---
title: "hv_vcpu_set_tsc_relative(_:_:)"
framework: Hypervisor
role: symbol
role_heading: Function
platforms: [macOS 11.0+]
path: "hypervisor/hv_vcpu_set_tsc_relative(_:_:)"
---

# hv_vcpu_set_tsc_relative(_:_:)

Sets the offset of the guest timestamp-counter (TSC) relative to the Hypervisor’s TSC clock.

## Declaration

```swift
func hv_vcpu_set_tsc_relative(_ vcpu: hv_vcpuid_t, _ offset: Int64) -> hv_return_t
```

## Parameters

- `vcpu`: The vCPU ID.
- `offset`: The relative offset value to apply to VMCS TSC-offset field.

## Return Value

Return Value 0 on success, or an hv_return_t error code.

## Discussion

Discussion Use this function to set the TSC-offset field in the VMCS so that the TSC value in the guest is the value of hv_tsc_clock() plus the given offset. The examples below show simple ways to calculate and set the TSC offset: // initialization - set guest TSC to zero int64_t vtsc_offset = - scale * hv_tsc_clock(); hv_vcpu_tsc_relative_offset(vcpu, vtsc_offset);

// compute the guest TSC at runtime from the host uint64_t vtsc = scale * hv_tsc_clock() + vtsc_offset;

// guest writing to the IA32_TSC MSR vtsc_offset = newTSCvalue - scale * hv_tsc_clock(); hv_vcpu_tsc_relative_offset(vcpu, vtsc_offset);

// guest writing to the IA32_TSC_ADJUST MSR vtsc_offset += adjustment; hv_vcpu_tsc_relative_offset(vcpu, vtsc_offset);

// at entry to inner hypervisor hv_vcpu_tsc_relative_offset(vcpu, vtsc_offset);

// at entry to inner guest hv_vcpu_tsc_relative_offset(vcpu, vtsc_offset + nested_offset);

## See Also

### Time-stamp counter functions

- [hv_tsc_clock()](hypervisor/hv_tsc_clock().md)