kbase: Add knowledge base for libvirt systemtap

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Han Han <hhan@redhat.com>

docs/kbase.rst

@@ -36,6 +36,9 @@ Knowledge base
       Examination of the security protections used for QEMU and how they need
       configuring to allow use of QEMU passthrough with host files/devices.
 
+   `Systemtap <kbase/systemtap.html>`__
+      Explanation of how to use systemtap for libvirt tracing.
+
    `Virtio-FS <kbase/virtiofs.html>`__
       Share a filesystem between the guest and the host
 

docs/kbase/meson.build

@@ -13,6 +13,7 @@ docs_kbase_files = [
   'rpm-deployment',
   's390_protected_virt',
   'secureusage',
+  'systemtap',
   'virtiofs',
 ]
 

docs/kbase/systemtap.rst

@@ -0,0 +1,113 @@
+====================
+Systemtap of Libvirt
+====================
+
+.. contents::
+
+`Systemtap <https://sourceware.org/systemtap/>`__ is a scripting
+language and tool for dynamically probing or tracing in Linux kernel
+space or user space. This page is about the usage of systemtap
+in libvirt tracing.
+
+Preparation
+===========
+
+Libvirt
+-------
+
+Libvirt should be configured with the systemtap option to support libvirt
+probing events in systemtap.
+
+For libvirt before **6.7.0**, it can be configured by:
+
+::
+
+    mkdir build
+    cd build
+   ../configure --with-dtrace
+
+For libvirt **6.7.0** or later, configure it by the ``meson`` (seeing
+`libvirt compiling <https://libvirt.org/compiling.html>`__):
+
+::
+
+   meson build -Ddtrace=enabled
+
+For the libvirt binaries installed by the package manager like ``dnf`` or
+``apt``, if libvirt systemtap tapset ``/usr/share/systemtap/tapset/libvirt_*``
+exists, it means the libvirt enables the systemtap feature.
+
+Systemtap
+---------
+
+For most of linux distributions, execute ``stap-prep`` by root to prepare the
+environment for systemtap after installing the systemtap. If your distribution
+doesn't have ``stap-prep``, install the ``kernel debuginfo`` packages manually.
+
+After these above, run this test command to confirm the systemtap works well:
+
+::
+
+   stap -e 'probe oneshot{ printf("hello world\n")}'
+
+
+Tracing events
+==============
+
+The libvirt systemtap tracing events are defined in tapset
+``/usr/share/systemtap/tapset/libvirt_*``. Libvirt support these type of tracing
+events: ``dbus``, ``event_glib``, ``object``, ``qemu``, ``rpc``.
+
+List all tracing events in libvirt:
+
+::
+
+   grep 'probe libvirt.[a-z_0-9.]*' /usr/share/systemtap/tapset/libvirt_* -o|cut -f 2 -d :
+
+
+Tracing examples
+================
+
+Here is an example of the systemtap script to trace the QMP messages sent from libvirtd
+daemon to the qemu process.
+``qmp.stp``:
+
+::
+
+   probe begin
+   {
+     printf("Start tracing\n")
+   }
+   probe libvirt.qemu.monitor_send_msg
+   {
+     printf("QMPs: %s", msg);
+   }
+
+Then run the systemtap script attaching to the libvirtd process:
+
+::
+
+   stap qmp.stp -x `pidof libvirtd`
+
+
+To trace a libvirtd started from command line, use the option ``-c``
+
+::
+
+   stap qmp.stp -c "/usr/sbin/libvirtd"
+
+
+Then after seeing the welcome message "Start tracing" from systemtap, then execute a virsh
+command associated with QMP, for example ``virsh domstats``. Then get the QMP tracing logs
+from systemtap. For example, the result from ``virsh domstats``
+
+::
+
+   QMPs: {"execute":"query-balloon","id":"libvirt-393"}
+   QMPs: {"execute":"qom-get","arguments":{"path":"/machine/peripheral/balloon0","property":"guest-stats"},"id":"libvirt-394"}
+   QMPs: {"execute":"query-blockstats","id":"libvirt-395"}
+   QMPs: {"execute":"query-named-block-nodes","id":"libvirt-396"}
+   QMPs: {"execute":"query-iothreads","id":"libvirt-397"}
+
+For more examples of libvirt systemtap scripts, see the scripts in ``/usr/share/doc/libvirt-docs/examples/systemtap``
+For more details of systemtap language, see `document of systemtap <https://sourceware.org/systemtap/documentation.html>`__