---
name: "kvmclock"
section: "4"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["freebsd"]
url: "https://new.carta.tech/kvmclock"
---

# kvmclock(4)

Para-virtualized clock driver for x86 KVM guests

## NAME

**kvmclock** – Para-virtualized clock driver for x86 KVM guests

## SYNOPSIS

To compile this driver into the kernel, place the following line in your kernel configuration file:

**device kvm\_clock**

## DESCRIPTION

This driver reads time-keeping information from the para-virtualized clock device provided by the KVM hypervisor on Linux hosts. The **kvmclock** driver is only implemented on i386 and amd64 platforms. It acts as a [timecounters(4)](/timecounters/4.md) device and is preferred over the Time Stamp Counter (TSC) when available. The driver exports timekeeping information via */dev/pvclock*, enabling the implementation of [clock\_gettime(2)](/clock_gettime/2.md) and related functions without entering the kernel.

The **kvmclock** driver works by accessing a per-vCPU timekeeping structure maintained by the hypervisor. It uses a combination of TSC readings and information from the shared structure to produce a high-resolution timecounter which is invariant under hypervisor events such as vCPU migration and live VM migration.

## SYSCTL VARIABLES

The following variables are available as both [sysctl(8)](/sysctl/8.md) variables and [loader(8)](/loader/8.md) tunables:

- *dev.kvmclock.0.vdso\_enable\_without\_rdtscp*

  By default, timekeeping information is exported to userspace only when the (virtual) CPU announces support for the “rdtscp” instruction. Setting this sysctl to 1 overrides this behavior, allowing timekeeping information to be exported even in the absence of “rdtscp” support. However, this breaks compatibility with copies of */lib/libc.so.7* released prior to FreeBSD 14.0, and statically linked binaries which embed a copy of the system C library. Thus, this sysctl value should not be changed if the system may execute binaries older than FreeBSD 14.0.

- *dev.kvmclock.0.vdso\_force\_unstable*

  Mark the timecounter as unstable for userspace consumers. This is mostly useful for debugging the driver and userspace timekeeping code, and generally should not be touched.

## SEE ALSO

[timecounters(4)](/timecounters/4.md)

## HISTORY

The **kvmclock** driver first appeared in FreeBSD 13.1.