From 9ae6ab27f44ee0da47520011afc04218f90e8b12 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Thu, 5 Aug 2021 17:49:54 +0200 Subject: [PATCH] static_call: Update API documentation Update the comment with the new features. Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Ingo Molnar Reviewed-by: Steven Rostedt (VMware) Acked-by: Josh Poimboeuf Link: https://lore.kernel.org/r/YQwIorQBHEq+s73b@hirez.programming.kicks-ass.net --- 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 fc94faa53b5b1..3e56a9751c062 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 -- 2.30.2