target/riscv: Implement Smdbltrp sret, mret and mnret behavior

When the Ssdbltrp extension is enabled, SSTATUS.MDT field is cleared
when executing sret if executed in M-mode. When executing mret/mnret,
SSTATUS.MDT is cleared.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250110125441.3208676-8-cleger@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

Author: clementleger

target/riscv/op_helper.c

@@ -306,6 +306,9 @@ target_ulong helper_sret(CPURISCVState *env)
         }
         mstatus = set_field(mstatus, MSTATUS_SDT, 0);
     }
+    if (riscv_cpu_cfg(env)->ext_smdbltrp && env->priv >= PRV_M) {
+        mstatus = set_field(mstatus, MSTATUS_MDT, 0);
+    }
     if (env->priv_ver >= PRIV_VERSION_1_12_0) {
         mstatus = set_field(mstatus, MSTATUS_MPRV, 0);
     }
@@ -391,6 +394,9 @@ target_ulong helper_mret(CPURISCVState *env)
     if (riscv_cpu_cfg(env)->ext_ssdbltrp) {
         mstatus = ssdbltrp_mxret(env, mstatus, prev_priv, prev_virt);
     }
+    if (riscv_cpu_cfg(env)->ext_smdbltrp) {
+        mstatus = set_field(mstatus, MSTATUS_MDT, 0);
+    }
     if ((env->priv_ver >= PRIV_VERSION_1_12_0) && (prev_priv != PRV_M)) {
         mstatus = set_field(mstatus, MSTATUS_MPRV, 0);
     }
@@ -436,6 +442,12 @@ target_ulong helper_mnret(CPURISCVState *env)
         env->mstatus = ssdbltrp_mxret(env, env->mstatus, prev_priv, prev_virt);
     }
 
+    if (riscv_cpu_cfg(env)->ext_smdbltrp) {
+        if (prev_priv < PRV_M) {
+            env->mstatus = set_field(env->mstatus, MSTATUS_MDT, 0);
+        }
+    }
+
     if (riscv_has_ext(env, RVH) && prev_virt) {
         riscv_cpu_swap_hypervisor_regs(env);
     }