The following commit has been merged into the locking/core branch of tip:
Commit-ID: 9ae6ab27f44ee0da47520011afc04218f90e8b12
Gitweb: https://git.kernel.org/tip/9ae6ab27f44ee0da47520011afc04218f90e8b12
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate: Thu, 05 Aug 2021 17:49:54 +02:00
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Tue, 17 Aug 2021 19:09:27 +02:00
static_call: Update API documentation
Update the comment with the new features.
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Reviewed-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Acked-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Link: https://lore.kernel.org/r/YQwIorQBHEq+s73b@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
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 fc94faa..3e56a97 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_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>
|