Scriptable KVM/QEMU guest agent implemented in Python

The Python packages negotiator-host, negotiator-guest and negotiator-common together implement a scriptable KVM/QEMU guest agent infrastructure in Python. This infrastructure supports realtime bidirectional communication between Linux hosts and guests which allows the hosts and guests to invoke user defined commands on 'the other side'.

Because the user defines the commands that hosts and guests can execute, the user controls the amount of influence that hosts and guests have over each other (there are several built-in commands, these are all read only).

Contents

• Status
• Installation
  • On KVM/QEMU hosts
  • On KVM/QEMU guests
• Getting started
• Debugging
  • Broken channels on KVM/QEMU hosts
  • Character device detection fails
• Why another guest agent?
• How does it work?
• Contact
• License

Status

Some points to consider:

• The Negotiator project does what I expect from it: realtime bidirectional communication between Linux based KVM/QEMU hosts and guests.
• The project doesn't have an automated test suite, although its functionality has been extensively tested during development and is being used in a production environment on more than 100 virtual machines (for non-critical tasks).
• The project has not been peer reviewed with regards to security. My primary use case is KVM/QEMU hosts and guests that trust each other to some extent (think private clouds, not shared hosting :-).

Installation

The negotiator packages and their dependencies are compatible with Python 2.6 and newer and are all pure Python. This means you don't need a compiler toolchain to install the negotiator packages. This is a design decision and so won't be changed.

• On KVM/QEMU hosts
• On KVM/QEMU guests

On KVM/QEMU hosts

Here's how to install the negotiator-host package on your host(s):

$ sudo pip install negotiator-host

If you prefer you can install the Python package in a virtual environment:

$ sudo apt-get install --yes python-virtualenv
$ virtualenv /tmp/negotiator-host
$ source /tmp/negotiator-host/bin/activate
$ pip install negotiator-host

After installation the negotiator-host program is available. The usage message will help you get started, try the --help option. Now you need to find a way to run the negotiator-host command as a daemon. I have good experiences with supervisord, here's how to set that up:

$ sudo apt-get install --yes supervisor
$ sudo tee /etc/supervisor/conf.d/negotiator-host.conf >/dev/null << EOF
[program:negotiator-host]
command = /usr/local/bin/negotiator-host --daemon
autostart = True
redirect_stderr = True
stdout_logfile = /var/log/negotiator-host.log
EOF
$ sudo supervisorctl update negotiator-host

On KVM/QEMU guests

Install the negotiator-guest package on your guest(s):

$ sudo pip install negotiator-guest

If you prefer you can install the Python package in a virtual environment:

$ sudo apt-get install --yes python-virtualenv
$ virtualenv /tmp/negotiator-guest
$ source /tmp/negotiator-guest/bin/activate
$ pip install negotiator-guest

After installation you need to find a way to run the negotiator-guest command as a daemon. I have good experiences with supervisord, here's how to set that up:

$ sudo apt-get install --yes supervisor
$ sudo tee /etc/supervisor/conf.d/negotiator-guest.conf >/dev/null << EOF
[program:negotiator-guest]
command = /usr/local/bin/negotiator-guest --daemon
autostart = True
redirect_stderr = True
stdout_logfile = /var/log/negotiator-guest.log
EOF
$ sudo supervisorctl update negotiator-guest

Getting started

If the instructions below are not enough to get you started, take a look at the Debugging section below for hints about what to do when things don't work as expected.

1. First you have to add two virtual devices to your QEMU guest. You can do so by editing the guest's XML definition file. On Ubuntu Linux KVM/QEMU hosts these files are found in the directory /etc/libvirt/qemu. Open the file in your favorite text editor (Vim? :-) and add the the following XML snippet inside the <devices> section:

   <channel type='unix'>
      <source mode='bind' path='/var/lib/libvirt/qemu/channel/target/GUEST_NAME.negotiator-host-to-guest.0' />
      <target type='virtio' name='negotiator-host-to-guest.0' />
   </channel>
   
   <channel type='unix'>
      <source mode='bind' path='/var/lib/libvirt/qemu/channel/target/GUEST_NAME.negotiator-guest-to-host.0' />
      <target type='virtio' name='negotiator-guest-to-host.0' />
   </channel>

   Replace GUEST_NAME with the name of your guest in both places. If you use libvirt 1.0.6 or newer (you can check with virsh --version) you can omit the path='...' attribute because libvirt will fill it in automatically when it reloads the guest's XML definition file (in step 2).

