docs/devel: add plugins.rst design document

This is mostly extracted from Emilio's more verbose commit comments
with some additional verbiage from me.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

docs/devel/index.rst

@@ -22,3 +22,4 @@ Contents:
    decodetree
    secure-coding-practices
    tcg
+   plugins

docs/devel/plugins.rst

@@ -0,0 +1,112 @@
+..
+   Copyright (C) 2017, Emilio G. Cota <cota@braap.org>
+   Copyright (c) 2019, Linaro Limited
+   Written by Emilio Cota and Alex Bennée
+
+================
+QEMU TCG Plugins
+================
+
+QEMU TCG plugins provide a way for users to run experiments taking
+advantage of the total system control emulation can have over a guest.
+It provides a mechanism for plugins to subscribe to events during
+translation and execution and optionally callback into the plugin
+during these events. TCG plugins are unable to change the system state
+only monitor it passively. However they can do this down to an
+individual instruction granularity including potentially subscribing
+to all load and store operations.
+
+API Stability
+=============
+
+This is a new feature for QEMU and it does allow people to develop
+out-of-tree plugins that can be dynamically linked into a running QEMU
+process. However the project reserves the right to change or break the
+API should it need to do so. The best way to avoid this is to submit
+your plugin upstream so they can be updated if/when the API changes.
+
+
+Exposure of QEMU internals
+--------------------------
+
+The plugin architecture actively avoids leaking implementation details
+about how QEMU's translation works to the plugins. While there are
+conceptions such as translation time and translation blocks the
+details are opaque to plugins. The plugin is able to query select
+details of instructions and system configuration only through the
+exported *qemu_plugin* functions. The types used to describe
+instructions and events are opaque to the plugins themselves.
+
+Usage
+=====
+
+The QEMU binary needs to be compiled for plugin support:
+
+::
+    configure --enable-plugins
+
+Once built a program can be run with multiple plugins loaded each with
+their own arguments:
+
+::
+    $QEMU $OTHER_QEMU_ARGS \
+      -plugin tests/plugin/libhowvec.so,arg=inline,arg=hint \
+      -plugin tests/plugin/libhotblocks.so
+
+Arguments are plugin specific and can be used to modify their
+behaviour. In this case the howvec plugin is being asked to use inline
+ops to count and break down the hint instructions by type.
+
+Plugin Life cycle
+=================
+
+First the plugin is loaded and the public qemu_plugin_install function
+is called. The plugin will then register callbacks for various plugin
+events. Generally plugins will register a handler for the *atexit*
+if they want to dump a summary of collected information once the
+program/system has finished running.
+
+When a registered event occurs the plugin callback is invoked. The
+callbacks may provide additional information. In the case of a
+translation event the plugin has an option to enumerate the
+instructions in a block of instructions and optionally register
+callbacks to some or all instructions when they are executed.
+
+There is also a facility to add an inline event where code to
+increment a counter can be directly inlined with the translation.
+Currently only a simple increment is supported. This is not atomic so
+can miss counts. If you want absolute precision you should use a
+callback which can then ensure atomicity itself.
+
+Finally when QEMU exits all the registered *atexit* callbacks are
+invoked.
+
+Internals
+=========
+
+Locking
+-------
+
+We have to ensure we cannot deadlock, particularly under MTTCG. For
+this we acquire a lock when called from plugin code. We also keep the
+list of callbacks under RCU so that we do not have to hold the lock
+when calling the callbacks. This is also for performance, since some
+callbacks (e.g. memory access callbacks) might be called very
+frequently.
+
+  * A consequence of this is that we keep our own list of CPUs, so that
+    we do not have to worry about locking order wrt cpu_list_lock.
+  * Use a recursive lock, since we can get registration calls from
+    callbacks.
+
+As a result registering/unregistering callbacks is "slow", since it
+takes a lock. But this is very infrequent; we want performance when
+calling (or not calling) callbacks, not when registering them. Using
+RCU is great for this.
+
+We support the uninstallation of a plugin at any time (e.g. from
+plugin callbacks). This allows plugins to remove themselves if they no
+longer want to instrument the code. This operation is asynchronous
+which means callbacks may still occur after the uninstall operation is
+requested. The plugin isn't completely uninstalled until the safe work
+has executed while all vCPUs are quiescent.