Re: [Qemu-devel] [PATCH 1/6] target-arm: correct CNTFRQ access rights

[Sergey Fedorov <serge.fdrv@gmail.com>]

On 05.02.2016 19:44, Peter Maydell wrote:
> Correct some corner cases we were getting wrong for
> CNTFRQ access rights:
>  * should UNDEF from 32-bit Secure EL1
>  * only writable from the highest implemented exception level,
>    which might not be EL1 now
>
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
>  target-arm/helper.c | 31 ++++++++++++++++++++++++++++---
>  1 file changed, 28 insertions(+), 3 deletions(-)
>
> diff --git a/target-arm/helper.c b/target-arm/helper.c
> index 7a8881a..082701a 100644
> --- a/target-arm/helper.c
> +++ b/target-arm/helper.c
> @@ -1217,9 +1217,34 @@ static const ARMCPRegInfo v6k_cp_reginfo[] = {
>  static CPAccessResult gt_cntfrq_access(CPUARMState *env, const ARMCPRegInfo *ri,
>                                         bool isread)
>  {
> -    /* CNTFRQ: not visible from PL0 if both PL0PCTEN and PL0VCTEN are zero */
> -    if (arm_current_el(env) == 0 && !extract32(env->cp15.c14_cntkctl, 0, 2)) {
> -        return CP_ACCESS_TRAP;
> +    /* CNTFRQ: not visible from PL0 if both PL0PCTEN and PL0VCTEN are zero.
> +     * Writable only at the highest implemented exception level.
> +     */
> +    switch (arm_current_el(env)) {
> +    case 0:
> +        if (!extract32(env->cp15.c14_cntkctl, 0, 2)) {
> +            return CP_ACCESS_TRAP;
> +        }
> +        /* EL0 reads are forbidden by the .access fields */

s/reads/writes/ ?

> +        break;
> +    case 1:
> +        if (!isread && (arm_feature(env, ARM_FEATURE_EL2)
> +                        || arm_feature(env, ARM_FEATURE_EL3))) {
> +            return CP_ACCESS_TRAP_UNCATEGORIZED;
> +        }
> +        if (!isread && ri->state == ARM_CP_STATE_AA32 &&
> +            arm_is_secure_below_el3(env)) {
> +            /* Accesses from 32-bit Secure EL1 UNDEF (*not* trap to EL3!) */
> +            return CP_ACCESS_TRAP_UNCATEGORIZED;
> +        }
> +        break;
> +    case 2:
> +        if (!isread && arm_feature(env, ARM_FEATURE_EL3)) {
> +            return CP_ACCESS_TRAP_UNCATEGORIZED;
> +        }
> +        break;
> +    case 3:
> +        break;
>      }
>      return CP_ACCESS_OK;
>  }

Maybe calculating "the highest implemented exception level" could
simplify reading of the code a bit? E.g.:

    int highest_el = arm_feature(env, ARM_FEATURE_EL3) ? 3 :
                     arm_feature(env, ARM_FEATURE_EL2) ? 2 : 1;

We would probably want to have a dedicated static inline function for
this similar to HighestEL() from ARMv8 ARM pseudocode.

Kind regards,
Sergey