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=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED 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 24E30C55ABD for ; Tue, 10 Nov 2020 15:23:27 +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 8282B20797 for ; Tue, 10 Nov 2020 15:23:26 +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="LhC2/MJK" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 8282B20797 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]:51274 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1kcVUT-0001Hu-9J for qemu-devel@archiver.kernel.org; Tue, 10 Nov 2020 10:23:25 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:48912) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1kcVSB-0007R5-Tb for qemu-devel@nongnu.org; Tue, 10 Nov 2020 10:21:04 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:58285) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1kcVS6-0005kK-ML for qemu-devel@nongnu.org; Tue, 10 Nov 2020 10:21:02 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1605021656; 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=BaFJxJU0QPMLxx2ykuBQwQBI1n9Y+JErL+jjOkQe/vc=; b=LhC2/MJKT7gvfjCajXAjnvA2oHjE36KaZY4kqpfwyowqPc48BLtJousQhXcz337i6h3PN2 /WAOG8UxLqf0cZpM+ax8MYANruh7fRuDMUEBrC7ESwG4+zx+nbyUI+UIy9itbpDtNk0lrO zACjwsJ1hcy/ypAjUgc5aZUYfm2S/KE= 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-351-hOEQ49uSN5GG8C1RRF7GxQ-1; Tue, 10 Nov 2020 10:20:52 -0500 X-MC-Unique: hOEQ49uSN5GG8C1RRF7GxQ-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 393471084C92; Tue, 10 Nov 2020 15:20:51 +0000 (UTC) Received: from w520.home (ovpn-112-213.phx2.redhat.com [10.3.112.213]) by smtp.corp.redhat.com (Postfix) with ESMTP id 8F5E421E98; Tue, 10 Nov 2020 15:20:50 +0000 (UTC) Date: Tue, 10 Nov 2020 08:20:50 -0700 From: Alex Williamson To: Kirti Wankhede Subject: Re: [RFC PATCH for-QEMU-5.2] vfio: Make migration support experimental Message-ID: <20201110082050.10ca74d4@w520.home> In-Reply-To: <898ba98f-9967-f3b3-737c-2d18b0281b51@nvidia.com> References: <160494787833.1473.10514376876696596117.stgit@gimli.home> <20201109194417.GR3024@work-vm> <20201109132950.6c2dfe02@w520.home> <20201110091037.GA3108@work-vm> <898ba98f-9967-f3b3-737c-2d18b0281b51@nvidia.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=alex.williamson@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=alex.williamson@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/11/10 02:00:53 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_H4=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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Malcolm Crossley , Neo Jia , Juan Quintela , Cornelia Huck , "Dr. David Alan Gilbert" , qemu-devel@nongnu.org, Dheeraj Nigam , Philippe =?UTF-8?B?TWF0aGlldS1EYXVkw6k=?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Tue, 10 Nov 2020 19:46:20 +0530 Kirti Wankhede wrote: > On 11/10/2020 2:40 PM, Dr. David Alan Gilbert wrote: > > * Alex Williamson (alex.williamson@redhat.com) wrote: > >> On Mon, 9 Nov 2020 19:44:17 +0000 > >> "Dr. David Alan Gilbert" wrote: > >> > >>> * Alex Williamson (alex.williamson@redhat.com) wrote: > >>>> Per the proposed documentation for vfio device migration: > >>>> > >>>> Dirty pages are tracked when device is in stop-and-copy phase > >>>> because if pages are marked dirty during pre-copy phase and > >>>> content is transfered from source to destination, there is no > >>>> way to know newly dirtied pages from the point they were copied > >>>> earlier until device stops. To avoid repeated copy of same > >>>> content, pinned pages are marked dirty only during > >>>> stop-and-copy phase. > >>>> > >>>> Essentially, since we don't have hardware dirty page tracking for > >>>> assigned devices at this point, we consider any page that is pinned > >>>> by an mdev vendor driver or pinned and mapped through the IOMMU to > >>>> be perpetually dirty. In the worst case, this may result in all of > >>>> guest memory being considered dirty during every iteration of live > >>>> migration. The current vfio implementation of migration has chosen > >>>> to mask device dirtied pages until the final stages of migration in > >>>> order to avoid this worst case scenario. > >>>> > >>>> Allowing the device to implement a policy decision to prioritize > >>>> reduced migration data like this jeopardizes QEMU's overall ability > >>>> to implement any degree of service level guarantees during migration. > >>>> For example, any estimates towards achieving acceptable downtime > >>>> margins cannot be trusted when such a device is present. The vfio > >>>> device