Add some basic documentation on the effects of KVM_ARM_SET_CNT_OFFSETS.
Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
---
Documentation/virt/kvm/api.rst | 38 ++++++++++++++++++++++++++++++++++
1 file changed, 38 insertions(+)
diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 62de0768d6aa..192adcb61add 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6029,6 +6029,44 @@ delivery must be provided via the "reg_aen" struct.
The "pad" and "reserved" fields may be used for future extensions and should be
set to 0s by userspace.
+4.138 KVM_ARM_SET_COUNTER_OFFSET
+--------------------------------
+
+:Capability: KVM_CAP_COUNTER_OFFSET
+:Architectures: arm64
+:Type: vm ioctl
+:Parameters: struct kvm_arm_counter_offset (in)
+:Returns: 0 on success, < 0 on error
+
+This capability indicates that userspace is able to apply a single VM-wide
+offset to both the virtual and physical counters as viewed by the guest
+using the KVM_ARM_SET_CNT_OFFSET ioctl and the following data structure:
+
+::
+
+ struct kvm_arm_counter_offset {
+ __u64 counter_offset;
+ __u64 reserved;
+ };
+
+The offset describes a number of counter cycles that are subtracted from
+both virtual and physical counter views (similar to the effects of the
+CNTVOFF_EL2 and CNTPOFF_EL2 system registers, but only global). The offset
+always applies to all vcpus (already created or created after this ioctl)
+for this VM.
+
+It is userspace's responsibility to compute the offset based, for example,
+on previous values of the guest counters.
+
+Any value other than 0 for the "reserved" field may result in an error
+(-EINVAL) being returned. This ioctl can also return -EBUSY if any vcpu
+ioctl is issued concurrently.
+
+Note that using this ioctl results in KVM ignoring subsequent userspace
+writes to the CNTVCT_EL0 and CNTPCT_EL0 registers using the SET_ONE_REG
+interface. No error will be returned, but the resulting offset will not be
+applied.
+
5. The kvm_run structure
========================
--
2.34.1
