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=-5.0 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 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 5BEB3C433DB for ; Fri, 19 Feb 2021 09:36:54 +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 9803260230 for ; Fri, 19 Feb 2021 09:36:53 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9803260230 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]:46898 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lD2DU-0005dT-Ds for qemu-devel@archiver.kernel.org; Fri, 19 Feb 2021 04:36:52 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:40906) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lD2CU-0005D6-3i for qemu-devel@nongnu.org; Fri, 19 Feb 2021 04:35:50 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:48123) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1lD2CQ-0003Pw-Nz for qemu-devel@nongnu.org; Fri, 19 Feb 2021 04:35:48 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1613727344; h=from:from:reply-to: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=B2hHtMHAdW4jmq4cmfY5XTkniC9n8xuU8oVFAaVqcv4=; b=apByw1G/RxyAT8A+y8f6tfl3+QB6Y2U4Us0wQSNrRcYwPmnVUjAtr2wZqgVtRVIpewVU/1 vNLGTNyogGcSILu4qweu+Z2xa+2MX0oCNxcu541FDzohMSm9LqN2Zfr9eNDq5knzCYBngC ErB7yfScpZbDN3UqeretvwigGXWMM7w= 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-237-Ry-81WYLNOe_tfb1ruVEEA-1; Fri, 19 Feb 2021 04:35:37 -0500 X-MC-Unique: Ry-81WYLNOe_tfb1ruVEEA-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 B907210082F6; Fri, 19 Feb 2021 09:35:36 +0000 (UTC) Received: from redhat.com (ovpn-113-184.ams2.redhat.com [10.36.113.184]) by smtp.corp.redhat.com (Postfix) with ESMTPS id E7AB0627DC; Fri, 19 Feb 2021 09:35:31 +0000 (UTC) Date: Fri, 19 Feb 2021 09:35:29 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Yuri Benditovich Subject: Re: [PATCH 0/3] virtio-net: graceful drop of vhost for TAP Message-ID: References: <20210204202915.15925-1-yuri.benditovich@daynix.com> <20210209093201-mutt-send-email-mst@kernel.org> <20210209145105.GP1166421@redhat.com> <20210209095553-mutt-send-email-mst@kernel.org> <0890bb17-9677-ff1d-bd08-c9be791e1c81@redhat.com> MIME-Version: 1.0 In-Reply-To: User-Agent: Mutt/2.0.5 (2021-01-21) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=berrange@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 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_LOW=-0.7, RCVD_IN_MSPIKE_H3=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: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: Yan Vugenfirer , Jason Wang , qemu-devel@nongnu.org, "Michael S. Tsirkin" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, Feb 18, 2021 at 09:55:25PM +0200, Yuri Benditovich wrote: > On Thu, Feb 18, 2021 at 11:35 AM Daniel P. Berrangé > wrote: > > > On Wed, Feb 10, 2021 at 02:19:59PM +0800, Jason Wang wrote: > > > > > > On 2021/2/9 下午11:04, Michael S. Tsirkin wrote: > > > > On Tue, Feb 09, 2021 at 02:51:05PM +0000, Daniel P. Berrangé wrote: > > > > > On Tue, Feb 09, 2021 at 09:34:20AM -0500, Michael S. Tsirkin wrote: > > > > > > On Thu, Feb 04, 2021 at 10:29:12PM +0200, Yuri Benditovich wrote: > > > > > > > This set of patches introduces graceful switch from tap-vhost to > > > > > > > tap-no-vhost depending on guest features. Before that the > > features > > > > > > > that vhost does not support were silently cleared in > > get_features. > > > > > > > This creates potential problem of migration from the machine > > where > > > > > > > some of virtio-net features are supported by the vhost kernel to > > the > > > > > > > machine where they are not supported (packed ring as an example). > > > > > > I still worry that adding new features will silently disable vhost > > for people. > > > > > > Can we limit the change to when a VM is migrated in? > > > > > Some management applications expect bi-directional live migration to > > > > > work, so taking specific actions on incoming migration only feels > > > > > dangerous. > > > > Could you be more specific? > > > > > > > > Bi-directional migration is currently broken > > > > when migrating new kernel->old kernel. > > > > > > > > This seems to be the motivation for this patch, though I wish > > > > it was spelled out more explicitly. > > > > > > > > People don't complain much, but I'm fine with fixing that > > > > with a userspace fallback. > > > > > > > > > > > > I'd rather not force the fallback on others though: vhost is generally > > > > specified explicitly by user while features are generally set > > > > automatically, so this patch will make us override what user specified, > > > > not nice. > > > > > > > > > > > > > IMHO if the features we're adding cannot be expected to exist in > > > > > host kernels in general, then the feature should defualt to off > > > > > and require explicit user config to enable. > > > > > Downstream distros which can guarantee newer kernels can flip the > > > > > default in their custom machine types if they desire. > > > > > > > > > > Regards, > > > > > Daniel > > > > Unfortunately that will basically mean we are stuck with no new > > features > > > > for years. We did what this patch is trying to change for years now, in > > > > particular KVM also seems to happily disable CPU features not supported > > > > by kernel so I wonder why we can't keep doing it, with tweaks for some > > > > corner cases. > > > > > > > > > It's probably not the corner case. > > > > > > So my understanding is when a feature is turned on via command line, it > > > should not be cleared silently otherwise we may break migration for sure. > > > > > > E.g when packed=on is specified, we should disable vhost instead of > > clear it > > > from the device. > > > > If something is explicitly turned on by the user, they expect that feature > > to be honoured, or an error to be raised. > > > > If something is not explicitly turned on by the user, the behaviour wrt the > > default should be stable for any given machine type version. > > > > IOW, if you disable vhost by default when packed=on is set, then you can't > > later switch to letting vhost be enabled with packed=on, unless you tie > > that change to a new machine type. > > > > If the user has explicitly said packed=on *and* vhost=on, then should > > must honour that, or raise an error if the combination is unsupportable. > > Silently disabling vhost, then vhost=on is not ok. > > > > If I'm not mistaken: > Inside qemu there is no possibility to determine whether the user > explicitly turned vhost on. > For qemu the vhost is off by default but libvirt creates a new profile with > vhost on. Yes, libvirt will always attempt to enable vhost if it is present on te host with /dev/vhost-net, except where the user explicitly told us not to. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|