target/riscv: Add Smrnmi mnret instruction

This patch adds a new instruction 'mnret'. 'mnret' is an M-mode-only
instruction that uses the values in `mnepc` and `mnstatus` to return to the
program counter, privilege mode, and virtualization mode of the
interrupted context.

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Signed-off-by: Tommy Wu <tommy.wu@sifive.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250106054336.1878291-5-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

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(mnret, tl, env)
 DEF_HELPER_1(wfi, void, env)
 DEF_HELPER_1(wrs_nto, void, env)
 DEF_HELPER_1(tlb_flush, void, env)

target/riscv/insn32.decode

@@ -121,6 +121,9 @@ 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
 {

target/riscv/insn_trans/trans_privileged.c.inc

@@ -18,6 +18,12 @@
  * this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+#define REQUIRE_SMRNMI(ctx) do {     \
+    if (!ctx->cfg_ptr->ext_smrnmi) { \
+        return false;                \
+    }                                \
+} while (0)
+
 static bool trans_ecall(DisasContext *ctx, arg_ecall *a)
 {
     /* always generates U-level ECALL, fixed in do_interrupt handler */
@@ -106,6 +112,20 @@ static bool trans_mret(DisasContext *ctx, arg_mret *a)
 #endif
 }
 
+static bool trans_mnret(DisasContext *ctx, arg_mnret *a)
+{
+#ifndef CONFIG_USER_ONLY
+    REQUIRE_SMRNMI(ctx);
+    decode_save_opc(ctx, 0);
+    gen_helper_mnret(cpu_pc, tcg_env);
+    tcg_gen_exit_tb(NULL, 0); /* 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

target/riscv/op_helper.c

@@ -328,24 +328,30 @@ target_ulong helper_sret(CPURISCVState *env)
     return retpc;
 }
 
-target_ulong helper_mret(CPURISCVState *env)
+static void check_ret_from_m_mode(CPURISCVState *env, target_ulong retpc,
+                                  target_ulong prev_priv)
 {
     if (!(env->priv >= PRV_M)) {
         riscv_raise_exception(env, RISCV_EXCP_ILLEGAL_INST, GETPC());
     }
 
-    target_ulong retpc = env->mepc;
     if (!riscv_has_ext(env, RVC) && (retpc & 0x3)) {
         riscv_raise_exception(env, RISCV_EXCP_INST_ADDR_MIS, GETPC());
     }
 
-    uint64_t mstatus = env->mstatus;
-    target_ulong prev_priv = get_field(mstatus, MSTATUS_MPP);
-
     if (riscv_cpu_cfg(env)->pmp &&
         !pmp_get_num_rules(env) && (prev_priv != PRV_M)) {
         riscv_raise_exception(env, RISCV_EXCP_INST_ACCESS_FAULT, GETPC());
     }
+}
+
+target_ulong helper_mret(CPURISCVState *env)
+{
+    target_ulong retpc = env->mepc;
+    uint64_t mstatus = env->mstatus;
+    target_ulong prev_priv = get_field(mstatus, MSTATUS_MPP);
+
+    check_ret_from_m_mode(env, retpc, prev_priv);
 
     target_ulong prev_virt = get_field(env->mstatus, MSTATUS_MPV) &&
                              (prev_priv != PRV_M);
@@ -377,6 +383,35 @@ target_ulong helper_mret(CPURISCVState *env)
     return retpc;
 }
 
+target_ulong helper_mnret(CPURISCVState *env)
+{
+    target_ulong retpc = env->mnepc;
+    target_ulong prev_priv = get_field(env->mnstatus, MNSTATUS_MNPP);
+    target_ulong prev_virt;
+
+    check_ret_from_m_mode(env, retpc, prev_priv);
+
+    prev_virt = get_field(env->mnstatus, MNSTATUS_MNPV) &&
+                (prev_priv != PRV_M);
+    env->mnstatus = set_field(env->mnstatus, MNSTATUS_NMIE, true);
+
+    /*
+     * If MNRET changes the privilege mode to a mode
+     * less privileged than M, it also sets mstatus.MPRV to 0.
+     */
+    if (prev_priv < PRV_M) {
+        env->mstatus = set_field(env->mstatus, MSTATUS_MPRV, false);
+    }
+
+    if (riscv_has_ext(env, RVH) && prev_virt) {
+        riscv_cpu_swap_hypervisor_regs(env);
+    }
+
+    riscv_cpu_set_mode(env, prev_priv, prev_virt);
+
+    return retpc;
+}
+
 void helper_wfi(CPURISCVState *env)
 {
     CPUState *cs = env_cpu(env);