Re: [PATCH v9 06/17] target/riscv: zicfilp `lpad` impl and branch tracking

[Alistair Francis]

On Tue, Aug 27, 2024 at 1:33 AM Deepak Gupta <debug@rivosinc.com> wrote:
>
> Implements setting lp expected when `jalr` is encountered and implements
> `lpad` instruction of zicfilp. `lpad` instruction is taken out of
> auipc x0, <imm_20>. This is an existing HINTNOP space. If `lpad` is
> target of an indirect branch, cpu checks for 20 bit value in x7 upper
> with 20 bit value embedded in `lpad`. If they don't match, cpu raises a
> sw check exception with tval = 2.
>
> Signed-off-by: Deepak Gupta <debug@rivosinc.com>
> Co-developed-by: Jim Shu <jim.shu@sifive.com>
> Co-developed-by: Andy Chiu <andy.chiu@sifive.com>
> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

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

Alistair

> ---
>  target/riscv/cpu_user.h                 |  1 +
>  target/riscv/insn32.decode              |  5 ++-
>  target/riscv/insn_trans/trans_rvi.c.inc | 55 +++++++++++++++++++++++++
>  3 files changed, 60 insertions(+), 1 deletion(-)
>
> 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
>
>  #endif
> diff --git a/target/riscv/insn32.decode b/target/riscv/insn32.decode
> index c45b8fa1d8..27108b992b 100644
> --- a/target/riscv/insn32.decode
> +++ b/target/riscv/insn32.decode
> @@ -123,7 +123,10 @@ sfence_vm   0001000    00100 ..... 000 00000 1110011 
> @sfence_vm
>
>  # *** RV32I Base Instruction Set ***
>  lui      ....................       ..... 0110111 @u
> -auipc    ....................       ..... 0010111 @u
> +{
> +  lpad   label:20                   00000 0010111
> +  auipc  ....................       ..... 0010111 @u
> +}
>  jal      ....................       ..... 1101111 @j
>  jalr     ............     ..... 000 ..... 1100111 @i
>  beq      ....... .....    ..... 000 ..... 1100011 @b
> diff --git a/target/riscv/insn_trans/trans_rvi.c.inc 
> b/target/riscv/insn_trans/trans_rvi.c.inc
> index 98e3806d5e..b427f3a939 100644
> --- a/target/riscv/insn_trans/trans_rvi.c.inc
> +++ b/target/riscv/insn_trans/trans_rvi.c.inc
> @@ -36,6 +36,49 @@ static bool trans_lui(DisasContext *ctx, arg_lui *a)
>      return true;
>  }
>
> +static bool trans_lpad(DisasContext *ctx, arg_lpad *a)
> +{
> +    /*
> +     * fcfi_lp_expected can set only if fcfi was eanbled.
> +     * translate further only if fcfi_lp_expected set.
> +     * lpad comes from NOP space anyways, so return true if
> +     * fcfi_lp_expected is false.
> +     */
> +    if (!ctx->fcfi_lp_expected) {
> +        return true;
> +    }
> +
> +    ctx->fcfi_lp_expected = false;
> +    if ((ctx->base.pc_next) & 0x3) {
> +        /*
> +         * misaligned, according to spec we should raise sw check exception
> +         */
> +        tcg_gen_st_tl(tcg_constant_tl(RISCV_EXCP_SW_CHECK_FCFI_TVAL),
> +                      tcg_env, offsetof(CPURISCVState, sw_check_code));
> +        gen_helper_raise_exception(tcg_env,
> +                      tcg_constant_i32(RISCV_EXCP_SW_CHECK));
> +        return true;
> +    }
> +
> +    /* per spec, label check performed only when embedded label non-zero */
> +    if (a->label != 0) {
> +        TCGLabel *skip = gen_new_label();
> +        TCGv tmp = tcg_temp_new();
> +        tcg_gen_extract_tl(tmp, get_gpr(ctx, xT2, EXT_NONE), 12, 20);
> +        tcg_gen_brcondi_tl(TCG_COND_EQ, tmp, a->label, skip);
> +        tcg_gen_st_tl(tcg_constant_tl(RISCV_EXCP_SW_CHECK_FCFI_TVAL),
> +                      tcg_env, offsetof(CPURISCVState, sw_check_code));
> +        gen_helper_raise_exception(tcg_env,
> +                      tcg_constant_i32(RISCV_EXCP_SW_CHECK));
> +        gen_set_label(skip);
> +    }
> +
> +    tcg_gen_st8_tl(tcg_constant_tl(0), tcg_env,
> +                  offsetof(CPURISCVState, elp));
> +
> +    return true;
> +}
> +
>  static bool trans_auipc(DisasContext *ctx, arg_auipc *a)
>  {
>      TCGv target_pc = dest_gpr(ctx, a->rd);
> @@ -75,6 +118,18 @@ 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->fcfi_enabled) {
> +        /*
> +         * return from functions (i.e. rs1 == xRA || rs1 == xT0) are not
> +         * tracked. zicfilp introduces sw guarded branch as well. sw guarded
> +         * branch are not tracked. rs1 == xT2 is a sw guarded branch.
> +         */
> +        if (a->rs1 != xRA && a->rs1 != xT0 && a->rs1 != xT2) {
> +            tcg_gen_st8_tl(tcg_constant_tl(1),
> +                          tcg_env, offsetof(CPURISCVState, elp));
> +        }
> +    }
> +
>      lookup_and_goto_ptr(ctx);
>
>      if (misaligned) {
> --
> 2.44.0
>
>