static_call: Update API documentation

Update the comment with the new features.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/YQwIorQBHEq+s73b@hirez.programming.kicks-ass.net

include/linux/static_call.h

@@ -17,11 +17,17 @@
  *   DECLARE_STATIC_CALL(name, func);
  *   DEFINE_STATIC_CALL(name, func);
  *   DEFINE_STATIC_CALL_NULL(name, typename);
+ *   DEFINE_STATIC_CALL_RET0(name, typename);
+ *
+ *   __static_call_return0;
+ *
  *   static_call(name)(args...);
  *   static_call_cond(name)(args...);
  *   static_call_update(name, func);
  *   static_call_query(name);
  *
+ *   EXPORT_STATIC_CALL{,_TRAMP}{,_GPL}()
+ *
  * Usage example:
  *
  *   # Start with the following functions (with identical prototypes):
@@ -96,6 +102,33 @@
  *   To query which function is currently set to be called, use:
  *
  *   func = static_call_query(name);
+ *
+ *
+ * DEFINE_STATIC_CALL_RET0 / __static_call_return0:
+ *
+ *   Just like how DEFINE_STATIC_CALL_NULL() / static_call_cond() optimize the
+ *   conditional void function call, DEFINE_STATIC_CALL_RET0 /
+ *   __static_call_return0 optimize the do nothing return 0 function.
+ *
+ *   This feature is strictly UB per the C standard (since it casts a function
+ *   pointer to a different signature) and relies on the architecture ABI to
+ *   make things work. In particular it relies on Caller Stack-cleanup and the
+ *   whole return register being clobbered for short return values. All normal
+ *   CDECL style ABIs conform.
+ *
+ *   In particular the x86_64 implementation replaces the 5 byte CALL
+ *   instruction at the callsite with a 5 byte clear of the RAX register,
+ *   completely eliding any function call overhead.
+ *
+ *   Notably argument setup is unconditional.
+ *
+ *
+ * EXPORT_STATIC_CALL() vs EXPORT_STATIC_CALL_TRAMP():
+ *
+ *   The difference is that the _TRAMP variant tries to only export the
+ *   trampoline with the result that a module can use static_call{,_cond}() but
+ *   not static_call_update().
+ *
  */
 
 #include <linux/types.h>