2. After adding the configuration snippet you have to activate it:

   $ sudo virsh define /etc/libvirt/qemu/GUEST_NAME.xml

3. Now you need to shut down the guest and then start it again:

   $ sudo virsh shutdown --mode acpi GUEST_NAME
   $ sudo virsh start GUEST_NAME

   Note that just rebooting the guest will not add the new virtual devices, you have to actually stop the guest and then start it again!

4. Now go and create some scripts in /usr/lib/negotiator/commands and try to execute them from the other side! Once you start writing your own commands it's useful to know that commands on the KVM/QEMU host side have access to some environment variables.

Debugging

This section contains hints about what to do when things don't work as expected.

• Broken channels on KVM/QEMU hosts
• Character device detection fails

Broken channels on KVM/QEMU hosts

Whether you want to get the official QEMU guest agent or the Negotiator project running, you will need a working bidirectional channel. I'm testing Negotiator on an Ubuntu 14.04 KVM/QEMU host and I needed several changes to get things working properly:

$ CHANNELS_DIRECTORY=/var/lib/libvirt/qemu/channel/target
$ sudo mkdir -p $CHANNELS_DIRECTORY
$ sudo chown libvirt-qemu:kvm $CHANNELS_DIRECTORY

The above should be done by the KVM/QEMU system packages if you ask me, but anyway. On top of this if you are running Ubuntu with AppArmor enabled (the default) you may need to apply the following patch:

$ diff -u /etc/apparmor.d/abstractions/libvirt-qemu.orig /etc/apparmor.d/abstractions/libvirt-qemu
--- /etc/apparmor.d/abstractions/libvirt-qemu.orig      2015-09-19 12:46:54.316593334 +0200
+++ /etc/apparmor.d/abstractions/libvirt-qemu   2015-09-24 14:43:43.642064576 +0200
@@ -49,6 +49,9 @@
   /run/shm/ r,
   owner /run/shm/spice.* rw,

+  # Local modification to enable the QEMU guest agent.
+  owner /var/lib/libvirt/qemu/channel/target/* rw,
+
   # 'kill' is not required for sound and is a security risk. Do not enable
   # unless you absolutely need it.
   deny capability kill,

Again this should just be part of the KVM/QEMU system packages, but whatever. The Negotiator project is playing with new-ish functionality so I pretty much know to expect sharp edges :-)

Character device detection fails

When the negotiator-guest program fails to detect the correct character devices it will complain loudly and point you here. Here are some of things I've run into that can cause this:

• The virtual channel(s) have not been correctly configured or the correct configuration hasn't been applied yet. Please carefully follow the instructions in the Getting started section above.
• The kernel module virtio_console is not loaded because it is not available in your kernel. You can check by using the lsmod command. If the module is not loaded you'll need to install and boot to a kernel that does have the module.

Why another guest agent?

The QEMU project provides an official guest agent and this agent is very useful to increase integration between QEMU hosts and guests. However the official QEMU guest agent has two notable shortcomings (for me at least):

Extensibility

The official QEMU guest agent has some generic mechanisms like being able to write files inside guests, but this is a far cry from a generic, extensible architecture. Ideally given the host and guest's permission we should be able to transfer arbitrary data and execute user defined logic on both sides.

Platform support

Despite considerable effort I haven't been able to get a recent version of the QEMU guest agent running on older Linux distributions (e.g. Ubuntu Linux 10.04). Older versions of the guest agent can be succesfully compiled for such distributions but don't support the features I require. By creating my own guest agent I have more control over platform support (given the primitives required for communication).

Note that my project in no way tries to replace the official QEMU guest agent. For example I have no intention of implementing freezing and thawing of file systems because the official agent already does that just fine :-). In other words the two projects share a lot of ideas but have very different goals.

How does it work?

The scriptable guest agent infrastructure uses the same mechanism that the official QEMU guest agent does:

• Inside the guest special character devices are created that allow reading and writing. These character devices are /dev/vport[0-9]p[0-9].
• On the host UNIX domain sockets are created that are connected to the character devices inside the guest. On Ubuntu Linux KVM/QEMU hosts, these UNIX domain sockets are created in the directory /var/lib/libvirt/qemu/channel/target.

Contact

The latest version of negotiator is available on PyPI and GitHub. You can find the documentation on Read The Docs. For bug reports please create an issue on GitHub. If you have questions, suggestions, etc. feel free to send me an e-mail at [email protected].

License

This software is licensed under the MIT license.

© 2015 Peter Odding.