1.\" SPDX-License-Identifier: BSD-2-Clause 2.\" 3.\" Copyright (c) 2023 Klara, Inc. 4.\" 5.\" Redistribution and use in source and binary forms, with or without 6.\" modification, are permitted provided that the following conditions 7.\" are met: 8.\" 1. Redistributions of source code must retain the above copyright 9.\" notice, this list of conditions and the following disclaimer. 10.\" 2. Redistributions in binary form must reproduce the above copyright 11.\" notice, this list of conditions and the following disclaimer in the 12.\" documentation and/or other materials provided with the distribution. 13.\" 14.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 15.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 16.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 17.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 18.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 19.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 20.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 21.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 22.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 23.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 24.\" SUCH DAMAGE. 25.\" 26.Dd February 1, 2023 27.Dt KVMCLOCK 4 28.Os 29.Sh NAME 30.Nm kvmclock 31.Nd Para-virtualized clock driver for x86 KVM guests 32.Sh SYNOPSIS 33To compile this driver into the kernel, 34place the following line in your 35kernel configuration file: 36.Bd -ragged -offset indent 37.Cd "device kvm_clock" 38.Ed 39.Sh DESCRIPTION 40This driver reads time-keeping information from the para-virtualized clock 41device provided by the KVM hypervisor on Linux hosts. 42The 43.Nm 44driver is only implemented on i386 and amd64 platforms. 45It acts as a 46.Xr timecounters 4 47device and is preferred over the Time Stamp Counter (TSC) when available. 48The driver exports timekeeping information via 49.Pa /dev/pvclock , 50enabling the implementation of 51.Xr clock_gettime 2 52and related functions without entering the kernel. 53.Pp 54The 55.Nm 56driver works by accessing a per-vCPU timekeeping structure maintained by the 57hypervisor. 58It uses a combination of TSC readings and information from the shared structure 59to produce a high-resolution timecounter which is invariant under hypervisor 60events such as vCPU migration and live VM migration. 61.Sh SYSCTL VARIABLES 62The following variables are available as both 63.Xr sysctl 8 64variables and 65.Xr loader 8 66tunables: 67.Bl -tag -width indent 68.It Va dev.kvmclock.0.vdso_enable_without_rdtscp 69By default, timekeeping information is exported to userspace only when the 70(virtual) CPU announces support for the 71.Dq rdtscp 72instruction. 73Setting this sysctl to 1 overrides this behavior, allowing timekeeping 74information to be exported even in the absence of 75.Dq rdtscp 76support. 77However, this breaks compatibility with copies of 78.Pa /lib/libc.so.7 79released prior to 80.Fx 14.0 , 81and statically linked binaries which embed a copy of the system C library. 82Thus, this sysctl value should not be changed if the system may execute 83binaries older than 84.Fx 14.0 . 85.It Va dev.kvmclock.0.vdso_force_unstable 86Mark the timecounter as unstable for userspace consumers. 87This is mostly useful for debugging the driver and userspace timekeeping code, 88and generally should not be touched. 89.El 90.Sh SEE ALSO 91.Xr timecounters 4 92.Sh HISTORY 93The 94.Nm 95driver first appeared in 96.Fx 13.1 . 97