DragonFly On-Line Manual Pages
VKERNEL(7) DragonFly Miscellaneous Information Manual VKERNEL(7)
vkernel, vcd, vkd, vke - virtual kernel architecture
platform vkernel64 # for 64 bit vkernels
/var/vkernel/boot/kernel/kernel [-hstUvz] [-c file] [-e
name=value:name=value:...] [-i file] [-I
interface[:address1[:address2][/netmask][=mac]]] [-l cpulock] [-m size]
[-n numcpus[:lbits[:cbits]]] [-p pidfile] [-r file[:serno]] [-R
The vkernel architecture allows for running DragonFly kernels in
The following options are available:
-c file Specify a readonly CD-ROM image file to be used by the
kernel, with the first -c option defining vcd0, the second
one vcd1, and so on. The first -r, -R, or -c option
specified on the command line will be the boot disk. The
CD9660 filesystem is assumed when booting from this media.
Specify an environment to be used by the kernel. This
option can be specified more than once.
-h Shows a list of available options, each with a short
-i file Specify a memory image file to be used by the virtual
kernel. If no -i option is given, the kernel will
generate a name of the form /var/vkernel/memimg.XXXXXX,
with the trailing `Xs' being replaced by a sequential
number, e.g. memimg.000001.
Create a virtual network device, with the first -I option
defining vke0, the second one vke1, and so on.
The interface argument is the name of a tap(4) device node
or the path to a vknetd(8) socket. The /dev/ path prefix
does not have to be specified and will be automatically
prepended for a device node. Specifying auto will pick
the first unused tap(4) device.
The address1 and address2 arguments are the IP addresses
of the tap(4) and vke interfaces. Optionally, address1
may be of the form bridgeX in which case the tap(4)
interface is added to the specified bridge(4) interface.
The vke address is not assigned until the interface is
brought up in the guest.
The netmask argument applies to all interfaces for which
an address is specified.
The MAC argument is the MAC address of the vke(4)
interface. If not specified, a pseudo-random one will be
When running multiple vkernels it is often more convenient
to simply connect to a vknetd(8) socket and let vknetd
deal with the tap and/or bridge. An example of this would
-l cpulock Specify which, if any, real CPUs to lock virtual CPUs to.
cpulock is one of any, map[,startCPU], or CPU.
any does not map virtual CPUs to real CPUs. This is the
map[,startCPU] maps each virtual CPU to a real CPU
starting with real CPU 0 or startCPU if specified.
CPU locks all virtual CPUs to the real CPU specified by
Locking the vkernel to a set of cpus is recommended on
multi-socket systems to improve NUMA locality of
-m size Specify the amount of memory to be used by the kernel in
bytes, K (kilobytes), M (megabytes) or G (gigabytes).
Lowercase versions of K, M, and G are allowed.
numcpus specifies the number of CPUs you wish to emulate.
Up to 16 CPUs are supported with 2 being the default
unless otherwise specified.
lbits specifies the number of bits within APICID(=CPUID)
needed for representing the logical ID. Controls the
number of threads/core (0 bits - 1 thread, 1 bit - 2
threads). This parameter is optional (mandatory only if
cbits is specified).
cbits specifies the number of bits within APICID(=CPUID)
needed for representing the core ID. Controls the number
of core/package (0 bits - 1 core, 1 bit - 2 cores). This
parameter is optional.
-p pidfile Specify a pidfile in which to store the process ID.
Scripts can use this file to locate the vkernel pid for
the purpose of shutting down or killing it.
The vkernel will hold a lock on the pidfile while running.
Scripts may test for the lock to determine if the pidfile
is valid or stale so as to avoid accidentally killing a
random process. Something like '/usr/bin/lockf -ks -t 0
pidfile echo -n' may be used to test the lock. A non-zero
exit code indicates that the pidfile represents a running
An error is issued and the vkernel exits if this file
cannot be opened for writing or if it is already locked by
an active vkernel process.
Specify a R/W disk image file to be used by the kernel,
with the first -r option defining vkd0, the second one
vkd1, and so on. A serial number for the virtual disk can
be specified in serno.
The first -r or -c option specified on the command line
will be the boot disk.
Works like -r but treats the disk image as copy-on-write.
This allows a private copy of the image to be modified but
does not modify the image file. The image file will not
be locked in this situation and multiple vkernels can run
off the same image file if desired.
Since modifications are thrown away, any data you wish to
retain across invocations needs to be exported over the
network prior to shutdown. This gives you the flexibility
to mount the disk image either read-only or read-write
depending on what is convenient. However, keep in mind
that when mounting a COW image read-write, modifications
will eat system memory and swap space until the vkernel is
-s Boot into single-user mode.
-t Tell the vkernel to use a precise host timer when
calculating clock values. If the TSC isn't used, this
will impose higher overhead on the vkernel as it will have
to make a system call to the real host every time it wants
to get the time. However, the more precise timer might be
necessary for your application.
