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=-15.1 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham 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 6EB6CC433DB for ; Tue, 16 Mar 2021 15:49:03 +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 E7CA7650DC for ; Tue, 16 Mar 2021 15:49:02 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E7CA7650DC 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]:42020 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lMBwM-0007bb-2d for qemu-devel@archiver.kernel.org; Tue, 16 Mar 2021 11:49:02 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:60786) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lMBuk-0006NU-P4 for qemu-devel@nongnu.org; Tue, 16 Mar 2021 11:47:22 -0400 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:59705) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1lMBuh-0001Ce-Nz for qemu-devel@nongnu.org; Tue, 16 Mar 2021 11:47:22 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1615909635; 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=uO4Oi54oe9o+i016MX4DIsX4psFWXLy4E1tpXis0Qvw=; b=Or1FJ/WmaAVn/bGRgF/Vw14HMrzHZPIGhFhkEzyHcAsseSrI7ziApFHLdWK9klRLzHpWjp ogsm8ZyJADzGoyn0w4S/89xW5TN/negaM8gAW8/nxb6dgpjTULym43TbP0kkRNj27EuSkI Gucj67JTiKmL8RyrU2PtS5gQPfZWSsk= 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-472-Fn6HXSPqNIiyuHHvzYWnAw-1; Tue, 16 Mar 2021 11:47:11 -0400 X-MC-Unique: Fn6HXSPqNIiyuHHvzYWnAw-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id A2D8DA40C1; Tue, 16 Mar 2021 15:47:08 +0000 (UTC) Received: from work-vm (ovpn-113-133.ams2.redhat.com [10.36.113.133]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 475F760C0F; Tue, 16 Mar 2021 15:47:01 +0000 (UTC) Date: Tue, 16 Mar 2021 15:46:57 +0000 From: "Dr. David Alan Gilbert" To: "Tian, Kevin" Subject: Re: [PATCH v2 1/1] docs/devel: Add VFIO device migration documentation Message-ID: References: <20210310192009.53848-1-targupta@nvidia.com> MIME-Version: 1.0 In-Reply-To: User-Agent: Mutt/2.0.5 (2021-01-21) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=dgilbert@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=63.128.21.124; envelope-from=dgilbert@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -29 X-Spam_score: -3.0 X-Spam_bar: --- X-Spam_report: (-3.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.25, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, 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: "alex.williamson@redhat.com" , "cjia@nvidia.com" , "quintela@redhat.com" , "cohuck@redhat.com" , "qemu-devel@nongnu.org" , "Zhao, Yan Y" , "lushenming@huawei.com" , Kirti Wankhede , Tarun Gupta , Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= , "philmd@redhat.com" , "dnigam@nvidia.com" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Tian, Kevin (kevin.tian@intel.com) wrote: > > From: Qemu-devel > > On Behalf Of Dr. David Alan Gilbert > > > > * Daniel P. Berrangé (berrange@redhat.com) wrote: > > > On Thu, Mar 11, 2021 at 12:50:09AM +0530, Tarun Gupta wrote: > > > > Document interfaces used for VFIO device migration. Added flow of state > > changes > > > > during live migration with VFIO device. Tested by building docs with the > > new > > > > vfio-migration.rst file. > > > > > > > > v2: > > > > - Included the new vfio-migration.rst file in index.rst > > > > - Updated dirty page tracking section, also added details about > > > > 'pre-copy-dirty-page-tracking' opt-out option. > > > > - Incorporated comments around wording of doc. > > > > > > > > Signed-off-by: Tarun Gupta > > > > Signed-off-by: Kirti Wankhede > > > > --- > > > > MAINTAINERS | 1 + > > > > docs/devel/index.rst | 1 + > > > > docs/devel/vfio-migration.rst | 135 > > ++++++++++++++++++++++++++++++++++ > > > > 3 files changed, 137 insertions(+) > > > > create mode 100644 docs/devel/vfio-migration.rst > > > > > > > > > > +Postcopy > > > > +======== > > > > + > > > > +Postcopy migration is not supported for VFIO devices. > > > > > > What is the problem here and is there any plan for how to address it ? > > > > There's no equivalent to userfaultfd for accesses to RAM made by a > > device. > > There's some potential for this to be doable with an IOMMU or the like, > > but: > > a) IOMMUs and devices aren't currently happy at recovering from > > failures > > b) the fragementation you get during a postcopy probably isn't pretty > > when you get to build IOMMU tables. > > To overcome such limitations one may adopt a prefault-and-pull scheme if > the vendor driver has the capability to track pending DMA buffers in the > migration process (with additional uAPI changes in VFIO or userfaultfd), > as discussed here: > > https://static.sched.com/hosted_files/kvmforum2019/7a/kvm-forum-postcopy-final.pdf Did that get any further? I can imagine that might be tricikier for a GPU than a network card; the shaders in a GPU are pretty random as to what they go off and access, so I can't see how you could prefault. Dave > > > > > Postcopy is essentially the only migration mechanism that can reliably > > > complete, so it really should be considered the default approach to > > > migration for all mgmt apps wanting to do migration, except in special > > > cases. IOW, if we want VFIO migration to be viable, we need postcopy > > > support. > > > > There's lots of other things postcopy doesn't work with; so hmm. > > > > Agree. Also given the amount of work even for pre-copy migration, it makes > more sense to do things step-by-step. > > Thanks > Kevin > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK