Re: [Qemu-devel] vhost-user graceful connect/disconnect

[Stefan Hajnoczi <stefanha@redhat.com>]

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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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