linux-kernel - [PATCH] static_call: Update comment

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Date:   Thu, 5 Aug 2021 17:49:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     jpoimboe@...hat.com, jbaron@...mai.com, rostedt@...dmis.org,
        ardb@...nel.org
Cc:     linux-kernel@...r.kernel.org, ltykernel@...il.com
Subject: [PATCH] static_call: Update comment


Update the comment with the new features.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 include/linux/static_call.h | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/include/linux/static_call.h b/include/linux/static_call.h
index fc94faa53b5b..35d28261b12b 100644
--- a/include/linux/static_call.h
+++ b/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_STAT_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>