KVMCLOCK(4)

📦 version 15.1, in freebsd 14.5, freebsd 15.1, nixpkgs unstable

🎀 KVMCLOCK(4) 🎀

Okay bestie, welcome to the Kernel Interfaces Manual page for KVMCLOCK(4) — the para-virtualized clock driver for x86 KVM guests. It's giving very demure kernel tea, no cap ☕💅

🌸 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) 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. Highkey iconic ✨

🎛️ SYSCTL VARIABLES 🎛️

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, bestie, heads up: 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 👀

🦋 HISTORY 🦋

The kvmclock driver first appeared in FreeBSD 13.1. ⭐

That's the whole tea, bestie 🫶 This page comes from Debian, dated February 1, 2023, and it's the KVMCLOCK(4) manual page. Go touch some grass now 🌱💅

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