Change double_fault detector to on by default, fatal error if triggered

Add a plusargs "+is_double_fault_detected_fatal" to the top-level cfg,
which is set to 1 by default.

Set the default for the "+enable_double_fault_detector" to 1.

dv/uvm/core_ibex/env/core_ibex_env_cfg.sv

@@ -21,12 +21,16 @@ class core_ibex_env_cfg extends uvm_object;
   rand scrambling_key_agent_cfg scrambling_key_cfg;
 
   // Double-Fault detection in scoreboard
-  bit                          enable_double_fault_detector = 0;
+  bit                          enable_double_fault_detector = 1;
   int unsigned                 double_fault_threshold_consecutive = 100;
   int unsigned                 double_fault_threshold_total = 1000;
+  // If '1', reaching either threshold fatally ends the test.
+  // If '0', we end the test with a pass.
+  bit                          is_double_fault_detected_fatal = 1;
 
   `uvm_object_utils_begin(core_ibex_env_cfg)
     `uvm_field_int(enable_double_fault_detector, UVM_DEFAULT)
+    `uvm_field_int(is_double_fault_detected_fatal, UVM_DEFAULT)
     `uvm_field_int(enable_irq_single_seq,   UVM_DEFAULT)
     `uvm_field_int(enable_irq_multiple_seq,   UVM_DEFAULT)
     `uvm_field_int(enable_irq_nmi_seq,   UVM_DEFAULT)
@@ -43,6 +47,7 @@ class core_ibex_env_cfg extends uvm_object;
   function new(string name = "");
     super.new(name);
     void'($value$plusargs("enable_double_fault_detector=%0d", enable_double_fault_detector));
+    void'($value$plusargs("is_double_fault_detected_fatal=%0d", is_double_fault_detected_fatal));
     void'($value$plusargs("enable_irq_single_seq=%0d", enable_irq_single_seq));
     void'($value$plusargs("enable_irq_multiple_seq=%0d", enable_irq_multiple_seq));
     void'($value$plusargs("enable_irq_nmi_seq=%0d", enable_irq_nmi_seq));

dv/uvm/core_ibex/env/core_ibex_scoreboard.sv

@@ -58,7 +58,6 @@ class core_ibex_scoreboard extends uvm_scoreboard;
             double_fault_cnt_consecutive = 0;
           end
 
-
           // Create an event if either counter reaches its threshold value, then reset the counter.
           if (double_fault_cnt_consecutive == cfg.double_fault_threshold_consecutive) begin
             fault_threshold_consecutive_reached.trigger();
@@ -71,8 +70,12 @@ class core_ibex_scoreboard extends uvm_scoreboard;
 
         end
       end
-      // Latch a signal to show that we have seen a double_fault.
-      // The pulse may be receieved sometime before the rvfi_seq_item.
+      // Latch the 'double_fault_seen_o' signal to catch the fault.
+      // The single pulse may be receieved sometime before the rvfi_seq_item
+      // corresponding to the faulting instruction is generated. Hence we
+      // latch that pulse when it is seen, and then reset above when the
+      // seq_item arrives.
+      // https://github.com/lowRISC/ibex/pull/1848#discussion_r995903762
       begin
         forever begin
           @(posedge cfg.ibex_dut_vif.double_fault_seen);

dv/uvm/core_ibex/riscv_dv_extension/testlist.yaml

@@ -744,9 +744,7 @@
     +instr_cnt=6000
     +pmp_max_offset=00024000
     +pmp_randomize=1
-    +pmp_allow_illegal_tor=1
-  sim_opts: >
-    +enable_double_fault_detector=1
+    +pmp_allow_addr_overlap=1
   rtl_test: core_ibex_base_test
   rtl_params:
     PMPEnable: 1

dv/uvm/core_ibex/tests/core_ibex_base_test.sv

@@ -236,6 +236,10 @@ class core_ibex_base_test extends uvm_test;
       begin
         if (cfg.enable_double_fault_detector) begin
           env.scoreboard.dfd_wait_for_pass_events();
+          if (cfg.is_double_fault_detected_fatal) begin
+            `uvm_fatal(`gfn, "Fatal threshold for double_fault detector reached.")
+          end
+          // If we get here, join this fork to end the test gracefully.
           `uvm_info(`gfn, "Test done due to double_fault detector.", UVM_LOW)
         end else begin
           wait (test_done == 1'b1);