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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]:42908 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1kcD6Q-0003mX-G3 for qemu-devel@archiver.kernel.org; Mon, 09 Nov 2020 14:45:22 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:39996) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1kcD5h-0003MK-V5 for qemu-devel@nongnu.org; Mon, 09 Nov 2020 14:44:37 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:47766) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1kcD5b-0001VL-Fi for qemu-devel@nongnu.org; Mon, 09 Nov 2020 14:44:37 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1604951069; 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=04zSg1wfjqeTTL4OfYZJDdptqJN4H0DhFzAZiKgAqek=; b=SjLG61f5XfxbpmxiP5fBb6BpiLCIbQz9u7NBSqX3YKkvdf4BuM8mY7Wa37v8znqFJRUNZ6 F9i4JSRRgsqO93BEJ9u7DVNFGket3J6D6UUx+Xh65pGCSDglsrVY6sodFWEu/ts0evLIzb F3upBE1VR5mXsrstpF2OKVSrIz3Tz+M= 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-355-asS4GHdzPtu2oLTqwRZXCg-1; Mon, 09 Nov 2020 14:44:25 -0500 X-MC-Unique: asS4GHdzPtu2oLTqwRZXCg-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id C2A3664087; Mon, 9 Nov 2020 19:44:24 +0000 (UTC) Received: from work-vm (ovpn-114-198.ams2.redhat.com [10.36.114.198]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 38D3A7B40D; Mon, 9 Nov 2020 19:44:20 +0000 (UTC) Date: Mon, 9 Nov 2020 19:44:17 +0000 From: "Dr. David Alan Gilbert" To: Alex Williamson Subject: Re: [RFC PATCH for-QEMU-5.2] vfio: Make migration support experimental Message-ID: <20201109194417.GR3024@work-vm> References: <160494787833.1473.10514376876696596117.stgit@gimli.home> MIME-Version: 1.0 In-Reply-To: <160494787833.1473.10514376876696596117.stgit@gimli.home> User-Agent: Mutt/1.14.6 (2020-07-11) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 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=216.205.24.124; envelope-from=dgilbert@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/11/09 00:04:29 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: Neo Jia , Juan Quintela , Cornelia Huck , qemu-devel@nongnu.org, Kirti Wankhede , Philippe =?iso-8859-1?Q?Mathieu-Daud=E9?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * 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. 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. Reviewed-by: Dr. David Alan Gilbert > 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-Daudé > Cc: Cornelia Huck > Signed-off-by: Alex Williamson > --- > > 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, > > Alex > > hw/vfio/migration.c | 2 +- > hw/vfio/pci.c | 2 ++ > include/hw/vfio/vfio-common.h | 1 + > 3 files changed, 4 insertions(+), 1 deletion(-) > > diff --git a/hw/vfio/migration.c b/hw/vfio/migration.c > index 3ce285ea395d..cd44d465a50b 100644 > --- a/hw/vfio/migration.c > +++ b/hw/vfio/migration.c > @@ -882,7 +882,7 @@ int vfio_migration_probe(VFIODevice *vbasedev, Error **errp) > Error *local_err = NULL; > int ret = -ENOTSUP; > > - if (!container->dirty_pages_supported) { > + if (!vbasedev->enable_migration || !container->dirty_pages_supported) { > goto add_blocker; > } > > diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c > index 58c0ce8971e3..1349b900e513 100644 > --- a/hw/vfio/pci.c > +++ b/hw/vfio/pci.c > @@ -3194,6 +3194,8 @@ static Property vfio_pci_dev_properties[] = { > VFIO_FEATURE_ENABLE_REQ_BIT, true), > DEFINE_PROP_BIT("x-igd-opregion", VFIOPCIDevice, features, > VFIO_FEATURE_ENABLE_IGD_OPREGION_BIT, false), > + DEFINE_PROP_BOOL("x-enable-migration", VFIOPCIDevice, > + vbasedev.enable_migration, false), > DEFINE_PROP_BOOL("x-no-mmap", VFIOPCIDevice, vbasedev.no_mmap, false), > DEFINE_PROP_BOOL("x-balloon-allowed", VFIOPCIDevice, > vbasedev.ram_block_discard_allowed, false), > diff --git a/include/hw/vfio/vfio-common.h b/include/hw/vfio/vfio-common.h > index baeb4dcff102..2119872c8af1 100644 > --- a/include/hw/vfio/vfio-common.h > +++ b/include/hw/vfio/vfio-common.h > @@ -123,6 +123,7 @@ typedef struct VFIODevice { > bool needs_reset; > bool no_mmap; > bool ram_block_discard_allowed; > + bool enable_migration; > VFIODeviceOps *ops; > unsigned int num_irqs; > unsigned int num_regions; > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK