docs: livepatch: convert docs to ReST and rename to *.rst

Convert livepatch documentation to ReST format. The changes
are mostly trivial, as the documents are already on a good
shape. Just a few markup changes are needed for Sphinx to
properly parse the docs.

The conversion is actually:
  - add blank lines and identation in order to identify paragraphs;
  - fix tables markups;
  - add some lists markups;
  - mark literal blocks;
  - The in-file TOC becomes a comment, in order to skip it from the
    output, as Sphinx already generates an index there.
  - adjust title markups.

At its new index.rst, let's add a :orphan: while this is not linked to
the main index.rst file, in order to avoid build warnings.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Joe Lawrence <joe.lawrence@redhat.com>
Reviewed-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 102 insertions, 0 deletions

diff --git a/Documentation/livepatch/cumulative-patches.rst b/Documentation/livepatch/cumulative-patches.rst
new file mode 100644
index 000000000000..1931f318976a
--- /dev/null
+++ b/Documentation/livepatch/cumulative-patches.rst
@@ -0,0 +1,102 @@
+===================================
+Atomic Replace & Cumulative Patches
+===================================
+
+There might be dependencies between livepatches. If multiple patches need
+to do different changes to the same function(s) then we need to define
+an order in which the patches will be installed. And function implementations
+from any newer livepatch must be done on top of the older ones.
+
+This might become a maintenance nightmare. Especially when more patches
+modified the same function in different ways.
+
+An elegant solution comes with the feature called "Atomic Replace". It allows
+creation of so called "Cumulative Patches". They include all wanted changes
+from all older livepatches and completely replace them in one transition.
+
+Usage
+-----
+
+The atomic replace can be enabled by setting "replace" flag in struct klp_patch,
+for example::
+
+	static struct klp_patch patch = {
+		.mod = THIS_MODULE,
+		.objs = objs,
+		.replace = true,
+	};
+
+All processes are then migrated to use the code only from the new patch.
+Once the transition is finished, all older patches are automatically
+disabled.
+
+Ftrace handlers are transparently removed from functions that are no
+longer modified by the new cumulative patch.
+
+As a result, the livepatch authors might maintain sources only for one
+cumulative patch. It helps to keep the patch consistent while adding or
+removing various fixes or features.
+
+Users could keep only the last patch installed on the system after
+the transition to has finished. It helps to clearly see what code is
+actually in use. Also the livepatch might then be seen as a "normal"
+module that modifies the kernel behavior. The only difference is that
+it can be updated at runtime without breaking its functionality.
+
+
+Features
+--------
+
+The atomic replace allows:
+
+  - Atomically revert some functions in a previous patch while
+    upgrading other functions.
+
+  - Remove eventual performance impact caused by core redirection
+    for functions that are no longer patched.
+
+  - Decrease user confusion about dependencies between livepatches.
+
+
+Limitations:
+------------
+
+  - Once the operation finishes, there is no straightforward way
+    to reverse it and restore the replaced patches atomically.
+
+    A good practice is to set .replace flag in any released livepatch.
+    Then re-adding an older livepatch is equivalent to downgrading
+    to that patch. This is safe as long as the livepatches do _not_ do
+    extra modifications in (un)patching callbacks or in the module_init()
+    or module_exit() functions, see below.
+
+    Also note that the replaced patch can be removed and loaded again
+    only when the transition was not forced.
+
+
+  - Only the (un)patching callbacks from the _new_ cumulative livepatch are
+    executed. Any callbacks from the replaced patches are ignored.
+
+    In other words, the cumulative patch is responsible for doing any actions
+    that are necessary to properly replace any older patch.
+
+    As a result, it might be dangerous to replace newer cumulative patches by
+    older ones. The old livepatches might not provide the necessary callbacks.
+
+    This might be seen as a limitation in some scenarios. But it makes life
+    easier in many others. Only the new cumulative livepatch knows what
+    fixes/features are added/removed and what special actions are necessary
+    for a smooth transition.
+
+    In any case, it would be a nightmare to think about the order of
+    the various callbacks and their interactions if the callbacks from all
+    enabled patches were called.
+
+
+  - There is no special handling of shadow variables. Livepatch authors
+    must create their own rules how to pass them from one cumulative
+    patch to the other. Especially that they should not blindly remove
+    them in module_exit() functions.
+
+    A good practice might be to remove shadow variables in the post-unpatch
+    callback. It is called only when the livepatch is properly disabled.