Re: [PATCH v2] target/arm: Fix CNTPCT_EL0 trapping from EL0 when HCR_EL2.E2H is 0

[Oleksandr Tyshchenko]

On 28.09.23 12:44, Michal Orzel wrote:

Hello Michal

> On an attempt to access CNTPCT_EL0 from EL0 using a guest running on top
> of Xen, a trap from EL2 was observed which is something not reproducible
> on HW (also, Xen does not trap accesses to physical counter).
> 
> This is because gt_counter_access() checks for an incorrect bit (1
> instead of 0) of CNTHCTL_EL2 if HCR_EL2.E2H is 0 and access is made to
> physical counter. Refer ARM ARM DDI 0487J.a, D19.12.2:
> When HCR_EL2.E2H is 0:
>   - EL1PCTEN, bit [0]: refers to physical counter
>   - EL1PCEN, bit [1]: refers to physical timer registers
> 
> Drop entire block "if (hcr & HCR_E2H) {...} else {...}" from EL0 case
> and fall through to EL1 case, given that after fixing checking for the
> correct bit, the handling is the same.
> 
> Fixes: 5bc8437136fb ("target/arm: Update timer access for VHE")
> Signed-off-by: Michal Orzel <michal.orzel@amd.com>
> ---
> This is now in conformance to ARM ARM CNTPCT_EL0 pseudocode:
> if PSTATE.EL == EL0 then
> ...
>      elif EL2Enabled() && HCR_EL2.E2H == '0' && CNTHCTL_EL2.EL1PCTEN == '0' 
> then
>          AArch64.SystemAccessTrap(EL2, 0x18);
> 
> Changes in v2:
>   - drop block and fall through to avoid duplication



The V2 also works, so my tag is still valid:

[with Zephyr running as Xen guest and accessing CNTPCT_EL0 from EL0]

Tested-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>



> ---
>   target/arm/helper.c | 17 +----------------
>   1 file changed, 1 insertion(+), 16 deletions(-)
> 
> diff --git a/target/arm/helper.c b/target/arm/helper.c
> index 83620787b453..59576aa1575a 100644
> --- a/target/arm/helper.c
> +++ b/target/arm/helper.c
> @@ -2475,22 +2475,7 @@ static CPAccessResult gt_counter_access(CPUARMState 
> *env, int timeridx,
>           if (!extract32(env->cp15.c14_cntkctl, timeridx, 1)) {
>               return CP_ACCESS_TRAP;
>           }
> -
> -        /* If HCR_EL2.<E2H,TGE> == '10': check CNTHCTL_EL2.EL1PCTEN. */
> -        if (hcr & HCR_E2H) {
> -            if (timeridx == GTIMER_PHYS &&
> -                !extract32(env->cp15.cnthctl_el2, 10, 1)) {
> -                return CP_ACCESS_TRAP_EL2;
> -            }
> -        } else {
> -            /* If HCR_EL2.<E2H> == 0: check CNTHCTL_EL2.EL1PCEN. */
> -            if (has_el2 && timeridx == GTIMER_PHYS &&
> -                !extract32(env->cp15.cnthctl_el2, 1, 1)) {
> -                return CP_ACCESS_TRAP_EL2;
> -            }
> -        }
> -        break;
> -
> +        /* fall through */
>       case 1:
>           /* Check CNTHCTL_EL2.EL1PCTEN, which changes location based on E2H. 
> */
>           if (has_el2 && timeridx == GTIMER_PHYS &&