Wed, 17 May 2023 08:22:11 -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 1pzGAQ-0002eZ-35 for qemu-devel@nongnu.org; Wed, 17 May 2023 08:22:06 -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 1pzGAO-0006Yp-9j for qemu-devel@nongnu.org; Wed, 17 May 2023 08:22:05 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1684326122; 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: in-reply-to:in-reply-to:references:references; bh=IHUfvFJhkDz3DqKJFkj3rngWOHllKoZuVNXQlgQ6EjQ=; b=Fdft2xanKgoCNgKcdQC9HbsEji8eVqy0kjS/ORgVZp2RqPs/NZflVQYG6/UghmqJtJqmzV KBkoENkoOXx6j+VnYOM9U4FocT9EIEgHywEnuYZYxx4cLz0wPRqQaJP6JgfnvdBmK3wlh7 vYfXa5WddeD3VEa++o2SZ62YHYYWykA= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-670--KuSUMsvPjCPB18WYzceaA-1; Wed, 17 May 2023 08:22:00 -0400 X-MC-Unique: -KuSUMsvPjCPB18WYzceaA-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.rdu2.redhat.com [10.11.54.1]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 40881867955; Wed, 17 May 2023 12:21:59 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.39.192.91]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 8EB3040C2063; Wed, 17 May 2023 12:21:58 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 526FE21E66E3; Wed, 17 May 2023 14:21:54 +0200 (CEST) From: Markus Armbruster To: Avihai Horon Cc: qemu-devel@nongnu.org, Alex Williamson , =?utf-8?Q?C=C3=A9dric?= Le Goater , Juan Quintela , Peter Xu , Leonardo Bras , Eric Blake , Thomas Huth , Laurent Vivier , Paolo Bonzini , Yishai Hadas , Jason Gunthorpe , Maor Gottlieb , Kirti Wankhede , Tarun Gupta , Joao Martins Subject: Re: [PATCH 1/8] migration: Add precopy initial data capability References: <20230501140141.11743-1-avihaih@nvidia.com> <20230501140141.11743-2-avihaih@nvidia.com> <87lehn71wm.fsf@pond.sub.org> <1836389d-0774-08a1-ef54-8487d238fe73@nvidia.com> Date: Wed, 17 May 2023 14:21:54 +0200 In-Reply-To: <1836389d-0774-08a1-ef54-8487d238fe73@nvidia.com> (Avihai Horon's message of "Wed, 17 May 2023 13:16:00 +0300") Message-ID: <87lehn2lnh.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.2 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.1 on 10.11.54.1 Received-SPF: pass client-ip=170.10.133.124; envelope-from=armbru@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_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 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 Avihai Horon writes: > On 17/05/2023 12:17, Markus Armbruster wrote: >> External email: Use caution opening links or attachments >> >> >> Avihai Horon writes: >> >>> Migration downtime estimation is calculated based on bandwidth and >>> remaining migration data. This assumes that loading of migration data in >>> the destination takes a negligible amount of time and that downtime >>> depends only on network speed. >>> >>> While this may be true for RAM, it's not necessarily true for other >>> migration users. For example, loading the data of a VFIO device in the >>> destination might require from the device to allocate resources, prepare >>> internal data structures and so on. These operations can take a >>> significant amount of time which can increase migration downtime. >>> >>> This patch adds a new capability "precopy initial data" that allows the >>> source to send initial precopy data and the destination to ACK that this >>> data has been loaded. Migration will not attempt to stop the source VM >>> and complete the migration until this ACK is received. >>> >>> This will allow migration users to send initial precopy data which can >>> be used to reduce downtime (e.g., by pre-allocating resources), while >>> making sure that the source will stop the VM and complete the migration >>> only after this initial precopy data is sent and loaded in the >>> destination so it will have full effect. >>> >>> This new capability relies on the return path capability to communicate >>> from the destination back to the source. >>> >>> The actual implementation of the capability will be added in the >>> following patches. >>> >>> Signed-off-by: Avihai Horon >>> --- >>> qapi/migration.json | 9 ++++++++- >>> migration/options.h | 1 + >>> migration/options.c | 20 ++++++++++++++++++++ >>> 3 files changed, 29 insertions(+), 1 deletion(-) >>> >>> diff --git a/qapi/migration.json b/qapi/migration.json >>> index 82000adce4..d496148386 100644 >>> --- a/qapi/migration.json >>> +++ b/qapi/migration.json >>> @@ -478,6 +478,13 @@ >>> # should not affect the correctness of postcopy migration. >>> # (since 7.1) >>> # >>> +# @precopy-initial-data: If enabled, migration will not attempt to stop source >>> +# VM and complete the migration until an ACK is received >>> +# from the destination that initial precopy data has >>> +# been loaded. This can improve downtime if there are >>> +# migration users that support precopy initial data. >>> +# (since 8.1) >>> +# >> Please format like >> >> # @precopy-initial-data: If enabled, migration will not attempt to >> # stop source VM and complete the migration until an ACK is >> # received from the destination that initial precopy data has been >> # loaded. This can improve downtime if there are migration users >> # that support precopy initial data. (since 8.1) >> >> to blend in with recent commit a937b6aa739 (qapi: Reformat doc comments >> to conform to current conventions). > > Sure. > >> >> What do you mean by "if there are migration users that support precopy >> initial data"? > > This capability only provides the framework to send precopy initial data and ACK that it was loaded in the destination. > To actually benefit from it, migration users (such as VFIO devices, RAM, etc.) must implement support for it and use it. > > What I wanted to say here is that there is no point to enable this capability if there are no migration users that support it. > For example, if you are migrating a VM without VFIO devices, then enabling this capability will have no effect. I see. Which "migration users" support it now? Which could support it in the future? Is the "initial precopy data" feature described in more detail anywhere? >> Do I have to ensure the ACK comes by configuring the destination VM in a >> certain way, and if yes, how exactly? > > In v2 of the series that I will send later you will have to enable this capability also in the destination. What happens when you enable it on the source and not on the destination? [...]