On Thu, Dec 21, 2017 at 06:01:29AM -0500, Marc-André Lureau wrote: > 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)
Yes, it's similar. Thanks for the link! > > > > 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!)... I'm not going to prototype this yet, I'm working on virtio-vhost-user first, but eventually I might get back to -object vhost-user(-backend). Stefan
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