[PATCH 8/8] livepatch: Atomic replace and cumulative patches documentation

[Petr Mladek]

User documentation for the atomic replace feature. It makes it easier
to maintain livepatches using so-called cumulative patches.

Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
---
 Documentation/livepatch/cumulative-patches.txt | 105 +++++++++++++++++++++++++
 1 file changed, 105 insertions(+)
 create mode 100644 Documentation/livepatch/cumulative-patches.txt

diff --git a/Documentation/livepatch/cumulative-patches.txt b/Documentation/livepatch/cumulative-patches.txt
new file mode 100644
index 000000000000..206b7f98d270
--- /dev/null
+++ b/Documentation/livepatch/cumulative-patches.txt
@@ -0,0 +1,105 @@
+===================================
+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 if anyone would want
+to remove a patch that is in the middle of the stack.
+
+An elegant solution comes with the feature called "Atomic Replace". It allows
+to create 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,
+	};
+
+Such a patch is added on top of the livepatch stack when registered. It can
+be enabled even when some earlier patches have not been enabled yet.
+
+All processes are then migrated to use the code only from the new patch.
+Once the transition is finished, all older patches are removed from the stack
+of patches. Even the older not-enabled patches mentioned above. They can
+even be unregistered and the related modules unloaded.
+
+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 stacking order and what patches are
+    currently in effect.
+
+
+Limitations:
+------------
+
+  + Replaced patches can no longer be enabled. But if the transition
+    to the cumulative patch was not forced, the kernel modules with
+    the older livepatches can be removed and eventually added again.
+
+    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.
+
+
+  + Only the (un)patching callbacks from the _new_ cumulative livepatch are
+    executed. Any callbacks from the replaced patches are ignored.
+
+    By 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 the 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 each 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 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.
-- 
2.13.6