diff --git a/target-arm/translate-a64.c b/target-arm/translate-a64.c
index c96ba68520..f6500e5181 100644
--- a/target-arm/translate-a64.c
+++ b/target-arm/translate-a64.c
@@ -1177,7 +1177,12 @@ static void handle_sys(DisasContext *s, uint32_t insn, bool isread,
                                                crn, crm, op0, op1, op2));
 
     if (!ri) {
-        /* Unknown register */
+        /* Unknown register; this might be a guest error or a QEMU
+         * unimplemented feature.
+         */
+        qemu_log_mask(LOG_UNIMP, "%s access to unsupported AArch64 "
+                      "system register op0:%d op1:%d crn:%d crm:%d op2:%d\n",
+                      isread ? "read" : "write", op0, op1, crn, crm, op2);
         unallocated_encoding(s);
         return;
     }
diff --git a/target-arm/translate.c b/target-arm/translate.c
index 782aab8b58..6d822c60b5 100644
--- a/target-arm/translate.c
+++ b/target-arm/translate.c
@@ -6962,6 +6962,19 @@ static int disas_coproc_insn(CPUARMState * env, DisasContext *s, uint32_t insn)
         return 0;
     }
 
+    /* Unknown register; this might be a guest error or a QEMU
+     * unimplemented feature.
+     */
+    if (is64) {
+        qemu_log_mask(LOG_UNIMP, "%s access to unsupported AArch32 "
+                      "64 bit system register cp:%d opc1: %d crm:%d\n",
+                      isread ? "read" : "write", cpnum, opc1, crm);
+    } else {
+        qemu_log_mask(LOG_UNIMP, "%s access to unsupported AArch32 "
+                      "system register cp:%d opc1:%d crn:%d crm:%d opc2:%d\n",
+                      isread ? "read" : "write", cpnum, opc1, crn, crm, opc2);
+    }
+
     return 1;
 }