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

[Suzuki K Poulose]

On 16/09/2026 09:16, Marc Zyngier wrote:
> On Tue, 15 Sep 2026 22:20:36 +0100,
> Suzuki K Poulose <suzuki.poulose@xxxxxxx> 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.