s390x: implement rrbe instruction properly

The rrbe instruction resets the reference bit in the given storage key.
So far, we merely made it a nop and also returned an invalid CC value,
so that the kernel never knew if a page actually got accessed.

This patch implements it properly, flushing the R bit and returning the
correct CC value.

Signed-off-by: Alexander Graf <agraf@suse.de>

target-s390x/op_helper.c

diff --git a/target-s390x/op_helper.c b/target-s390x/op_helper.c
index 440e871..364b100 100644
--- a/target-s390x/op_helper.c
+++ b/target-s390x/op_helper.c
@@ -2779,14 +2779,15 @@
 /* reset reference bit extended */
 uint32_t HELPER(rrbe)(uint32_t r1, uint64_t r2)
 {
+    uint8_t re;
+    uint8_t key;
     if (r2 > ram_size) {
         return 0;
     }
 
-    /* XXX implement */
-#if 0
-    env->storage_keys[r2 / TARGET_PAGE_SIZE] &= ~SK_REFERENCED;
-#endif
+    key = env->storage_keys[r2 / TARGET_PAGE_SIZE];
+    re = key & (SK_R | SK_C);
+    env->storage_keys[r2 / TARGET_PAGE_SIZE] = (key & ~SK_R);
 
     /*
      * cc
@@ -2796,7 +2797,8 @@
      * 2  Reference bit one; change bit zero
      * 3  Reference bit one; change bit one
      */
-    return 0;
+
+    return re >> 1;
 }
 
 /* compare and swap and purge */