Re: [PATCH v18 01/23] KVM: arm64: protected VM: Handle set_one_reg CNTVCT_EL0/CNTPCT_EL0

[Suzuki K Poulose <suzuki.poulose@arm.com>]

On 16/09/2026 09:16, Marc Zyngier wrote:
> On Tue, 15 Sep 2026 22:20:36 +0100,
> Suzuki K Poulose <suzuki.poulose@arm.com> wrote:
>>
>> This is how it looks like now, reworked.
>>
>>
>> diff --git a/arch/arm64/kvm/arch_timer.c b/arch/arm64/kvm/arch_timer.c
>> index 6ac3321f4c575..1c8fac222a643 100644
>> --- a/arch/arm64/kvm/arch_timer.c
>> +++ b/arch/arm64/kvm/arch_timer.c
>> @@ -1110,8 +1110,7 @@ void kvm_timer_vcpu_init(struct kvm_vcpu *vcpu)
>>                  timer_context_init(vcpu, i);
>>
>>          /* Synchronize offsets across timers of a VM if not already
>> provided */
>> -       if (!vcpu_is_protected(vcpu) &&
>> -           !test_bit(KVM_ARCH_FLAG_VM_COUNTER_OFFSET,
>> &vcpu->kvm->arch.flags)) {
>> +       if (!test_bit(KVM_ARCH_FLAG_VM_COUNTER_OFFSET,
>> &vcpu->kvm->arch.flags)) {
>>                  timer_set_offset(vcpu_vtimer(vcpu), kvm_phys_timer_read());
>>                  timer_set_offset(vcpu_ptimer(vcpu), 0);
>>          }
>> @@ -1133,6 +1132,15 @@ void kvm_timer_init_vm(struct kvm *kvm)
>>           */
>>          for (int i = 0; i < NR_KVM_TIMERS; i++)
>>                  kvm->arch.timer_data.ppi[i] = get_vgic_ppi(kvm,
>> default_ppi[i]);
>> +
>> +       /*
>> +        * For the protected VMs, fix the counter offset to 0 by marking the
>> +        * KVM_ARCH_FLAG_VM_COUNTER_OFFSET. This would also prevent
>> userspace
>> +        * writes to CNTVCT_ELO & CNTPCT_EL0.
>> +        * The user can't set the offset via vm ioctl
>> KVM_ARM_SET_COUNTER_OFFSET.
>> +        */
> 
> This is a bit wordy. Something like:
> 
> 	Protected VMs don't allow any offset being set from userspace,
> 	either set via writes to the counters or using the dedicated
> 	ioctl. Pretend the offset has already been set and rely on the
> 	default offset being 0.
> 
>> +       if (kvm_vm_is_protected(kvm))
>> +               set_bit(KVM_ARCH_FLAG_VM_COUNTER_OFFSET, &kvm->arch.flags);
>>   }
>>
>>   void kvm_timer_cpu_up(void)
> 
> Otherwise LGTM.

Thanks,

Cheers
Suzuki

> 
> 	M.
>