[libvirt] RFC decoupling VM NIC provisioning from VM NIC connection to backend networks

[Daniel P. Berrange]

On Fri, Oct 28, 2011 at 04:15:41PM -0700, Sumit Naiksatam (snaiksat) wrote:
> Hi,
> 
> In its current implementation Libvirt makes sure that the network
> interfaces that it passes/provision to a VM (for example to qemu[-kvm])
> are already connected to its backend (interfaces/networks) by the time
> the VM starts its boot process. In a non virtualized setup it would be
> like booting a machine with the Ethernet cable already plugged into a
> router/switch port. While in a non virtualized setup you can boot a
> machine first (with no physical connection to a router/switch) and later
> connect its NIC/s to the switch/router, when you boot a VM via Libvirt
> it is not possible to decouple the two actions (VM boot, cable
> plug/unplug).
> 
> An example of case where the capability of decoupling the two actions
> mentioned above is a requirement in Quantum/NetStack which is the
> network service leveraged by OpenStack. The modular design of OpenStack
> allows you to:
> - provision VMs with NIC/s
> - create networks
> - create ports on networks
> - plug/unplug a VM NIC into/from a given port on a network (at runtime)
> 
> Note that this runtime plug/unplug requirement has nothing to do with
> hot plug/unplug of NICs.
> The idea is more that of decoupling the provisioning of a VM from the
> connection of the VM to the network/s.
> This would make it possible to change (at run-time too) the networks the
> NIC/s of a given VM are connected to.
> 
> For example, when a VM boots, its interfaces should be in link down
> state if the network admin has not connected the VM NIC/s to any
> "network" yet.
> Even though libvirt already provides a way to change the link state of
> an a VM NIC, link state and physical connection are two different things
> and should be manageable independently.
> 
> Ideally the configuration syntax should be interface type and hypervisor
> type agnostic.
> 
> Let's take QEMU[-kvm] as an example - when Libvirt starts a QEMU VM, it
> passes to QEMU a number of file descriptors that map to host backend
> interfaces (for example macvtap interfaces).
> 
> In order to introduce this runtime plug/unplug capability, we need a
> mechanism that permits to delay the binding between the host macvtap
> interfaces and the guest taps (because you cannot know the fd of the
> macvtap interfaces before you create them). This means you need a
> mechanism that allows you to change such fd/s at runtime:
> 
> - you can close/reset an fd (ie, when you disconnect a VM NIC from its
> network)
> - you can open/set an fd (ie, when you connect a VM NIC to a network)
> 
> This could probably be a libvirt command that translates to a QEMU
> monitor command.
> 
> Can the runtime plug/unplug capability described above be achieved
> (cleanly) with another mechanism?
> 
> Is anybody working on implementing something similar?

No, but I've long thought about doing this & it is quite straightforward
todo really. Ordinarily when we start QEMU we do

   qemu ...  -device e1000,id=nic0,netdev=netdevnic0 \
             -netdev user,id=netdevnic0

Todo what you describe we need to be able to:

 1. Start QEMU with a NIC, but no netdev
 2. Add a netdev to running QEMU.
 3. Remove a netdev from a running QEMU
 4. Associate a netdev with a NIC in running QEMU

We can do 1:

  $ qemu ...  -device e1000,id=nic0

But QEMU prints an annoying warning

  Warning: nic nic0 has no peer

We can do 2 via the monitor:

  (qemu) netdev_add type=user,id=netdevnic0

We can do 3 via the monitor:

  (qemu) netdev_del netdevnic0


The problem is 4 - AFAICT we can't connect the existing NIC upto the newly
hotplugged netdev, since we can't update the 'netdev' property in the NIC
device. Also if we delete the netdev, we can't clear out the 'netdev'
property in the NIC, so its dangling to a netdev that no longer exists.
The latter is fairly harmless, since we can just re-use the name if adding
a new backend later. The first problem is a bit of a pain, unless we plug
in a 'user' backend on the CLI, and immediately netdev_del it before starting
the CPUs. Ideally we'd have some way to set qdev properties for devices so we
can associate the NIC with the new netdev.

eg when adding a netdev:

   (qemu) netdev_add type=user,id=netdevnic0
   (qemu) set nic0 netdev=netdevnic0

Or removing one

   (qemu) netdev_add netdevnic0
   (qemu) unset nic0 netdev


WRT to libvirt XML config. Normally you specifiy a NIC like

     <interface type='network'>
      <mac address='52:54:00:0f:7d:ad'/>
      <source network='default'/>
      <model type='virtio'/>
    </interface>

To boot a guest without any netdev backend present, we'd introduce a
new network type="none". eg

     <interface type='none'>
       <mac address='52:54:00:0f:7d:ad'/>
       <model type='virtio'/>
     </interface>

The existing API  'virDomainUpdateDevice', can then be used to change
the interface config on the fly, adding or removing the netdev by
passing in new XML with a different 'type' attribute & <source>
element.

Finally, when adding & removing the netdev backends to a running guest,
we likely want to be able to set the NIC's  link carrier, so the guest
OS sees that it has lost / gain its network connection & will thus
retry DHCP / IPv6 autoconfig. There is already a QEMU montior command
'set_link' for changing the NIC link carrier. A minor problem is that,
AFAICT, we can't specify the link carrier state on the command line
when specifying the NIC hardware, eg we would want something like
this when starting a guest without a netdev back

  qemu ... -device e1000,id=nic0,link=down

And when adding a netdev we would do

  (qemu) netdev_add user,id=netdevnic0
  (qemu) set nic0 netdev=netdevnic0
  (qemu) set_link nic0 up

Or when removing a netdev

  (qemu) set_link nic0 down
  (qemu) unset nic0 netdev
  (qemu) netdev_del user,id=netdevnic0


Regards,
Daniel
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