[PATCH v17 10/20] KVM: arm64: coco: Disable Steal time accounting for coco guests

[Fuad Tabba]

Hi Suzuki,

On Tue, 8 Sept 2026 at 17:23, Suzuki K Poulose <suzuki.poulose at arm.com> wrote:
>
> PVTIME support is advertised by KVM_CAP_STEAL_TIME, which doesn't take into
> account the kvm instance. Even with that, a VMM could skip the CAP check and
> proceed to configure the PVTIME as we don't do further check on the DEVICE_CTRL.
> Tighten this up by passing the KVM instance around wherever possible and
> catch things early
>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose at arm.com>

...

> -bool kvm_arm_pvtime_supported(void)
> +bool kvm_arm_pvtime_supported(struct kvm *kvm)
>  {
> -       return !!sched_info_on();
> +       return !!sched_info_on() && (!kvm || !kvm_vm_is_confidential(kvm));
>  }

I think it would be better to leave KVM_CAP_STEAL_TIME reported and
instead reject the configuration for these VMs, returning -EPERM from
the KVM_ARM_VCPU_PVTIME_CTRL attr. A capability query should report
what the kernel supports, not whether a given VM may use it; gating it
per-VM here is also inconsistent, since the !kvm clause leaves the
system-fd cap at 1 while the VM-fd cap reads 0. And -EPERM says that
steal-time exists but isn't permitted for a confidential VM, whereas
-ENXIO reads as "no such feature".

Cheers,
/fuad

>
>  int kvm_arm_pvtime_set_attr(struct kvm_vcpu *vcpu,
> @@ -81,7 +81,7 @@ int kvm_arm_pvtime_set_attr(struct kvm_vcpu *vcpu,
>         int ret = 0;
>         int idx;
>
> -       if (!kvm_arm_pvtime_supported() ||
> +       if (!kvm_arm_pvtime_supported(kvm) ||
>             attr->attr != KVM_ARM_VCPU_PVTIME_IPA)
>                 return -ENXIO;
>
> @@ -110,7 +110,7 @@ int kvm_arm_pvtime_get_attr(struct kvm_vcpu *vcpu,
>         u64 __user *user = (u64 __user *)attr->addr;
>         u64 ipa;
>
> -       if (!kvm_arm_pvtime_supported() ||
> +       if (!kvm_arm_pvtime_supported(vcpu->kvm) ||
>             attr->attr != KVM_ARM_VCPU_PVTIME_IPA)
>                 return -ENXIO;
>
> @@ -126,7 +126,7 @@ int kvm_arm_pvtime_has_attr(struct kvm_vcpu *vcpu,
>  {
>         switch (attr->attr) {
>         case KVM_ARM_VCPU_PVTIME_IPA:
> -               if (kvm_arm_pvtime_supported())
> +               if (kvm_arm_pvtime_supported(vcpu->kvm))
>                         return 0;
>         }
>         return -ENXIO;
> --
> 2.43.0
>