Re: [PATCH v4] target/riscv: update checks on writing pmpcfg for Smepmp to version 1.0

[Alistair Francis]

On Mon, Sep 25, 2023 at 2:11 AM Alvin Chang <alvinga@andestech.com> wrote:
>
> Current checks on writing pmpcfg for Smepmp follows Smepmp version
> 0.9.1. However, Smepmp 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 Smepmp 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 Smepmp
> truth table, and checks the rules by aforementioned specification
> changes.
>
> Signed-off-by: Alvin Chang <alvinga@andestech.com>
> ---
> Changes from v3: Modify "epmp_operation" to "smepmp_operation".

This has the same issue as all the previous versions.

QEMU is currently not shipping with Smepmp support. So updating some
of the code to support Smepmp is confusing.

As I pointed out for the v3, we currently only support ePMP 0.9.3. So
that is what the code must work with.

In order for this change to go in, we also need to upgrade QEMU to
support Smepmp 1.0.

This patch is an attempt to do that:
https://www.mail-archive.com/qemu-devel@nongnu.org/msg967676.html

You basically need to combine the changes from
https://www.mail-archive.com/qemu-devel@nongnu.org/msg967676.html into
this patch. So there is a single patch that updates to Smepmp.

Alistair

>
> Changes from v2: Adopt switch case ranges and numerical order.
>
> Changes from v1: Convert ePMP over to Smepmp.
>
>  target/riscv/pmp.c | 40 ++++++++++++++++++++++++++++++++--------
>  1 file changed, 32 insertions(+), 8 deletions(-)
>
> diff --git a/target/riscv/pmp.c b/target/riscv/pmp.c
> index a08cd95658..2ebf18c941 100644
> --- a/target/riscv/pmp.c
> +++ b/target/riscv/pmp.c
> @@ -99,16 +99,40 @@ 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) {
> +            /*
> +             * 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
> +                 * Smepmp spec.
> +                 */
> +                const uint8_t smepmp_operation =
> +                    ((val & PMP_LOCK) >> 4) | ((val & PMP_READ) << 2) |
> +                    (val & PMP_WRITE) | ((val & PMP_EXEC) >> 2);
> +
> +                switch (smepmp_operation) {
> +                case 0 ... 8:
>                      locked = false;
> -                }
> -                /* shared region and not adding X bit */
> -                if ((val & PMP_LOCK) != PMP_LOCK &&
> -                    (val & 0x7) != (PMP_WRITE | PMP_EXEC)) {
> +                    break;
> +                case 9 ... 11:
> +                    break;
> +                case 12:
> +                    locked = false;
> +                    break;
> +                case 13:
> +                    break;
> +                case 14:
> +                case 15:
>                      locked = false;
> +                    break;
> +                default:
> +                    g_assert_not_reached();
>                  }
>              }
>          } else {
> --
> 2.34.1
>
>