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

[Michal Orzel]

Hi,

On 27/09/2023 17:49, Peter Maydell wrote:
> 
> 
> On Fri, 22 Sept 2023 at 14:21, Michal Orzel <michal.orzel@amd.com> wrote:
>>
>> 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
>>
>> Fix it by checking for the right bit (i.e. 0) and update the comment
>> referring to incorrect bit name.
>>
>> 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);
>> ---
>>  target/arm/helper.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/target/arm/helper.c b/target/arm/helper.c
>> index 3b22596eabf3..3a2d77b3f81e 100644
>> --- a/target/arm/helper.c
>> +++ b/target/arm/helper.c
>> @@ -2483,9 +2483,9 @@ static CPAccessResult gt_counter_access(CPUARMState 
>> *env, int timeridx,
>>                  return CP_ACCESS_TRAP_EL2;
>>              }
>>          } else {
>> -            /* If HCR_EL2.<E2H> == 0: check CNTHCTL_EL2.EL1PCEN. */
>> +            /* If HCR_EL2.<E2H> == 0: check CNTHCTL_EL2.EL1PCTEN. */
>>              if (has_el2 && timeridx == GTIMER_PHYS &&
>> -                !extract32(env->cp15.cnthctl_el2, 1, 1)) {
>> +                !extract32(env->cp15.cnthctl_el2, 0, 1)) {
>>                  return CP_ACCESS_TRAP_EL2;
>>              }
>>          }
> 
> I agree that the current logic is not correct, but this change
> makes this code identical to the "case 1" handling, so we
> can delete the whole "if (hcr & HCR_E2H) { ... } else { ...  }"
> block and instead fall through, as we already do in gt_timer_access().
Ok, will do.

~Michal