target/riscv: Implement Smdbltrp behavior

When the Smsdbltrp ISA extension is enabled, if a trap happens while
MSTATUS.MDT is already set, it will trigger an abort or an NMI is the
Smrnmi extension is available.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250110125441.3208676-9-cleger@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

Author: clementleger

target/riscv/cpu_helper.c

@@ -1938,6 +1938,24 @@ static target_ulong promote_load_fault(target_ulong orig_cause)
     /* if no promotion, return original cause */
     return orig_cause;
 }
+
+static void riscv_do_nmi(CPURISCVState *env, target_ulong cause, bool virt)
+{
+    env->mnstatus = set_field(env->mnstatus, MNSTATUS_NMIE, false);
+    env->mnstatus = set_field(env->mnstatus, MNSTATUS_MNPV, virt);
+    env->mnstatus = set_field(env->mnstatus, MNSTATUS_MNPP, env->priv);
+    env->mncause = cause;
+    env->mnepc = env->pc;
+    env->pc = env->rnmi_irqvec;
+
+    if (cpu_get_fcfien(env)) {
+        env->mnstatus = set_field(env->mnstatus, MNSTATUS_MNPELP, env->elp);
+    }
+
+    /* Trapping to M mode, virt is disabled */
+    riscv_cpu_set_mode(env, PRV_M, false);
+}
+
 /*
  * Handle Traps
  *
@@ -1977,22 +1995,8 @@ void riscv_cpu_do_interrupt(CPUState *cs)
     bool nnmi_excep = false;
 
     if (cpu->cfg.ext_smrnmi && env->rnmip && async) {
-        env->mnstatus = set_field(env->mnstatus, MNSTATUS_NMIE, false);
-        env->mnstatus = set_field(env->mnstatus, MNSTATUS_MNPV,
-                                  env->virt_enabled);
-        env->mnstatus = set_field(env->mnstatus, MNSTATUS_MNPP,
-                                  env->priv);
-        env->mncause = cause | ((target_ulong)1U << (mxlen - 1));
-        env->mnepc = env->pc;
-        env->pc = env->rnmi_irqvec;
-
-        if (cpu_get_fcfien(env)) {
-            env->mnstatus = set_field(env->mnstatus, MNSTATUS_MNPELP, env->elp);
-        }
-
-        /* Trapping to M mode, virt is disabled */
-        riscv_cpu_set_mode(env, PRV_M, false);
-
+        riscv_do_nmi(env, cause | ((target_ulong)1U << (mxlen - 1)),
+                     env->virt_enabled);
         return;
     }
 
@@ -2204,11 +2208,32 @@ void riscv_cpu_do_interrupt(CPUState *cs)
             /* Trapping to M mode, virt is disabled */
             virt = false;
         }
+        /*
+         * If the hart encounters an exception while executing in M-mode,
+         * with the mnstatus.NMIE bit clear, the program counter is set to
+         * the RNMI exception trap handler address.
+         */
+        nnmi_excep = cpu->cfg.ext_smrnmi &&
+                     !get_field(env->mnstatus, MNSTATUS_NMIE) &&
+                     !async;
 
         s = env->mstatus;
         s = set_field(s, MSTATUS_MPIE, get_field(s, MSTATUS_MIE));
         s = set_field(s, MSTATUS_MPP, env->priv);
         s = set_field(s, MSTATUS_MIE, 0);
+        if (cpu->cfg.ext_smdbltrp) {
+            if (env->mstatus & MSTATUS_MDT) {
+                assert(env->priv == PRV_M);
+                if (!cpu->cfg.ext_smrnmi || nnmi_excep) {
+                    cpu_abort(CPU(cpu), "M-mode double trap\n");
+                } else {
+                    riscv_do_nmi(env, cause, false);
+                    return;
+                }
+            }
+
+            s = set_field(s, MSTATUS_MDT, 1);
+        }
         env->mstatus = s;
         env->mcause = cause | ((target_ulong)async << (mxlen - 1));
         if (smode_double_trap) {