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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]:41774 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1kcSOa-0005Gq-Uq for qemu-devel@archiver.kernel.org; Tue, 10 Nov 2020 07:05:08 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:56434) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1kcSNK-0004kV-LB for qemu-devel@nongnu.org; Tue, 10 Nov 2020 07:03:50 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:49733) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1kcSNH-0004tV-FF for qemu-devel@nongnu.org; Tue, 10 Nov 2020 07:03:49 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1605009823; 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=xb5NDBYolxZDBZGyeWCqauLRVuynEynXiKgHa0bzNjI=; b=gnI/NOrBza3rqBLLUq7j2nbrlXoIvXJJaA7KD0NlWiDBWa+tOKRgo2w+JVD8FOemofxLm5 j+5PwfOSFPajCc75JEV6I7QQLa867FkpjkNBNHh2t/N52Qe0fUz8xo2NyuIREO4MEAvdj6 IUvRCsFaOA+izBHB6X+ZLFHJBwcCzcc= 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-168-vw_7oHk7NP2Otl4GVW_I1g-1; Tue, 10 Nov 2020 07:03:41 -0500 X-MC-Unique: vw_7oHk7NP2Otl4GVW_I1g-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 19F451009E37; Tue, 10 Nov 2020 12:03:40 +0000 (UTC) Received: from gondolin (ovpn-112-243.ams2.redhat.com [10.36.112.243]) by smtp.corp.redhat.com (Postfix) with ESMTP id 5CB5C4149; Tue, 10 Nov 2020 12:03:35 +0000 (UTC) Date: Tue, 10 Nov 2020 13:03:14 +0100 From: Cornelia Huck To: Alex Williamson Subject: Re: [RFC PATCH for-QEMU-5.2] vfio: Make migration support experimental Message-ID: <20201110130314.5621be1c.cohuck@redhat.com> In-Reply-To: <160494787833.1473.10514376876696596117.stgit@gimli.home> References: <160494787833.1473.10514376876696596117.stgit@gimli.home> Organization: Red Hat GmbH MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=cohuck@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Received-SPF: pass client-ip=63.128.21.124; envelope-from=cohuck@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/11/10 00:21:06 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_H5=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: Neo Jia , Juan Quintela , qemu-devel@nongnu.org, "Dr. David Alan Gilbert" , Kirti Wankhede , Philippe =?UTF-8?B?TWF0aGlldS1EYXVkw6k=?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Mon, 09 Nov 2020 11:56:02 -0700 Alex Williamson wrote: > Per the proposed documentation for vfio device migration: >=20 > 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. >=20 > 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. >=20 > 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. >=20 > Link: https://lists.gnu.org/archive/html/qemu-devel/2020-11/msg00807.html > Cc: Kirti Wankhede > Cc: Neo Jia > Cc: Dr. David Alan Gilbert > Cc: Juan Quintela > Cc: Philippe Mathieu-Daud=C3=A9 > Cc: Cornelia Huck > Signed-off-by: Alex Williamson > --- >=20 > Given that our discussion in the link above seems to be going in > circles, I'm afraid it seems necessary to both have a contigency > plan and to raise the visibility of the current behavior to > determine whether others agree that this is a sufficiently > troubling behavior to consider migration support experimental > at this stage. Please voice your opinion or contribute patches > to resolve this before QEMU 5.2. Thanks, >=20 > Alex >=20 > hw/vfio/migration.c | 2 +- > hw/vfio/pci.c | 2 ++ > include/hw/vfio/vfio-common.h | 1 + > 3 files changed, 4 insertions(+), 1 deletion(-) Given the ongoing discussions, I'd be rather more comfortable making this experimental for the upcoming release and spent some time getting this into a state that everyone is happy to live with, so Acked-by: Cornelia Huck