static_call: Add function to query current function

Some users of paravirtualized functions need to query which function
has been specified in a pv_ops vector element. In order to be able to
switch such paravirtualized functions to static_calls instead, there
needs to be a function to query the function which will be called via
static_call().

Signed-off-by: Juergen Gross <[e-mail]>
Signed-off-by: Borislav Petkov <[e-mail]>
Acked-by: Peter Zijlstra (Intel) <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]

include/linux/static_call.h

···
  *   static_call(name)(args...);
  *   static_call_cond(name)(args...);
  *   static_call_update(name, func);
+ *   static_call_query(name);
  *
  * Usage example:
  *
···
  *
  *   which will include the required value tests to avoid NULL-pointer
  *   dereferences.
+ *
+ *   To query which function is currently set to be called, use:
+ *
+ *   func = static_call_query(name);
  */
 
 #include <linux/types.h>
···
 	__static_call_update(&STATIC_CALL_KEY(name),			\
 			     STATIC_CALL_TRAMP_ADDR(name), func);	\
 })
+
+#define static_call_query(name) (READ_ONCE(STATIC_CALL_KEY(name).func))
 
 #ifdef CONFIG_HAVE_STATIC_CALL_INLINE
 
···
 		.func = NULL,						\
 	};								\
 	ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name)
+
 
 #define static_call_cond(name)	(void)__static_call(name)