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

On 09/09/2026 12:45, Fuad Tabba wrote:

Hi Suzuki,

On Tue, 8 Sept 2026 at 17:23, Suzuki K Poulose [off-list ref] wrote:

quoted

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>

...

quoted

-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

quoted

  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