[RFC v2 4/4] target/riscv: add RNMI mnret instruction

[frank . chang]

From: Frank Chang <frank.ch...@sifive.com>

Signed-off-by: Frank Chang <frank.ch...@sifive.com>
---
 target/riscv/helper.h                         |  1 +
 target/riscv/insn32.decode                    |  3 ++
 .../riscv/insn_trans/trans_privileged.c.inc   | 13 ++++++++
 target/riscv/op_helper.c                      | 31 +++++++++++++++++++
 4 files changed, 48 insertions(+)

diff --git a/target/riscv/helper.h b/target/riscv/helper.h
index e3f3f41e891..0914d777d6d 100644
--- a/target/riscv/helper.h
+++ b/target/riscv/helper.h
@@ -65,6 +65,7 @@ DEF_HELPER_4(csrrc, tl, env, tl, tl, tl)
 #ifndef CONFIG_USER_ONLY
 DEF_HELPER_2(sret, tl, env, tl)
 DEF_HELPER_2(mret, tl, env, tl)
+DEF_HELPER_2(mnret, tl, env, tl)
 DEF_HELPER_1(wfi, void, env)
 DEF_HELPER_1(tlb_flush, void, env)
 #endif
diff --git a/target/riscv/insn32.decode b/target/riscv/insn32.decode
index 84080dd18ca..557f3394276 100644
--- a/target/riscv/insn32.decode
+++ b/target/riscv/insn32.decode
@@ -97,6 +97,9 @@ wfi         0001000    00101 00000 000 00000 1110011
 sfence_vma  0001001    ..... ..... 000 00000 1110011 @sfence_vma
 sfence_vm   0001000    00100 ..... 000 00000 1110011 @sfence_vm
 
+# *** NMI ***
+mnret       0111000    00010 00000 000 00000 1110011
+
 # *** RV32I Base Instruction Set ***
 lui      ....................       ..... 0110111 @u
 auipc    ....................       ..... 0010111 @u
diff --git a/target/riscv/insn_trans/trans_privileged.c.inc 
b/target/riscv/insn_trans/trans_privileged.c.inc
index 32312be2024..63c49dfe6fb 100644
--- a/target/riscv/insn_trans/trans_privileged.c.inc
+++ b/target/riscv/insn_trans/trans_privileged.c.inc
@@ -106,6 +106,19 @@ static bool trans_mret(DisasContext *ctx, arg_mret *a)
 #endif
 }
 
+static bool trans_mnret(DisasContext *ctx, arg_mnret *a)
+{
+#ifndef CONFIG_USER_ONLY
+    tcg_gen_movi_tl(cpu_pc, ctx->base.pc_next);
+    gen_helper_mnret(cpu_pc, cpu_env, cpu_pc);
+    exit_tb(ctx); /* no chaining */
+    ctx->base.is_jmp = DISAS_NORETURN;
+    return true;
+#else
+    return false;
+#endif
+}
+
 static bool trans_wfi(DisasContext *ctx, arg_wfi *a)
 {
 #ifndef CONFIG_USER_ONLY
diff --git a/target/riscv/op_helper.c b/target/riscv/op_helper.c
index 1eddcb94de7..b9601776153 100644
--- a/target/riscv/op_helper.c
+++ b/target/riscv/op_helper.c
@@ -175,6 +175,37 @@ target_ulong helper_mret(CPURISCVState *env, target_ulong 
cpu_pc_deb)
     return retpc;
 }
 
+target_ulong helper_mnret(CPURISCVState *env, target_ulong cpu_pc_deb)
+{
+    if (!riscv_feature(env, RISCV_FEATURE_RNMI)) {
+        /* RNMI feature is not presented. */
+        riscv_raise_exception(env, RISCV_EXCP_ILLEGAL_INST, GETPC());
+    }
+
+    if (!(env->priv >= PRV_M)) {
+        riscv_raise_exception(env, RISCV_EXCP_ILLEGAL_INST, GETPC());
+    }
+
+    /* Get return PC from mnepc CSR. */
+    target_ulong retpc = env->mnepc;
+    if (!riscv_has_ext(env, RVC) && (retpc & 0x3)) {
+        riscv_raise_exception(env, RISCV_EXCP_INST_ADDR_MIS, GETPC());
+    }
+
+    /* Get previous privilege level from mnstatus CSR. */
+    target_ulong prev_priv = get_field(env->mnstatus, MNSTATUS_MPP);
+
+    if (!pmp_get_num_rules(env) && (prev_priv != PRV_M)) {
+        riscv_raise_exception(env, RISCV_EXCP_ILLEGAL_INST, GETPC());
+    }
+
+    riscv_cpu_set_mode(env, prev_priv);
+
+    env->nmie = true;
+
+    return retpc;
+}
+
 void helper_wfi(CPURISCVState *env)
 {
     CPUState *cs = env_cpu(env);
-- 
2.17.1