RISC-V Debug Specification:
https://github.com/riscv/riscv-debug-spec/releases/tag/1.0

Add DRET decode/translate and a helper to leave Debug Mode and return
to dpc. Executing DRET outside Debug Mode raises illegal instruction.

Signed-off-by: Chao Liu <chao.liu.zevorn@gmail.com>
---
 target/riscv/helper.h                          |  1 +
 target/riscv/insn32.decode                     |  1 +
 target/riscv/insn_trans/trans_privileged.c.inc | 18 ++++++++++++++++++
 target/riscv/op_helper.c                       | 16 ++++++++++++++++
 4 files changed, 36 insertions(+)

diff --git a/target/riscv/helper.h b/target/riscv/helper.h
index b785456ee0..6140b6340d 100644
--- a/target/riscv/helper.h
+++ b/target/riscv/helper.h
@@ -131,6 +131,7 @@ DEF_HELPER_6(csrrw_i128, tl, env, int, tl, tl, tl, tl)
 #ifndef CONFIG_USER_ONLY
 DEF_HELPER_1(sret, tl, env)
 DEF_HELPER_1(mret, tl, env)
+DEF_HELPER_1(dret, tl, env)
 DEF_HELPER_1(mnret, tl, env)
 DEF_HELPER_1(ctr_clear, void, env)
 DEF_HELPER_1(wfi, void, env)
diff --git a/target/riscv/insn32.decode b/target/riscv/insn32.decode
index 6e35c4b1e6..4db842d5d9 100644
--- a/target/riscv/insn32.decode
+++ b/target/riscv/insn32.decode
@@ -118,6 +118,7 @@ sctrclr     000100000100     00000 000 00000 1110011
 uret        0000000    00010 00000 000 00000 1110011
 sret        0001000    00010 00000 000 00000 1110011
 mret        0011000    00010 00000 000 00000 1110011
+dret        0111101    10010 00000 000 00000 1110011
 wfi         0001000    00101 00000 000 00000 1110011
 sfence_vma  0001001    ..... ..... 000 00000 1110011 @sfence_vma
 
diff --git a/target/riscv/insn_trans/trans_privileged.c.inc b/target/riscv/insn_trans/trans_privileged.c.inc
index 8a62b4cfcd..f8641b1977 100644
--- a/target/riscv/insn_trans/trans_privileged.c.inc
+++ b/target/riscv/insn_trans/trans_privileged.c.inc
@@ -125,6 +125,24 @@ static bool trans_mret(DisasContext *ctx, arg_mret *a)
 #endif
 }
 
+static bool trans_dret(DisasContext *ctx, arg_dret *a)
+{
+#ifndef CONFIG_USER_ONLY
+    if (!ctx->cfg_ptr->ext_sdext) {
+        return false;
+    }
+    decode_save_opc(ctx, 0);
+    translator_io_start(&ctx->base);
+    gen_update_pc(ctx, 0);
+    gen_helper_dret(cpu_pc, tcg_env);
+    exit_tb(ctx); /* no chaining */
+    ctx->base.is_jmp = DISAS_NORETURN;
+    return true;
+#else
+    return false;
+#endif
+}
+
 static bool trans_mnret(DisasContext *ctx, arg_mnret *a)
 {
 #ifndef CONFIG_USER_ONLY
diff --git a/target/riscv/op_helper.c b/target/riscv/op_helper.c
index 6ccc127c30..99736bbebb 100644
--- a/target/riscv/op_helper.c
+++ b/target/riscv/op_helper.c
@@ -454,6 +454,22 @@ target_ulong helper_mret(CPURISCVState *env)
     return retpc;
 }
 
+target_ulong helper_dret(CPURISCVState *env)
+{
+    uintptr_t ra = GETPC();
+#ifdef CONFIG_USER_ONLY
+    riscv_raise_exception(env, RISCV_EXCP_ILLEGAL_INST, ra);
+    return 0;
+#else
+    if (!riscv_cpu_cfg(env)->ext_sdext || !env->debug_mode) {
+        riscv_raise_exception(env, RISCV_EXCP_ILLEGAL_INST, ra);
+    }
+    target_ulong retpc = env->dpc & get_xepc_mask(env);
+    riscv_cpu_leave_debug_mode(env);
+    return retpc;
+#endif
+}
+
 target_ulong helper_mnret(CPURISCVState *env)
 {
     target_ulong retpc = env->mnepc;
-- 
2.52.0