From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mga02.intel.com (mga02.intel.com [134.134.136.20]) by dpdk.org (Postfix) with ESMTP id EBEB32C39 for ; Wed, 28 Sep 2016 04:28:13 +0200 (CEST) Received: from fmsmga006.fm.intel.com ([10.253.24.20]) by orsmga101.jf.intel.com with ESMTP; 27 Sep 2016 19:28:12 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.30,408,1470726000"; d="scan'208";a="13932485" Received: from yliu-dev.sh.intel.com (HELO yliu-dev) ([10.239.67.162]) by fmsmga006.fm.intel.com with ESMTP; 27 Sep 2016 19:28:11 -0700 Date: Wed, 28 Sep 2016 10:28:48 +0800 From: Yuanhan Liu To: "Michael S. Tsirkin" Cc: Stephen Hemminger , Maxime Coquelin , dev@dpdk.org, qemu-devel@nongnu.org Message-ID: <20160928022848.GE1597@yliu-dev.sh.intel.com> References: <1474872056-24665-1-git-send-email-yuanhan.liu@linux.intel.com> <1474872056-24665-2-git-send-email-yuanhan.liu@linux.intel.com> <20160926221112-mutt-send-email-mst@kernel.org> <20160927031158.GA25823@yliu-dev.sh.intel.com> <20160927224935-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160927224935-mutt-send-email-mst@kernel.org> User-Agent: Mutt/1.5.23 (2014-03-12) Subject: Re: [dpdk-dev] [PATCH 1/2] vhost: enable any layout feature X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: patches and discussions about DPDK List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 28 Sep 2016 02:28:14 -0000 On Tue, Sep 27, 2016 at 10:56:40PM +0300, Michael S. Tsirkin wrote: > On Tue, Sep 27, 2016 at 11:11:58AM +0800, Yuanhan Liu wrote: > > On Mon, Sep 26, 2016 at 10:24:55PM +0300, Michael S. Tsirkin wrote: > > > On Mon, Sep 26, 2016 at 11:01:58AM -0700, Stephen Hemminger wrote: > > > > I assume that if using Version 1 that the bit will be ignored > > > > Yes, but I will just quote what you just said: what if the guest > > virtio device is a legacy device? I also gave my reasons in another > > email why I consistently set this flag: > > > > - we have to return all features we support to the guest. > > > > We don't know the guest is a modern or legacy device. That means > > we should claim we support both: VERSION_1 and ANY_LAYOUT. > > > > Assume guest is a legacy device and we just set VERSION_1 (the current > > case), ANY_LAYOUT will never be negotiated. > > > > - I'm following the way Linux kernel takes: it also set both features. > > > > Maybe, we could unset ANY_LAYOUT when VERSION_1 is _negotiated_? > > > > The unset after negotiation I proposed turned out it won't work: the > > feature is already negotiated; unsetting it only in vhost side doesn't > > change anything. Besides, it may break the migration as Michael stated > > below. > > I think the reverse. Teach vhost user that for future machine types > only VERSION_1 implies ANY_LAYOUT. > > > > > Therein lies a problem. If dpdk tweaks flags, updating it > > > will break guest migration. > > > > > > One way is to require that users specify all flags fully when > > > creating the virtio net device. > > > > Like how? By a new command line option? And user has to type > > all those features? > > Make libvirt do this. users use management normally. those that don't > likely don't migrate VMs. Fair enough. > > > > QEMU could verify that all required > > > flags are set, and fail init if not. > > > > > > This has other advantages, e.g. it adds ability to > > > init device without waiting for dpdk to connect. Will the feature negotiation between DPDK and QEMU still exist in your proposal? > > > > > > However, enabling each new feature would now require > > > management work. How about dpdk ships the list > > > of supported features instead? > > > Management tools could read them on source and destination > > > and select features supported on both sides. > > > > That means the management tool would somehow has a dependency on > > DPDK project, which I have no objection at all. But, is that > > a good idea? > > It already starts the bridge somehow, does it not? Indeed. I was firstly thinking about reading the dpdk source file to determine the DPDK supported feature list, with which the bind is too tight. I later realized you may ask DPDK to provide a binary to dump the list, or something like that. > > > BTW, I'm not quite sure I followed your idea. I mean, how it supposed > > to fix the ANY_LAYOUT issue here? How this flag will be set for > > legacy device? > > > > --yliu > > For ANY_LAYOUT, I think we should just set in in qemu, > but only for new machine types. What do you mean by "new machine types"? Virtio device with newer virtio-spec version? > This addresses migration > concerns. To make sure I followed you, do you mean the migration issue from an older "dpdk + qemu" combo to a newer "dpdk + qemu" combo (that more new features might be shipped)? Besides that, your proposal looks like a big work to accomplish. Are you okay to make it simple first: set it consistently like what Linux kernel does? This would at least make the ANY_LAYOUT actually be enabled for legacy device (which is also the default one that's widely used so far). --yliu > > But there will be more new features in the future and > it is necessary to think how we will enable them without > breaking migration. > > -- > MST