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

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

On 09/09/2026 12:45, Fuad Tabba wrote:
> Hi Suzuki,
> 
> On Tue, 8 Sept 2026 at 17:23, Suzuki K Poulose <suzuki.poulose@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@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


But isn't that we want from a VMM perspective ? i.e., enable PVTIME if
it is supported for the given VM (type). Rather than PVTIME is supported
by the KVM, so I can go enable it by default. But, we endup getting a
failure while we enable it. I encountered this with kvmtool, where
CAP_STEAL_TIME is reported true and even the HAS_DEVICE_ATTR goes
through fine, but SET_DEVICE_ATTR seemed to fail.

Isn't it better to allow the VM to decide if the capability is supported
for the given VM type, like we are doing for other capabilities ?

> steal-time exists but isn't permitted for a confidential VM, whereas
> -ENXIO reads as "no such feature".

Cheers
Suzuki


> 
> 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
>>