Re: [PATCH] target/riscv: pmp: Ignore writes when RW=01

[Alistair Francis]

On Thu, Sep 14, 2023 at 10:35 PM Mayuresh Chitale
<mchitale@ventanamicro.com> wrote:
>
> As per the Priv spec: "The R, W, and X fields form a collective WARL
> field for which the combinations with R=0 and W=1 are reserved."
> However currently such writes are not ignored as ought to be. The
> combinations with RW=01 are allowed only when the Smepmp extension
> is enabled and mseccfg.MML is set.
>
> Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
> ---
>  target/riscv/pmp.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/target/riscv/pmp.c b/target/riscv/pmp.c
> index f3eb6e6585..5b430be18c 100644
> --- a/target/riscv/pmp.c
> +++ b/target/riscv/pmp.c
> @@ -119,6 +119,14 @@ static bool pmp_write_cfg(CPURISCVState *env, uint32_t 
> pmp_index, uint8_t val)
>          if (locked) {
>              qemu_log_mask(LOG_GUEST_ERROR, "ignoring pmpcfg write - 
> locked\n");
>          } else if (env->pmp_state.pmp[pmp_index].cfg_reg != val) {
> +            /* If !mseccfg.MML then ignore writes with encoding RW=01 */
> +            if ((val & PMP_WRITE) && !(val & PMP_READ) &&
> +                (!riscv_cpu_cfg(env)->ext_smepmp ||
> +                !MSECCFG_MML_ISSET(env))) {

MSECCFG_MML can only be set if ext_smepmp exists, so I don't think we
need both checks here

> +                val &= ~(PMP_WRITE | PMP_READ);
> +                val |= env->pmp_state.pmp[pmp_index].cfg_reg &
> +                       (PMP_WRITE | PMP_READ);

Why do we restore the previous value? It's a WARL so we should just
guarantee a legal value (which would be 0x0 I guess)

Alistair

> +            }
>              env->pmp_state.pmp[pmp_index].cfg_reg = val;
>              pmp_update_rule_addr(env, pmp_index);
>              return true;
> --
> 2.34.1
>
>