Re: [PATCH] target/riscv: Implement WARL behaviour for mcountinhibit/mcounteren

[Alistair Francis]

On Wed, Aug 2, 2023 at 8:50 AM Rob Bradford <rbradford@rivosinc.com> wrote:
>
> These are WARL fields - zero out the bits for unavailable counters and
> special case the TM bit in mcountinhibit which is hardwired to zero.
> This patch achieves this by modifying the value written so that any use
> of the field will see the correctly masked bits.
>
> Tested by modifying OpenSBI to write max value to these CSRs and upon
> subsequent read the appropriate number of bits for number of PMUs is
> enabled and the TM bit is zero in mcountinhibit.
>
> Signed-off-by: Rob Bradford <rbradford@rivosinc.com>

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

Alistair

> ---
>  target/riscv/csr.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/target/riscv/csr.c b/target/riscv/csr.c
> index ea7585329e..495ff6a9c2 100644
> --- a/target/riscv/csr.c
> +++ b/target/riscv/csr.c
> @@ -1834,8 +1834,11 @@ static RISCVException 
> write_mcountinhibit(CPURISCVState *env, int csrno,
>  {
>      int cidx;
>      PMUCTRState *counter;
> +    RISCVCPU *cpu = env_archcpu(env);
>
> -    env->mcountinhibit = val;
> +    /* WARL register - disable unavailable counters; TM bit is always 0 */
> +    env->mcountinhibit =
> +        val & (cpu->pmu_avail_ctrs | COUNTEREN_CY | COUNTEREN_IR);
>
>      /* Check if any other counter is also monitoring cycles/instructions */
>      for (cidx = 0; cidx < RV_MAX_MHPMCOUNTERS; cidx++) {
> @@ -1858,7 +1861,11 @@ static RISCVException read_mcounteren(CPURISCVState 
> *env, int csrno,
>  static RISCVException write_mcounteren(CPURISCVState *env, int csrno,
>                                         target_ulong val)
>  {
> -    env->mcounteren = val;
> +    RISCVCPU *cpu = env_archcpu(env);
> +
> +    /* WARL register - disable unavailable counters */
> +    env->mcounteren = val & (cpu->pmu_avail_ctrs | COUNTEREN_CY | 
> COUNTEREN_TM |
> +                             COUNTEREN_IR);
>      return RISCV_EXCP_NONE;
>  }
>
> --
> 2.41.0
>
>