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 <jgross@suse.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210311142319.4723-4-jgross@suse.com

include/linux/static_call.h

@@ -20,6 +20,7 @@
  *   static_call(name)(args...);
  *   static_call_cond(name)(args...);
  *   static_call_update(name, func);
+ *   static_call_query(name);
  *
  * Usage example:
  *
@@ -91,6 +92,10 @@
  *
  *   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>
@@ -118,6 +123,8 @@ extern void arch_static_call_transform(void *site, void *tramp, void *func, bool
 			     STATIC_CALL_TRAMP_ADDR(name), func);	\
 })
 
+#define static_call_query(name) (READ_ONCE(STATIC_CALL_KEY(name).func))
+
 #ifdef CONFIG_HAVE_STATIC_CALL_INLINE
 
 extern int __init static_call_init(void);
@@ -191,6 +198,7 @@ static inline int static_call_init(void) { return 0; }
 	};								\
 	ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name)
 
+
 #define static_call_cond(name)	(void)__static_call(name)
 
 static inline