should participate in dirty page tracking to the best of its > >>>> ability throughout migration, even if that means the dirty footprint > >>>> of the device impedes migration progress, allowing both QEMU and > >>>> higher level management tools to decide whether to continue the > >>>> migration or abort due to failure to achieve the desired behavior. > >>> > >>> I don't feel particularly badly about the decision to squash it in > >>> during the stop-and-copy phase; for devices where the pinned memory > >>> is large, I don't think doing it during the main phase makes much sense; > >>> especially if you then have to deal with tracking changes in pinning. > >> > >> > >> AFAIK the kernel support for tracking changes in page pinning already > >> exists, this is largely the vfio device in QEMU that decides when to > >> start exposing the device dirty footprint to QEMU. I'm a bit surprised > >> by this answer though, we don't really know what the device memory > >> footprint is. It might be large, it might be nothing, but by not > >> participating in dirty page tracking until the VM is stopped, we can't > >> know what the footprint is and how it will affect downtime. Is it > >> really the place of a QEMU device driver to impose this sort of policy? > > > > If it could actually track changes then I'd agree we shouldn't impose > > any policy; but if it's just marking the whole area as dirty we're going > > to need a bodge somewhere; this bodge doesn't look any worse than the > > others to me. > > > >> > >>> Having said that, I agree with marking it as experimental, because > >>> I'm dubious how useful it will be for the same reason, I worry > >>> about whether the downtime will be so large to make it pointless. > >> > > Not all device state is large, for example NIC might only report > currently mapped RX buffers which usually not more than a 1GB and could > be as low as 10's of MB. GPU might or might not have large data, that > depends on its use cases. Right, it's only if we have a vendor driver that doesn't pin any memory when dirty tracking is enabled and we're running without a viommu that we would expect all of guest memory to be continuously dirty. > >> TBH I think that's the wrong reason to mark it experimental. There's > >> clearly demand for vfio device migration and even if the practical use > >> cases are initially small, they will expand over time and hardware will > >> get better. My objection is that the current behavior masks the > >> hardware and device limitations, leading to unrealistic expectations. > >> If the user expects minimal downtime, configures convergence to account > >> for that, QEMU thinks it can achieve it, and then the device marks > >> everything dirty, that's not supportable. > > > > Yes, agreed. > > Yes, there is demand for vfio device migration and many devices owners > started scoping and development for migration support. > Instead of making whole migration support as experimental, we can have > opt-in option to decide to mark sys mem pages dirty during iterative > phase (pre-copy phase) of migration. Per my previous suggestion, I'd think an opt-out would be more appropriate, ie. implementing pre-copy dirty page tracking by default. > >> OTOH if the vfio device > >> participates in dirty tracking through pre-copy, then the practical use > >> cases will find themselves as migrations will either be aborted because > >> downtime tolerances cannot be achieved or downtimes will be configured > >> to match reality. Thanks, > > > > Without a way to prioritise the unpinned memory during that period, > > we're going to be repeatedly sending the pinned memory which is going to > > lead to a much larger bandwidth usage that required; so that's going in > > completely the wrong direction and also wrong from the point of view of > > the user. Who decides which is the wrong direction for the user? If the user wants minimal bandwidth regardless of downtime, can't they create a procedure to pause the VM, do the migration, then resume? Are there already migration tunables to effectively achieve this? If a user attempts to do a "live" migration, isn't our priority then shifted to managing the downtime constraints over the bandwidth? IOW the policy decision is implied by the user actions and configuration of the migration, I don't think that at the device level we should be guessing which feature to prioritize, just like a vCPU doesn't to stop marking dirty pages during pre-copy because it's touching too much memory. Higher level policies and configurations should determine inflection points... imo. Thanks, Alex