triggers: native triggers (action=0) should prevent causing a breakpoint exception while already in a trap handler

This commit implements Debug Specification Section 5.4 Native Triggers.

The specification allows two solutions for solving the reentrancy
problem. This commit chooses the first solution because the second one
targets implementations without S-mode.

riscv/triggers.cc

@@ -106,6 +106,21 @@ bool trigger_t::textra_match(processor_t * const proc) const noexcept
   return true;
 }
 
+bool trigger_t::allow_action(const state_t * const state) const
+{
+  if (get_action() == ACTION_DEBUG_EXCEPTION) {
+    const bool mstatus_mie = state->mstatus->read() & MSTATUS_MIE;
+    const bool sstatus_sie = state->sstatus->read() & MSTATUS_SIE;
+    const bool vsstatus_sie = state->vsstatus->read() & MSTATUS_SIE;
+    const bool medeleg_breakpoint = (state->medeleg->read() >> CAUSE_BREAKPOINT) & 1;
+    const bool hedeleg_breakpoint = (state->hedeleg->read() >> CAUSE_BREAKPOINT) & 1;
+    return (state->prv != PRV_M || mstatus_mie) &&
+           (state->prv != PRV_S || state->v || !medeleg_breakpoint || sstatus_sie) &&
+           (state->prv != PRV_S || !state->v || !medeleg_breakpoint || !hedeleg_breakpoint || vsstatus_sie);
+  }
+  return true;
+}
+
 reg_t disabled_trigger_t::tdata1_read(const processor_t * const proc) const noexcept
 {
   auto xlen = proc->get_xlen();
@@ -212,7 +227,7 @@ std::optional<match_result_t> mcontrol_common_t::detect_memory_access_match(proc
     value &= 0xffffffff;
   }
 
-  if (simple_match(xlen, value)) {
+  if (simple_match(xlen, value) && allow_action(proc->get_state())) {
     /* This is OK because this function is only called if the trigger was not
      * inhibited by the previous trigger in the chain. */
     hit = true;
@@ -289,7 +304,7 @@ void mcontrol6_t::tdata1_write(processor_t * const proc, const reg_t val, const
 
 std::optional<match_result_t> icount_t::detect_icount_match(processor_t * const proc) noexcept
 {
-  if (!common_match(proc))
+  if (!common_match(proc) || !allow_action(proc->get_state()))
     return std::nullopt;
 
   std::optional<match_result_t> ret = std::nullopt;
@@ -389,7 +404,7 @@ std::optional<match_result_t> trap_common_t::detect_trap_match(processor_t * con
   bool interrupt = (t.cause() & ((reg_t)1 << (xlen - 1))) != 0;
   reg_t bit = t.cause() & ~((reg_t)1 << (xlen - 1));
   assert(bit < xlen);
-  if (simple_match(interrupt, bit)) {
+  if (simple_match(interrupt, bit) && allow_action(proc->get_state())) {
     hit = true;
     return match_result_t(TIMING_AFTER, action);
   }

riscv/triggers.h

@@ -90,6 +90,7 @@ public:
 protected:
   static action_t legalize_action(reg_t val, reg_t action_mask, reg_t dmode_mask) noexcept;
   bool common_match(processor_t * const proc) const noexcept;
+  bool allow_action(const state_t * const state) const;
   reg_t tdata2;
 
   bool vs = false;