Re: [PATCH 1/3] target/riscv: Optional feature to provide trapped instruction in CSRs

[Alistair Francis]

On Wed, Jul 29, 2020 at 4:29 AM Anup Patel <anup.patel@wdc.com> wrote:
>
> The RISC-V spec allows implementations to provide trapped instruction
> opcode in MTVAL/STVAL CSR for illegal/virtual instruction traps. This
> is totally optional and most RISC-V implementations always set zero
> in the MTVAL/STVAL CSR for illegal/virtual instruction traps.
>
> When trapped instruction opcode is available in MTVAL/STVAL CSR, the
> M-mode runtime firmware (and Hypervisors) can skip unprivlege access
> for reading trapped instruction opcode which in-turn will speed-up
> the illegal/virtual instruction trap handling.
>
> This patch implements RISCV_FEATURE_TINST feature which when enabled
> provides original trapped instruction opcode in MTVAL/STVAL CSRs for
> illegal/virtual instruction trap.
>
> Signed-off-by: Anup Patel <anup.patel@wdc.com>

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>

Alistair

> ---
>  target/riscv/cpu.c        |  7 +++++++
>  target/riscv/cpu.h        | 11 ++++++++++-
>  target/riscv/cpu_helper.c |  6 ++++++
>  target/riscv/translate.c  | 14 +++++++++++++-
>  4 files changed, 36 insertions(+), 2 deletions(-)
>
> diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
> index eeb91f8513..ec098e445e 100644
> --- a/target/riscv/cpu.c
> +++ b/target/riscv/cpu.c
> @@ -317,6 +317,7 @@ void restore_state_to_opc(CPURISCVState *env, 
> TranslationBlock *tb,
>                            target_ulong *data)
>  {
>      env->pc = data[0];
> +    env->trap_insn = data[1];
>  }
>
>  static void riscv_cpu_reset(DeviceState *dev)
> @@ -332,6 +333,7 @@ static void riscv_cpu_reset(DeviceState *dev)
>      env->mstatus &= ~(MSTATUS_MIE | MSTATUS_MPRV);
>      env->mcause = 0;
>      env->pc = env->resetvec;
> +    env->trap_insn = 0;
>  #endif
>      cs->exception_index = EXCP_NONE;
>      env->load_res = -1;
> @@ -387,6 +389,10 @@ static void riscv_cpu_realize(DeviceState *dev, Error 
> **errp)
>          set_feature(env, RISCV_FEATURE_PMP);
>      }
>
> +    if (cpu->cfg.tinst) {
> +        set_feature(env, RISCV_FEATURE_TINST);
> +    }
> +
>      /* If misa isn't set (rv32 and rv64 machines) set it here */
>      if (!env->misa) {
>          /* Do some ISA extension error checking */
> @@ -487,6 +493,7 @@ static Property riscv_cpu_properties[] = {
>      DEFINE_PROP_STRING("priv_spec", RISCVCPU, cfg.priv_spec),
>      DEFINE_PROP_BOOL("mmu", RISCVCPU, cfg.mmu, true),
>      DEFINE_PROP_BOOL("pmp", RISCVCPU, cfg.pmp, true),
> +    DEFINE_PROP_BOOL("tinst", RISCVCPU, cfg.tinst, false),
>      DEFINE_PROP_END_OF_LIST(),
>  };
>
> diff --git a/target/riscv/cpu.h b/target/riscv/cpu.h
> index 1bb5271511..33984539d7 100644
> --- a/target/riscv/cpu.h
> +++ b/target/riscv/cpu.h
> @@ -26,6 +26,12 @@
>
>  #define TCG_GUEST_DEFAULT_MO 0
>
> +/*
> + * RISC-V-specific extra insn start words:
> + * 1: Original instruction opcode
> + */
> +#define TARGET_INSN_START_EXTRA_WORDS 1
> +
>  #define TYPE_RISCV_CPU "riscv-cpu"
>
>  #define RISCV_CPU_TYPE_SUFFIX "-" TYPE_RISCV_CPU
> @@ -70,7 +76,8 @@
>  enum {
>      RISCV_FEATURE_MMU,
>      RISCV_FEATURE_PMP,
> -    RISCV_FEATURE_MISA
> +    RISCV_FEATURE_MISA,
> +    RISCV_FEATURE_TINST
