Re: [PATCH v3 07/20] target/riscv: zicfilp `lpad` impl and branch tracking

[Richard Henderson]

On 8/7/24 10:06, Deepak Gupta wrote:
> diff --git a/target/riscv/cpu_user.h b/target/riscv/cpu_user.h
> index 02afad608b..e6927ff847 100644
> --- a/target/riscv/cpu_user.h
> +++ b/target/riscv/cpu_user.h
> @@ -15,5 +15,6 @@
>   #define xA6 16
>   #define xA7 17  /* syscall number for RVI ABI */
>   #define xT0 5   /* syscall number for RVE ABI */
> +#define xT2 7

Maybe just add them all?

> diff --git a/target/riscv/insn_trans/trans_rvi.c.inc 
> b/target/riscv/insn_trans/trans_rvi.c.inc
> index 98e3806d5e..cbd7d5c395 100644
> --- a/target/riscv/insn_trans/trans_rvi.c.inc
> +++ b/target/riscv/insn_trans/trans_rvi.c.inc
> @@ -36,6 +36,58 @@ static bool trans_lui(DisasContext *ctx, arg_lui *a)
>       return true;
>   }
>   
> +static bool trans_lpad(DisasContext *ctx, arg_lpad *a)
> +{
> +    bool lp_expected;
> +    /* zicfilp only supported on 32bit and 64bit */
> +    if (get_xl(ctx) != MXL_RV32 && get_xl(ctx) != MXL_RV64) {
> +        return false;
> +    }

Where does it say that?
There doesn't seem to be anything in the spec that excludes rv128.

> +    lp_expected = ctx->fcfi_lp_expected;
> +    /* forward cfi not enabled or lp not expected, return false */
> +    if (!ctx->fcfi_enabled) {
> +        return false;
> +    }

Comments should explain why, not exactly mirror the code.
In any case, better as:

  /*
   * If zicfilp not present, the encoding is a nop.
   * If forward cfi not enabled, the implementation is a nop.
   */
  if (!ctx->fcfi_enabled) {
      return true;
  }

No need to fall through into AUIPC to re-discover nop-ness.

> +
> +    /*
> +     * If this is the first instruction of the TB, let the translator
> +     * know the landing pad requirement was satisfied. No need to bother
> +     * checking for CFI feature or enablement.
> +     */

Comment is strange, because you have just checked enablement.

> +    /* if lp was expected, do label check */
> +    if (lp_expected) {
> +        TCGLabel *skip = gen_new_label();
> +        TCGv tmp = tcg_temp_new();
> +        tcg_gen_st_tl(tcg_constant_tl(NO_LP_EXPECTED),
> +                      tcg_env, offsetof(CPURISCVState, elp));

This placement is wrong, according to the LPAD pseudocode.
It must go last.

> +        tcg_gen_extract_tl(tmp, get_gpr(ctx, xT2, EXT_NONE), 12, 20);
> +        tcg_gen_brcondi_tl(TCG_COND_EQ, tcg_constant_tl(a->imm_cfi20), 0, 
> skip);

This one you check at translation time:

  if (a->imm_cfi20 != 0)


> +        tcg_gen_brcondi_tl(TCG_COND_EQ, tmp, a->imm_cfi20, skip);
> +        gen_helper_raise_sw_check_excep(tcg_env,
> +            tcg_constant_tl(RISCV_EXCP_SW_CHECK_FCFI_TVAL),
> +            tcg_constant_tl(LABEL_MISMATCH_LPAD), tcg_constant_tl(0));
> +        gen_set_label(skip);
> +    }
> +
> +    return true;
> +}
> +
>   static bool trans_auipc(DisasContext *ctx, arg_auipc *a)
>   {
>       TCGv target_pc = dest_gpr(ctx, a->rd);
> @@ -75,6 +127,20 @@ static bool trans_jalr(DisasContext *ctx, arg_jalr *a)
>       gen_set_gpr(ctx, a->rd, succ_pc);
>   
>       tcg_gen_mov_tl(cpu_pc, target_pc);
> +    if (ctx->cfg_ptr->ext_zicfilp && ctx->fcfi_enabled) {

No need to check ext_zicfilp, as fcfi_enabled includes that already.

> +        /*
> +         * Rely on a helper to check the forward CFI enable for the
> +         * current process mode. The alternatives would be (1) include
> +         * "fcfi enabled" in the cflags or (2) maintain a "fcfi
> +         * currently enabled" in tcg_env and emit TCG code to access
> +         * and test it.
> +         */

Comment is out of date.

> +        if (a->rs1 != xRA && a->rs1 != xT0 && a->rs1 != xT2) {
> +            tcg_gen_st_tl(tcg_constant_tl(LP_EXPECTED),
> +                          tcg_env, offsetof(CPURISCVState, elp));
> +        }
> +    }
> +
>       lookup_and_goto_ptr(ctx);
>   
>       if (misaligned) {


r~