Documentation/rv: Add deterministic automata monitor synthesis documentation

Add the da_monitor_synthesis.rst introduces some concepts behind the
Deterministic Automata (DA) monitor synthesis and interface.

Link: https://lkml.kernel.org/r/7873bdb7b2e5d2bc0b2eb6ca0b324af9a0ba27a0.1659052063.git.bristot@kernel.org

Cc: Wim Van Sebroeck <wim@linux-watchdog.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marco Elver <elver@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Gabriele Paoloni <gpaoloni@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Clark Williams <williams@redhat.com>
Cc: Tao Zhou <tao.zhou@linux.dev>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-trace-devel@vger.kernel.org
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

2 files changed, 148 insertions, 0 deletions

diff --git a/Documentation/trace/rv/da_monitor_synthesis.rst b/Documentation/trace/rv/da_monitor_synthesis.rst
new file mode 100644
index 000000000000..0dbdcd1e62b9
--- /dev/null
+++ b/Documentation/trace/rv/da_monitor_synthesis.rst
@@ -0,0 +1,147 @@
+Deterministic Automata Monitor Synthesis
+========================================
+
+The starting point for the application of runtime verification (RV) technics
+is the *specification* or *modeling* of the desired (or undesired) behavior
+of the system under scrutiny.
+
+The formal representation needs to be then *synthesized* into a *monitor*
+that can then be used in the analysis of the trace of the system. The
+*monitor* connects to the system via an *instrumentation* that converts
+the events from the *system* to the events of the *specification*.
+
+
+In Linux terms, the runtime verification monitors are encapsulated inside
+the *RV monitor* abstraction. The RV monitor includes a set of instances
+of the monitor (per-cpu monitor, per-task monitor, and so on), the helper
+functions that glue the monitor to the system reference model, and the
+trace output as a reaction to event parsing and exceptions, as depicted
+below::
+
+ Linux  +----- RV Monitor ----------------------------------+ Formal
+  Realm |                                                   |  Realm
+  +-------------------+     +----------------+     +-----------------+
+  |   Linux kernel    |     |     Monitor    |     |     Reference   |
+  |     Tracing       |  -> |   Instance(s)  | <-  |       Model     |
+  | (instrumentation) |     | (verification) |     | (specification) |
+  +-------------------+     +----------------+     +-----------------+
+         |                          |                       |
+         |                          V                       |
+         |                     +----------+                 |
+         |                     | Reaction |                 |
+         |                     +--+--+--+-+                 |
+         |                        |  |  |                   |
+         |                        |  |  +-> trace output ?  |
+         +------------------------|--|----------------------+
+                                  |  +----> panic ?
+                                  +-------> <user-specified>
+
+DA monitor synthesis
+--------------------
+
+The synthesis of automata-based models into the Linux *RV monitor* abstraction
+is automated by the dot2k tool and the rv/da_monitor.h header file that
+contains a set of macros that automatically generate the monitor's code.
+
+dot2k
+-----
+
+The dot2k utility leverages dot2c by converting an automaton model in
+the DOT format into the C representation [1] and creating the skeleton of
+a kernel monitor in C.
+
+For example, it is possible to transform the wip.dot model present in
+[1] into a per-cpu monitor with the following command::
+
+  $ dot2k -d wip.dot -t per_cpu
+
+This will create a directory named wip/ with the following files:
+
+- wip.h: the wip model in C
+- wip.c: the RV monitor
+
+The wip.c file contains the monitor declaration and the starting point for
+the system instrumentation.
+
+Monitor macros
+--------------
+
+The rv/da_monitor.h enables automatic code generation for the *Monitor
+Instance(s)* using C macros.
+
+The benefits of the usage of macro for monitor synthesis are 3-fold as it:
+
+- Reduces the code duplication;
+- Facilitates the bug fix/improvement;
+- Avoids the case of developers changing the core of the monitor code
+  to manipulate the model in a (let's say) non-standard way.
+
+This initial implementation presents three different types of monitor instances:
+
+- ``#define DECLARE_DA_MON_GLOBAL(name, type)``
+- ``#define DECLARE_DA_MON_PER_CPU(name, type)``
+- ``#define DECLARE_DA_MON_PER_TASK(name, type)``
+
+The first declares the functions for a global deterministic automata monitor,
+the second for monitors with per-cpu instances, and the third with per-task
+instances.
+
+In all cases, the 'name' argument is a string that identifies the monitor, and
+the 'type' argument is the data type used by dot2k on the representation of
+the model in C.
+
+For example, the wip model with two states and three events can be
+stored in an 'unsigned char' type. Considering that the preemption control
+is a per-cpu behavior, the monitor declaration in the 'wip.c' file is::
+
+  DECLARE_DA_MON_PER_CPU(wip, unsigned char);
+
+The monitor is executed by sending events to be processed via the functions
+presented below::
+
+  da_handle_event_$(MONITOR_NAME)($(event from event enum));
+  da_handle_start_event_$(MONITOR_NAME)($(event from event enum));
+  da_handle_start_run_event_$(MONITOR_NAME)($(event from event enum));
+
+The function ``da_handle_event_$(MONITOR_NAME)()`` is the regular case where
+the event will be processed if the monitor is processing events.
+
+When a monitor is enabled, it is placed in the initial state of the automata.
+However, the monitor does not know if the system is in the *initial state*.
+
+The ``da_handle_start_event_$(MONITOR_NAME)()`` function is used to notify the
+monitor that the system is returning to the initial state, so the monitor can
+start monitoring the next event.
+
+The ``da_handle_start_run_event_$(MONITOR_NAME)()`` function is used to notify
+the monitor that the system is known to be in the initial state, so the
+monitor can start monitoring and monitor the current event.
+
+Using the wip model as example, the events "preempt_disable" and
+"sched_waking" should be sent to monitor, respectively, via [2]::
+
+  da_handle_event_wip(preempt_disable_wip);
+  da_handle_event_wip(sched_waking_wip);
+
+While the event "preempt_enabled" will use::
+
+  da_handle_start_event_wip(preempt_enable_wip);
+
+To notify the monitor that the system will be returning to the initial state,
+so the system and the monitor should be in sync.
+
+Final remarks
+-------------
+
+With the monitor synthesis in place using the rv/da_monitor.h and
+dot2k, the developer's work should be limited to the instrumentation
+of the system, increasing the confidence in the overall approach.
+
+[1] For details about deterministic automata format and the translation
+from one representation to another, see::
+
+  Documentation/trace/rv/deterministic_automata.rst
+
+[2] dot2k appends the monitor's name suffix to the events enums to
+avoid conflicting variables when exporting the global vmlinux.h
+use by BPF programs.
diff --git a/Documentation/trace/rv/index.rst b/Documentation/trace/rv/index.rst
index 013a41a410cf..46d47f33052c 100644
--- a/Documentation/trace/rv/index.rst
+++ b/Documentation/trace/rv/index.rst
@@ -8,3 +8,4 @@ Runtime Verification
 
    runtime-verification.rst
    deterministic_automata.rst
+   da_monitor_synthesis.rst