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 | 47 ++++++++++++++++++++++++++++++++++
1 file changed, 47 insertions(+)
diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 0a67cb738013..bdd361fd90f4 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -5937,6 +5937,53 @@ 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_CNT_OFFSETS
+-----------------------------
+
+:Capability: KVM_CAP_COUNTER_OFFSETS
+:Architectures: arm64
+:Type: vm ioctl
+:Parameters: struct kvm_arm_counter_offsets (in)
+:Returns: 0 on success, <0 on error
+
+This capability indicates that userspace is able to apply a set of
+VM-wide offsets to the virtual and physical counters as viewed by the
+guest using the KVM_ARM_SET_CNT_OFFSETS ioctl and the following data
+structure:
+
+ struct kvm_arm_counter_offsets {
+ __u64 virtual_offset;
+ __u64 physical_offset;
+
+ #define KVM_COUNTER_SET_VOFFSET_FLAG (1UL << 0)
+ #define KVM_COUNTER_SET_POFFSET_FLAG (1UL << 1)
+
+ __u64 flags;
+ __u64 reserved;
+ };
+
+Each of the two offsets describe a number of counter cycles that are
+subtracted from the corresponding counter (similar to the effects of
+the CNTVOFF_EL2 and CNTPOFF_EL2 system registers). For each offset
+that userspace wishes to change, it must set the corresponding flag in
+the "flag" field. These values always apply to all vcpus (already
+created or created after this ioctl) in this VM.
+
+It is userspace's responsibility to compute the offsets based, for
+example, on previous values of the guest counters.
+
+With nested virtualisation, the virtual offset as no effect on the
+execution of the guest, and the nested hypervisor is responsible for
+the offset that is presented to its own guests.
+
+Any flag value that isn't described here, or any value other than 0
+for the "reserved" field may result in an error being returned.
+
+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