>  };
>
>  #define PRIV_VERSION_1_10_0 0x00011000
> @@ -97,6 +104,7 @@ struct CPURISCVState {
>      target_ulong frm;
>
>      target_ulong badaddr;
> +    target_ulong trap_insn;
>      target_ulong guest_phys_fault_addr;
>
>      target_ulong priv_ver;
> @@ -264,6 +272,7 @@ typedef struct RISCVCPU {
>          char *user_spec;
>          bool mmu;
>          bool pmp;
> +        bool tinst;
>      } cfg;
>  } RISCVCPU;
>
> diff --git a/target/riscv/cpu_helper.c b/target/riscv/cpu_helper.c
> index eccd80cfef..e4bd45d66a 100644
> --- a/target/riscv/cpu_helper.c
> +++ b/target/riscv/cpu_helper.c
> @@ -864,6 +864,12 @@ void riscv_cpu_do_interrupt(CPUState *cs)
>          case RISCV_EXCP_STORE_PAGE_FAULT:
>              tval = env->badaddr;
>              break;
> +        case RISCV_EXCP_VIRT_INSTRUCTION_FAULT:
> +        case RISCV_EXCP_ILLEGAL_INST:
> +            if (riscv_feature(env, RISCV_FEATURE_TINST)) {
> +                tval = env->trap_insn;
> +            }
> +            break;
>          default:
>              break;
>          }
> diff --git a/target/riscv/translate.c b/target/riscv/translate.c
> index 1d973b62e9..03954bff62 100644
> --- a/target/riscv/translate.c
> +++ b/target/riscv/translate.c
> @@ -56,6 +56,8 @@ typedef struct DisasContext {
>         to reset this known value.  */
>      int frm;
>      bool ext_ifencei;
> +    /* TCG op of the current insn_start.  */
> +    TCGOp *insn_start;
>  } DisasContext;
>
>  #ifdef TARGET_RISCV64
> @@ -717,6 +719,13 @@ static bool gen_shift(DisasContext *ctx, arg_r *a,
>  /* Include the auto-generated decoder for 16 bit insn */
>  #include "decode_insn16.inc.c"
>
> +static inline void decode_save_opc(DisasContext *ctx, target_ulong opc)
> +{
> +    assert(ctx->insn_start != NULL);
> +    tcg_set_insn_start_param(ctx->insn_start, 1, opc);
> +    ctx->insn_start = NULL;
> +}
> +
>  static void decode_opc(CPURISCVState *env, DisasContext *ctx, uint16_t 
> opcode)
>  {
>      /* check for compressed insn */
> @@ -724,6 +733,7 @@ static void decode_opc(CPURISCVState *env, DisasContext 
> *ctx, uint16_t opcode)
>          if (!has_ext(ctx, RVC)) {
>              gen_exception_illegal(ctx);
>          } else {
> +            decode_save_opc(ctx, opcode);
>              ctx->pc_succ_insn = ctx->base.pc_next + 2;
>              if (!decode_insn16(ctx, opcode)) {
>                  /* fall back to old decoder */
> @@ -734,6 +744,7 @@ static void decode_opc(CPURISCVState *env, DisasContext 
> *ctx, uint16_t opcode)
>          uint32_t opcode32 = opcode;
>          opcode32 = deposit32(opcode32, 16, 16,
>                               translator_lduw(env, ctx->base.pc_next + 2));
> +        decode_save_opc(ctx, opcode32);
>          ctx->pc_succ_insn = ctx->base.pc_next + 4;
>          if (!decode_insn32(ctx, opcode32)) {
>              gen_exception_illegal(ctx);
> @@ -773,7 +784,8 @@ static void riscv_tr_insn_start(DisasContextBase *dcbase, 
> CPUState *cpu)
>  {
>      DisasContext *ctx = container_of(dcbase, DisasContext, base);
>
> -    tcg_gen_insn_start(ctx->base.pc_next);
> +    tcg_gen_insn_start(ctx->base.pc_next, 0);
> +    ctx->insn_start = tcg_last_op();
>  }
>
>  static bool riscv_tr_breakpoint_check(DisasContextBase *dcbase, CPUState 
> *cpu,
> --
> 2.25.1
>
>