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static_call: Update API documentation

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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
This commit is contained in:
Peter Zijlstra 2021-08-05 17:49:54 +02:00 committed by Ingo Molnar
parent 026659b977
commit 9ae6ab27f4
1 changed files with 33 additions and 0 deletions
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33
include/linux/static_call.h
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@ -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>