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KVMCLOCK(4)		     Kernel Interfaces Manual		     KVMCLOCK(4)

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 timecoun-
     ters(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 with-
     out 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  time-
     counter  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)  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"	instruc-
	     tion.   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)

HISTORY
     The kvmclock driver first appeared in FreeBSD 13.1.

FreeBSD 16.0 CURRENT		February 1, 2023		     KVMCLOCK(4)
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