powerpc/64s/perf: perf interrupt does not have to get_user_pages to access user memory

read_user_stack_slow that walks user address translation by hand is
only required on hash, because a hash fault can not be serviced from
"NMI" context (to avoid re-entering the hash code) so the user stack
can be mapped into Linux page tables but not accessible by the CPU.

Radix MMU mode does not have this restriction. A page fault failure
would indicate the page is not accessible via get_user_pages either,
so avoid this on radix.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201111120151.3150658-1-npiggin@gmail.com

arch/powerpc/perf/callchain.h

@@ -33,7 +33,7 @@ static inline int __read_user_stack(const void __user *ptr, void *ret,
 
 	rc = copy_from_user_nofault(ret, ptr, size);
 
-	if (IS_ENABLED(CONFIG_PPC64) && rc)
+	if (IS_ENABLED(CONFIG_PPC64) && !radix_enabled() && rc)
 		return read_user_stack_slow(ptr, ret, size);
 
 	return rc;

arch/powerpc/perf/callchain_64.c

@@ -21,7 +21,8 @@
 /*
  * On 64-bit we don't want to invoke hash_page on user addresses from
  * interrupt context, so if the access faults, we read the page tables
- * to find which page (if any) is mapped and access it directly.
+ * to find which page (if any) is mapped and access it directly. Radix
+ * has no need for this so it doesn't use read_user_stack_slow.
  */
 int read_user_stack_slow(const void __user *ptr, void *buf, int nb)
 {