stapvirt — prepare libvirt domains for systemtap probing
Synopsis
Description
The stapvirt program can be used to add ports to domains managed by libvirt (see <http://libvirt.org/>). These ports can then be used by stap to run scripts inside the domains (see the '--remote' option in stap(1) for more information).
Ports are added to the definition of the domain using the port-add command. These ports can later be removed using the port-remove command. Note that there can only be as many simultaneous stap sessions as there are ports.
Starting from libvirt v1.1.1 and QEMU v0.10.0, SystemTap ports can be hotplugged and thus do not need to be added first using the port-add command. However, you need to ensure that there is a virtio-serial controller in place so that hotplugged ports can be connected. If creating a domain using virt-install, you can do this by adding this option:
$ virt-install [...] --controller=virtio-serial
If the domain has already been created, you can simply do port-add followed immediately by port-remove, and then power off and restart the domain. The port will be removed, but the controller will remain.
Options
The following options are supported. Any other option prints a short help message.
- -c URI
Specify the libvirt driver URI to which to connect (e.g. 'qemu:///system'). The default value is NULL, which indicates to libvirt to connect to the default driver. See the page at <http://libvirt.org/uri.html> for supported values.
- -d PATH
Specify the directory in which UNIX sockets should be created when SystemTap ports are added. The default directory is '/var/lib/libvirt/qemu'.
- -v
Increase verbosity. This option may be repeated for more verbosity.
Commands
The following commands are recognized by stapvirt. Any other command prints a short help message.
- help
Display the help message.
- list
List available domains.
- port-add DOMAIN
Add a permanent SystemTap port to the domain's definition. If the domain is currently running, it must be powered off before changes take effect.
- port-list DOMAIN
List the UNIX socket paths of the permanent SystemTap ports in the domain's definition.
- port-remove DOMAIN
Remove a permanent SystemTap port from the domain's definition. If the domain is currently running, it must be powered off before changes take effect.
- query DOMAIN
Display the following information about the domain: its name, its UUID, its state, the number of permanent SystemTap ports installed, and whether hotplugging is supported.
Tutorial
This tutorial will help you get started with stapvirt. Let's start by listing all the privileged domains on the machine with the list command:
$ stapvirt -c 'qemu:///system' list Available domains on URI 'qemu:///system': ID State Type Name 2 running persistent TestVM
Note that we specified the libvirt URI using the -c switch. Otherwise libvirt might have defaulted to e.g. 'qemu:///session'.
Rather than typing the URI everytime, it might be easier to instead set the LIBVIRT_DEFAULT_URI environment variable and omit the -c switch. Note that this is a libvirt functionality (see <libvirt.org/uri.html> for more details).
The list command indicates that we have a running domain named 'TestVM' with ID 2. Let's use the query command to retrieve more information:
$ stapvirt query TestVM # by name
$ stapvirt query 2 # by ID
Name: TestVM
UUID: 905951c0-fa4f-409b-079c-c91ddda27028
State: running
ID: 2
Type: persistent
Permanent Ports: 0
Hotplugging: not supported
The query command gives us some basic information about the domain, such as its name, UUID, and state. More importantly, it gives us two pieces of information: the number of permanent ports installed, and whether hotplugging is supported. Technically, hotplugging support depends on libvirt and qemu, and is not related to the domain in itself.
If hotplugging were supported, we could stop here and run stap directly (assuming we have a virtio-serial controller already in place, see Description). Since in our case hotplugging is not supported, we need to add SystemTap ports. To do this, we use the port-add command:
$ stapvirt port-add TestVM Added new port org.systemtap.stapsh.0 The domain must be powered off before changes take effect.
We can confirm that a port was added by running the query command again:
$ stapvirt query TestVM
...
Permanent Ports: 1
Hotplugging: not supported
It now indicates that there is 1 permanent port. We can also use the port-list command to know exactly where the port will be created:
$ stapvirt port-list TestVM /var/lib/libvirt/qemu/TestVM.org.systemtap.stapsh.0.sock
After powering off and restarting the domain, we are now ready to use the port with stap:
$ stap -e 'probe begin { printf("Hello from TestVM!\n"); exit() }' \
--remote=libvirt://TestVM
Hello from TestVM!
Finally, if we'd like to remove the port, we can use the port-remove command:
$ stapvirt port-remove TestVM Removed port org.systemtap.stapsh.0 The domain must be powered off before changes take effect.
And that's all there is to it!
See Also
stap(1), virt-install(1)
Bugs
Use the Bugzilla link of the project web page or our mailing list. http://sourceware.org/systemtap/,<systemtap@sourceware.org>.