triggers: legalize timing=1(after) for load data trigger

As recommended in the debug spec table "Suggested Trigger Timings", to
avoid the footgun of replaying a load (which may have side effects) when
the breakpoint trap handler returns.

reference: https://github.com/riscv-software-src/riscv-isa-sim/pull/1208#issuecomment-1373035906

-----------------------------------------------------------------------

The legalize_timing() depends on select, execution, load, and store,
which are updated in the same function tdata1_write(). As a result,
reordering statements in the tdata1_write() may break the functionality.

Passing those variables as parameters to legalize_timing() does not
solve the problem. Thus, we give the original write value and the masks
of the variables to the legalize_timing(). This makes the legalization
function independent of the updating variables and resolves the issue.

reference: https://github.com/riscv-software-src/riscv-isa-sim/pull/1214

riscv/triggers.cc

@@ -148,7 +148,7 @@ void mcontrol_t::tdata1_write(processor_t * const proc, const reg_t val, const b
   dmode = get_field(val, CSR_MCONTROL_DMODE(xlen));
   hit = get_field(val, CSR_MCONTROL_HIT);
   select = get_field(val, MCONTROL_SELECT);
-  timing = legalize_timing(val, MCONTROL_TIMING, MCONTROL_EXECUTE);
+  timing = legalize_timing(val, MCONTROL_TIMING, MCONTROL_SELECT, MCONTROL_EXECUTE, MCONTROL_LOAD);
   action = legalize_action(get_field(val, MCONTROL_ACTION));
   chain = allow_chain ? get_field(val, MCONTROL_CHAIN) : 0;
   match = legalize_match(get_field(val, MCONTROL_MATCH));
@@ -235,7 +235,10 @@ mcontrol_common_t::match_t mcontrol_common_t::legalize_match(reg_t val) const no
   }
 }
 
-bool mcontrol_common_t::legalize_timing(reg_t val, reg_t timing_mask, reg_t execute_mask) noexcept {
+bool mcontrol_common_t::legalize_timing(reg_t val, reg_t timing_mask, reg_t select_mask, reg_t execute_mask, reg_t load_mask) noexcept {
+  // For load data triggers, force timing=after to avoid debugger having to repeat loads which may have side effects.
+  if (get_field(val, select_mask) && get_field(val, load_mask))
+    return TIMING_AFTER;
   if (get_field(val, execute_mask))
     return TIMING_BEFORE;
   return get_field(val, timing_mask);
@@ -271,7 +274,7 @@ void mcontrol6_t::tdata1_write(processor_t * const proc, const reg_t val, const
   vu = get_field(val, CSR_MCONTROL6_VU);
   hit = get_field(val, CSR_MCONTROL6_HIT);
   select = get_field(val, CSR_MCONTROL6_SELECT);
-  timing = legalize_timing(val, CSR_MCONTROL6_TIMING, CSR_MCONTROL6_EXECUTE);
+  timing = legalize_timing(val, CSR_MCONTROL6_TIMING, CSR_MCONTROL6_SELECT, CSR_MCONTROL6_EXECUTE, CSR_MCONTROL6_LOAD);
   action = legalize_action(get_field(val, CSR_MCONTROL6_ACTION));
   chain = allow_chain ? get_field(val, CSR_MCONTROL6_CHAIN) : 0;
   match = legalize_match(get_field(val, CSR_MCONTROL6_MATCH));

riscv/triggers.h

@@ -208,7 +208,7 @@ private:
 
 protected:
   match_t legalize_match(reg_t val) const noexcept;
-  static bool legalize_timing(reg_t val, reg_t timing_mask, reg_t execute_mask) noexcept;
+  static bool legalize_timing(reg_t val, reg_t timing_mask, reg_t select_mask, reg_t execute_mask, reg_t load_mask) noexcept;
   bool dmode = false;
   action_t action = ACTION_DEBUG_EXCEPTION;
   bool hit = false;