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Re: [Qemu-devel] vhost-user graceful connect/disconnect
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] vhost-user graceful connect/disconnect |
Date: |
Thu, 4 Jan 2018 10:47:44 +0000 |
User-agent: |
Mutt/1.9.1 (2017-09-22) |
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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Stefan Hajnoczi <=