diff --git a/target-s390x/insn-data.def b/target-s390x/insn-data.def
index b0bee9baf0..0a0ec0a0c7 100644
--- a/target-s390x/insn-data.def
+++ b/target-s390x/insn-data.def
@@ -379,6 +379,8 @@
     D(0xa701, TMLL,    RI_a,  Z,   r1_o, i2_16u_shl, 0, 0, 0, tm64, 0)
 
 #ifndef CONFIG_USER_ONLY
+/* DIAGNOSE (KVM hypercall) */
+    C(0x8300, DIAG,    RX_a,  Z,   0, 0, 0, 0, diag, 0)
 /* LOAD PSW */
     C(0x8200, LPSW,    S,     Z,   0, a2, 0, 0, lpsw, 0)
 /* SET ADDRESSING MODE */
diff --git a/target-s390x/translate.c b/target-s390x/translate.c
index cbfcf41427..6848e31432 100644
--- a/target-s390x/translate.c
+++ b/target-s390x/translate.c
@@ -2355,23 +2355,6 @@ static void disas_s390_insn(CPUS390XState *env, DisasContext *s)
     LOG_DISAS("opc 0x%x\n", opc);
 
     switch (opc) {
-#ifndef CONFIG_USER_ONLY
-    case 0x83: /* DIAG     R1,R3,D2     [RS] */
-        /* Diagnose call (KVM hypercall) */
-        check_privileged(s);
-        potential_page_fault(s);
-        insn = ld_code4(env, s->pc);
-        decode_rs(s, insn, &r1, &r3, &b2, &d2);
-        tmp32_1 = tcg_const_i32(insn & 0xfff);
-        tmp2 = load_reg(2);
-        tmp3 = load_reg(1);
-        gen_helper_diag(tmp2, cpu_env, tmp32_1, tmp2, tmp3);
-        store_reg(2, tmp2);
-        tcg_temp_free_i32(tmp32_1);
-        tcg_temp_free_i64(tmp2);
-        tcg_temp_free_i64(tmp3);
-        break;
-#endif
     case 0x88: /* SRL    R1,D2(B2)        [RS] */
     case 0x89: /* SLL    R1,D2(B2)        [RS] */
     case 0x8a: /* SRA    R1,D2(B2)        [RS] */
@@ -3327,6 +3310,22 @@ static ExitStatus op_cvd(DisasContext *s, DisasOps *o)
     return NO_EXIT;
 }
 
+#ifndef CONFIG_USER_ONLY
+static ExitStatus op_diag(DisasContext *s, DisasOps *o)
+{
+    TCGv_i32 tmp;
+
+    check_privileged(s);
+    potential_page_fault(s);
+
+    /* We pretend the format is RX_a so that D2 is the field we want.  */
+    tmp = tcg_const_i32(get_field(s->fields, d2) & 0xfff);
+    gen_helper_diag(regs[2], cpu_env, tmp, regs[2], regs[1]);
+    tcg_temp_free_i32(tmp);
+    return NO_EXIT;
+}
+#endif
+
 static ExitStatus op_divs32(DisasContext *s, DisasOps *o)
 {
     gen_helper_divs32(o->out2, cpu_env, o->in1, o->in2);