Add `ext_smdbltrp`in RISCVCPUConfig and implement MSTATUS.MDT behavior.
Also set MDT to 1 at reset according to the specification.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
---
 target/riscv/cpu.c      |  3 +++
 target/riscv/cpu_bits.h |  1 +
 target/riscv/cpu_cfg.h  |  1 +
 target/riscv/csr.c      | 13 +++++++++++++
 4 files changed, 18 insertions(+)

diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
index 39555364bf..15b21e4f7d 100644
--- a/target/riscv/cpu.c
+++ b/target/riscv/cpu.c
@@ -959,6 +959,9 @@ static void riscv_cpu_reset_hold(Object *obj, ResetType type)
             env->mstatus_hs = set_field(env->mstatus_hs,
                                         MSTATUS64_UXL, env->misa_mxl);
         }
+        if (riscv_cpu_cfg(env)->ext_smdbltrp) {
+            env->mstatus = set_field(env->mstatus, MSTATUS_MDT, 1);
+        }
     }
     env->mcause = 0;
     env->miclaim = MIP_SGEIP;
diff --git a/target/riscv/cpu_bits.h b/target/riscv/cpu_bits.h
index 0d0f253fcb..b368e27ca0 100644
--- a/target/riscv/cpu_bits.h
+++ b/target/riscv/cpu_bits.h
@@ -561,6 +561,7 @@
 #define MSTATUS_SDT         0x01000000
 #define MSTATUS_GVA         0x4000000000ULL
 #define MSTATUS_MPV         0x8000000000ULL
+#define MSTATUS_MDT         0x40000000000ULL /* Smdbltrp extension */
 
 #define MSTATUS64_UXL       0x0000000300000000ULL
 #define MSTATUS64_SXL       0x0000000C00000000ULL
diff --git a/target/riscv/cpu_cfg.h b/target/riscv/cpu_cfg.h
index 518102d748..8ac1e7fce3 100644
--- a/target/riscv/cpu_cfg.h
+++ b/target/riscv/cpu_cfg.h
@@ -78,6 +78,7 @@ struct RISCVCPUConfig {
     bool ext_sstc;
     bool ext_smcntrpmf;
     bool ext_ssdbltrp;
+    bool ext_smdbltrp;
     bool ext_svadu;
     bool ext_svinval;
     bool ext_svnapot;
diff --git a/target/riscv/csr.c b/target/riscv/csr.c
index 054418ff54..1ac5731d32 100644
--- a/target/riscv/csr.c
+++ b/target/riscv/csr.c
@@ -1616,6 +1616,13 @@ static RISCVException write_mstatus(CPURISCVState *env, int csrno,
         }
     }
 
+    if (riscv_cpu_cfg(env)->ext_smdbltrp) {
+        mask |= MSTATUS_MDT;
+        if ((val & MSTATUS_MDT) != 0) {
+            val &= ~MSTATUS_MIE;
+        }
+    }
+
     if (xl != MXL_RV32 || env->debugger) {
         if (riscv_has_ext(env, RVH)) {
             mask |= MSTATUS_MPV | MSTATUS_GVA;
@@ -1654,6 +1661,12 @@ static RISCVException write_mstatush(CPURISCVState *env, int csrno,
     uint64_t valh = (uint64_t)val << 32;
     uint64_t mask = riscv_has_ext(env, RVH) ? MSTATUS_MPV | MSTATUS_GVA : 0;
 
+    if (riscv_cpu_cfg(env)->ext_smdbltrp) {
+        mask |= MSTATUS_MDT;
+        if ((valh & MSTATUS_MDT) != 0) {
+            mask |= MSTATUS_MIE;
+        }
+    }
     env->mstatus = (env->mstatus & ~mask) | (valh & mask);
 
     return RISCV_EXCP_NONE;
-- 
2.45.2