On Tue, Dec 19, 2017 at 04:21:51PM +0000, Stefan Hajnoczi wrote: > 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.
I'm not sure how important or feasible this last part is. > 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? > > Stefan This sounds like an interesting set of ideas to me. Something to keep in mind for the future. Thanks! -- MST
