Xen Test VM

This repository contains OCaml code to build a minimal para-virtualised kernel to run on the Xen hypervisor for testing Xen Server. The kernel is built using the Mirage unikernel framework.

Building

This relies on the Mirage framework which generates the build environment by installing packages in the local switch. In that regard it is different from typical OCaml projects.

$ make

This runs mirage, installs packages, and compiles the sources. As such, this does not work in a sandboxed environment because it relies on installing more OCaml packages.

Installing the VM

Use

$ ./install.sh host

to install the kernel on a XenServer host using ssh root access on host. See the script for how it uses the xe command to register the kernel as a VM.

Out-of-Band Control Messages

In addition to the shutdown messages sent by Xen, the kernel monitors the Xen Store for messages. These are used to control the response to shutdown messages.

Shutdown Messages

The kernel responds to these messages in "control/shutdown". Usually the hypervisor only sends these.

suspend  
poweroff 
reboot   
halt     
crash    

All other messages are logged and ignored.

Testing Messages

The kernel reads messages in "control/testing". It acknowledges a message by replacing the read message with the empty string.

A message in "control/testing" is a JSON object:

{ "when":       "now"           // when to react
, "ack":        "ok"            // how to ack control/shutdown
, "action":     "reboot"        // how to react to control/shutdown
}

Note that proper JSON does not permit //-style comments. The message describes three aspects:

1. "when": either "now" or "onshutdown". The kernel will either immediately or when then next shutdown message arrives perform the "action".

2. "ack": either "ok", "none", "delete", or something else. This controls, how the kernel acknowledges the next shutdown message.

   • "ok": regular behavior
   • "none": don't acknowledge the message
   • "delete": delete "control/shutdown"
   • "something": write the string read to "control/shutdown"

3. "action": what do do (either now or on shutdown). The message in control/shutdown is ignored and superseded by the action field:

   • "suspend": suspend
   • "poweroff": power off
   • "reboot": reboot
   • "halt": halt
   • "crash": crash
   • "ignore": do nothing - ignore the message

To write to control/testing, use:

msg='{"when":"now","ack":"ok","action":"reboot"}'
xenstore write /local/domain/<domid>/control/testing "$msg"

The domid is logged to the console and can be obtained through the Xen API.

Debugging the VM

To direct console output of the VM to a file, you can tell the $HOST:

xenstore write /local/logconsole/@ "/tmp/console.%d"

Output then goes to /tmp/console.<domid>.