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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]:56130 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1kcPgW-0005nM-NY for qemu-devel@archiver.kernel.org; Tue, 10 Nov 2020 04:11:28 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:47328) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1kcPfv-0005Mq-Dp for qemu-devel@nongnu.org; Tue, 10 Nov 2020 04:10:51 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:45770) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1kcPft-0003lR-7A for qemu-devel@nongnu.org; Tue, 10 Nov 2020 04:10:51 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1604999448; 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=rK0MUXq6Dtt1nuLrklZfRi2vT7uZ4oFftHQ9pE7j84o=; b=QtwDu+SCHdTQTabe3fNQ93loooxkNRe4NwwPaDe/S5v00ZqShRbvHikkwOCFM9XjRRQZ/F CSzELyQe2gCDHhf3f3iA9C3uXhGotWIQv6z//A4osZzl3tbOKlobAFo918u+M6GVb0q3c9 kRrSho0Y+W8QtfxEtBthk/KHB6NDbfY= 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-499-jYFDmTYgOLW8XWCuPXqZhw-1; Tue, 10 Nov 2020 04:10:45 -0500 X-MC-Unique: jYFDmTYgOLW8XWCuPXqZhw-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 8D82218CB720; Tue, 10 Nov 2020 09:10:44 +0000 (UTC) Received: from work-vm (ovpn-115-49.ams2.redhat.com [10.36.115.49]) by smtp.corp.redhat.com (Postfix) with ESMTPS id D003875135; Tue, 10 Nov 2020 09:10:39 +0000 (UTC) Date: Tue, 10 Nov 2020 09:10:37 +0000 From: "Dr. David Alan Gilbert" To: Alex Williamson Subject: Re: [RFC PATCH for-QEMU-5.2] vfio: Make migration support experimental Message-ID: <20201110091037.GA3108@work-vm> References: <160494787833.1473.10514376876696596117.stgit@gimli.home> <20201109194417.GR3024@work-vm> <20201109132950.6c2dfe02@w520.home> MIME-Version: 1.0 In-Reply-To: <20201109132950.6c2dfe02@w520.home> User-Agent: Mutt/1.14.6 (2020-07-11) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 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 = 2020/11/10 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: 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: > 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. > > > 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. > 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. Dave > > Alex > > > 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