Re: [PATCH] target/riscv: update checks on writing pmpcfg for ePMP to version 1.0

[Alistair Francis]

On Tue, Sep 5, 2023 at 2:30 AM Alvin Chang <vivahavey@gmail.com> wrote:
>
> Current checks on writing pmpcfg for ePMP follows ePMP version 0.9.1.
> However, ePMP specification has already been ratified, and there are
> some differences between version 0.9.1 and 1.0. In this commit we update
> the checks of writing pmpcfg to follow ePMP version 1.0.
>
> When mseccfg.MML is set, the constraints to modify PMP rules are:
> 1. Locked rules cannot be removed or modified until a PMP reset, unless
>    mseccfg.RLB is set.
> 2. From Smepmp specification version 1.0, chapter 2 section 4b:
>    Adding a rule with executable privileges that either is M-mode-only
>    or a locked Shared-Region is not possible and such pmpcfg writes are
>    ignored, leaving pmpcfg unchanged.
>
> The commit transfers the value of pmpcfg into the index of the ePMP
> truth table, and checks the rules by aforementioned specification
> changes.
>
> Signed-off-by: Alvin Chang <alvinga@andestech.com>

Thanks for the patch!

As part of this change we should convert ePMP over to Smepmp and drop
the experimental status

Alistair

> ---
>  target/riscv/pmp.c | 51 ++++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 42 insertions(+), 9 deletions(-)
>
> diff --git a/target/riscv/pmp.c b/target/riscv/pmp.c
> index a08cd95658..c036ca3e70 100644
> --- a/target/riscv/pmp.c
> +++ b/target/riscv/pmp.c
> @@ -99,16 +99,49 @@ static void pmp_write_cfg(CPURISCVState *env, uint32_t 
> pmp_index, uint8_t val)
>                  locked = false;
>              }
>
> -            /* mseccfg.MML is set */
> -            if (MSECCFG_MML_ISSET(env)) {
> -                /* not adding execute bit */
> -                if ((val & PMP_LOCK) != 0 && (val & PMP_EXEC) != PMP_EXEC) {
> -                    locked = false;
> -                }
> -                /* shared region and not adding X bit */
> -                if ((val & PMP_LOCK) != PMP_LOCK &&
> -                    (val & 0x7) != (PMP_WRITE | PMP_EXEC)) {
> +            /*
> +             * mseccfg.MML is set. Locked rules cannot be removed or modified
> +             * until a PMP reset. Besides, from Smepmp specification version 
> 1.0
> +             * , chapter 2 section 4b says:
> +             * Adding a rule with executable privileges that either is
> +             * M-mode-only or a locked Shared-Region is not possible and such
> +             * pmpcfg writes are ignored, leaving pmpcfg unchanged.
> +             */
> +            if (MSECCFG_MML_ISSET(env) && !pmp_is_locked(env, pmp_index)) {
> +                /*
> +                 * Convert the PMP permissions to match the truth table in 
> the
> +                 * ePMP spec.
> +                 */
> +                const uint8_t epmp_operation =
> +                    ((val & PMP_LOCK) >> 4) | ((val & PMP_READ) << 2) |
> +                    (val & PMP_WRITE) | ((val & PMP_EXEC) >> 2);
> +
> +                switch (epmp_operation) {
> +                /* pmpcfg.L = 0. Neither M-mode-only nor locked 
> Shared-Region */
> +                case 0:
> +                case 1:
> +                case 2:
> +                case 3:
> +                case 4:
> +                case 5:
> +                case 6:
> +                case 7:
> +                /* pmpcfg.L = 1 and pmpcfg.X = 0 (but case 10 is not 
> allowed) */
> +                case 8:
> +                case 12:
> +                case 14:
> +                /* pmpcfg.LRWX = 1111 */
> +                case 15:  /* Read-only locked Shared-Region on all modes */
>                      locked = false;
> +                    break;
> +                /* Other rules which add new code regions are not allowed */
> +                case 9:
> +                case 10:  /* Execute-only locked Shared-Region on all modes 
> */
> +                case 11:
> +                case 13:
> +                    break;
> +                default:
> +                    g_assert_not_reached();
>                  }
>              }
>          } else {
> --
> 2.34.1
>
>