Manual Pages  — KVMCLOCK

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) 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) 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.

The following variables are available as both sysctl(8) variables and loader(8) 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.

timecounters(4)