Re: [PATCH v11 4/6] target/riscv: Add Smrnmi mnret instruction

[Alistair Francis]

On Tue, Dec 31, 2024 at 1:28 PM <frank.chang@sifive.com> wrote:
>
> From: Tommy Wu <tommy.wu@sifive.com>
>
> 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>

Alistair

> ---
>  target/riscv/helper.h                         |  1 +
>  target/riscv/insn32.decode                    |  3 ++
>  .../riscv/insn_trans/trans_privileged.c.inc   | 20 +++++++++
>  target/riscv/op_helper.c                      | 45 ++++++++++++++++---
>  4 files changed, 64 insertions(+), 5 deletions(-)
>
> diff --git a/target/riscv/helper.h b/target/riscv/helper.h
> index 451261ce5a..16ea240d26 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(mnret, tl, env)
>  DEF_HELPER_1(wfi, void, env)
>  DEF_HELPER_1(wrs_nto, void, env)
>  DEF_HELPER_1(tlb_flush, void, env)
> diff --git a/target/riscv/insn32.decode b/target/riscv/insn32.decode
> index e9139ec1b9..942c434c6e 100644
> --- a/target/riscv/insn32.decode
> +++ b/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
>  {
> diff --git a/target/riscv/insn_trans/trans_privileged.c.inc 
> b/target/riscv/insn_trans/trans_privileged.c.inc
> index ecd3b8b2c9..73f940d406 100644
> --- a/target/riscv/insn_trans/trans_privileged.c.inc
> +++ b/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
> diff --git a/target/riscv/op_helper.c b/target/riscv/op_helper.c
> index eddedacf4b..63ec53e992 100644
> --- a/target/riscv/op_helper.c
> +++ b/target/riscv/op_helper.c
> @@ -321,24 +321,30 @@ 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);
> @@ -370,6 +376,35 @@ 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);
> --
> 2.34.1
>
>