From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:52878) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1eRybj-0001yg-Uz for qemu-devel@nongnu.org; Thu, 21 Dec 2017 06:01:53 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1eRybV-0001sG-0E for qemu-devel@nongnu.org; Thu, 21 Dec 2017 06:01:42 -0500 Received: from mx1.redhat.com ([209.132.183.28]:52418) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1eRybU-0001q9-NI for qemu-devel@nongnu.org; Thu, 21 Dec 2017 06:01:32 -0500 Date: Thu, 21 Dec 2017 06:01:29 -0500 (EST) From: =?utf-8?Q?Marc-Andr=C3=A9?= Lureau Message-ID: <1512291492.2209478.1513854089861.JavaMail.zimbra@redhat.com> In-Reply-To: <20171219162151.GA10889@stefanha-x1.localdomain> References: <20171219162151.GA10889@stefanha-x1.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] vhost-user graceful connect/disconnect List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Hajnoczi Cc: qemu-devel@nongnu.org, mukawa@igel.co.jp, "Michael S. Tsirkin" , Wei Wang , Maxime Coquelin , n nikolaev Hi ----- Original Message ----- > Hi, > Vhost-user implementations assume the slave is already running before > the master starts. The slave is required during virtio device > initialization (e.g. feature bit negotiation) and so it is simplest to > assume that the master is already available and will respond immediately > to the VHOST_USER_GET_FEATURES message. > > I have thought about how to let master and slave start in any order. > Some approaches involve changes to the VIRTIO specification so that > guest drivers can wait until the vhost-user connection is established. > > We can avoid spec changes using PCI hotplug: > > 1. Introduce a new vhost-user object that manages a connection: > > -chardev ... > -object vhost-user,id=vhost-user0,chardev=chr0 > > Note this object is *not* a NetClient. It's a resource for managing > a vhost-user connection and can be used with any device type (net, > scsi, blk, etc). > > This object tries to establish a connection. When a connection is > established it sends vhost-user protocol messages to fetch > information needed for virtio device initialization (like the number > of virtqueues supported, features bits, etc). This information is > stashed so that vhost_*() calls later on do not require synchronous > communication with the vhost-user slave. This share similarities with vhost-user-backend I proposed here: https://lists.gnu.org/archive/html/qemu-devel/2016-06/msg01014.html (instead of spawning a slave, talk to a chardev - both could eventually co-exist) > > 2. When the vhost-user connection is established the object emits a QMP > event so management software can hotplug the virtio device: > > VHOST_USER_CONNECTED source=vhost-user0 > > 3. The management software hotplugs the virtio device: > > (qmp) netdev_add vhost-user,id=netdev0,vhost-user=vhost-user0 > (qmp) device_add virtio-net-pci,netdev=netdev0 > > Advantages of this approach: > > * Does not require spec changes. > * Can be implemented without vhost-user.c qemu_chr_fe_read/write_all() > calls that hang QEMU until the slave sends data. > * Allows slave to set the number of queues, feature bits, etc via the > vhost-user socket. It is not necessary to manually specify feature > bitmasks and other slave-specific information on the QEMU > command-line. > > I haven't thought through disconnection but I imagine the vhost-user > object would emit a VHOST_USER_DISCONNECTED event so the management > software can hot unplug the device. > > Thoughts? I am sorry, I can't think through all the potential issues easily. But that sounds interesting enough to start a proof-of-concept. And we need more tests for vhost-user (including fixing the existing tests!)... Thanks