Re: [PATCH 1/1] target/riscv: pmp: Ignore writes when RW=01 and MML=0

[Alistair Francis]

On Thu, Dec 21, 2023 at 1:32 AM Ivan Klokov <ivan.klokov@syntacore.com> wrote:
>
> This patch changes behavior on writing RW=01 to pmpcfg with MML=0.
> RWX filed is form of collective WARL with the combination of
> pmpcfg.RW=01 remains reserved for future standard use.
>
> According to definition of WARL writing the CSR has no other side
> effect. But current implementation change architectural state and
> change system behavior. After writing we will get unreadable-unwriteble
> region regardless on the previous state.
>
> On the other side WARL said that we should read legal value and nothing
> says about what we should write. Current behavior change system state
> regardless of whether we read this register or not.
>
> Fixes: ac66f2f0 ("target/riscv: pmp: Ignore writes when RW=01")
>
> Signed-off-by: Ivan Klokov <ivan.klokov@syntacore.com>

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

Alistair

> ---
>  target/riscv/pmp.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/target/riscv/pmp.c b/target/riscv/pmp.c
> index 162e88a90a..c0b814699e 100644
> --- a/target/riscv/pmp.c
> +++ b/target/riscv/pmp.c
> @@ -126,7 +126,7 @@ static bool pmp_write_cfg(CPURISCVState *env, uint32_t 
> pmp_index, uint8_t val)
>              /* If !mseccfg.MML then ignore writes with encoding RW=01 */
>              if ((val & PMP_WRITE) && !(val & PMP_READ) &&
>                  !MSECCFG_MML_ISSET(env)) {
> -                val &= ~(PMP_WRITE | PMP_READ);
> +                return false;
>              }
>              env->pmp_state.pmp[pmp_index].cfg_reg = val;
>              pmp_update_rule_addr(env, pmp_index);
> --
> 2.34.1
>
>