By default, the vkernel uses the TSC cpu timer if
possible, or an imprecise (host-tick-resolution) timer
which uses a user-mapped kernel page and does not have any
syscall overhead. To disable the TSC cpu timer, use the
-e hw.tsc_cputimer_enable=0 flag.
-U Enable writing to kernel memory and module loading. By
default, those are disabled for security reasons.
-v Turn on verbose booting.
-z Force the vkernel's ram to be pre-zerod. Useful for
benchmarking on single-socket systems where the memory
allocation does not have to be NUMA-friendly. This
options is not recommended on multi-socket systems or when
the -l option is used.
A number of virtual device drivers exist to supplement the virtual
The vkd driver allows for up to 16 vn(4) based disk devices. The root
device will be vkd0 (see EXAMPLES for further information on how to
prepare a root image).
The vcd driver allows for up to 16 virtual CD-ROM devices. Basically
this is a read only vkd device with a block size of 2048.
The vke driver supports up to 16 virtual network interfaces which are
associated with tap(4) devices on the host. For each vke device, the
per-interface read only sysctl(3) variable hw.vkeX.tap_unit holds the
unit number of the associated tap(4) device.
By default, half of the total mbuf clusters available is distributed
equally among all the vke devices up to 256. This can be overridden with
the tunable hw.vke.max_ringsize. Take into account the number passed
will be aligned to the lower power of two.
The virtual kernel only enables SIGQUIT and SIGTERM while operating in
regular console mode. Sending `^\' (SIGQUIT) to the virtual kernel
causes the virtual kernel to enter its internal ddb(4) debugger and re-
enable all other terminal signals. Sending SIGTERM to the virtual kernel
triggers a clean shutdown by passing a SIGUSR2 to the virtual kernel's
It is possible to directly gdb the virtual kernel's process. It is
recommended that you do a `handle SIGSEGV noprint' to ignore page faults
processed by the virtual kernel itself and `handle SIGUSR1 noprint' to
ignore signals used for simulating inter-processor interrupts.
/dev/vcdX vcd device nodes
/dev/vkdX vkd device nodes
vkernel configuration file, for config(8).
Your virtual kernel is a complete DragonFly system, but you might not
want to run all the services a normal kernel runs. Here is what a
typical virtual kernel's /etc/rc.conf file looks like, with some
additional possibilities commented out.
BOOT DRIVE SELECTION
You can override the default boot drive selection and filesystem using a
kernel environment variable. Note that the filesystem selected must be
compiled into the vkernel and not loaded as a module. You need to escape
some quotes around the variable data to avoid mis-interpretation of the
colon in the -e option. For example:
To boot a vkernel from a NFS root, a number of tunables need to be set:
IP address to be set in the vkernel interface.
Netmask for the IP to be set.
Network interface name inside the vkernel.
Host running nfsd(8).
Host path where a world and distribution targets are properly
See an example on how to boot a diskless vkernel in the EXAMPLES section.
A couple of steps are necessary in order to prepare the system to build
and run a virtual kernel.
Setting up the filesystem
The vkernel architecture needs a number of files which reside in
/var/vkernel. Since these files tend to get rather big and the /var
partition is usually of limited size, we recommend the directory to be
created in the /home partition with a link to it in /var:
mkdir -p /home/var.vkernel/boot
ln -s /home/var.vkernel /var/vkernel
Next, a filesystem image to be used by the virtual kernel has to be
created and populated (assuming world has been built previously). If the
image is created on a UFS filesystem you might want to pre-zero it. On a
HAMMER filesystem you should just truncate-extend to the image size as
HAMMER does not re-use data blocks already present in the file.
vnconfig -c -S 2g -T vn0 /var/vkernel/rootimg.01
disklabel -r -w vn0s0 auto
disklabel -e vn0s0 # add `a' partition with fstype `4.2BSD'
mount /dev/vn0s0a /mnt
make installworld DESTDIR=/mnt
make distribution DESTDIR=/mnt
echo '/dev/vkd0s0a / ufs rw 1 1' >/mnt/etc/fstab
echo 'proc /proc procfs rw 0 0' >>/mnt/etc/fstab
Edit /mnt/etc/ttys and replace the console entry with the following line
and turn off all other gettys.
console "/usr/libexec/getty Pc" cons25 on secure
Replace Pc with al.Pc if you would like to automatically log in as root.
Then, unmount the disk.
vnconfig -u vn0
Compiling the virtual kernel
In order to compile a virtual kernel use the VKERNEL64 kernel
configuration file residing in /sys/config (or a configuration file
make -DNO_MODULES buildkernel KERNCONF=VKERNEL64
make -DNO_MODULES installkernel KERNCONF=VKERNEL64 DESTDIR=/var/vkernel
Enabling virtual kernel operation
A special sysctl(8), vm.vkernel_enable, must be set to enable vkernel
Configuring the network on the host system
In order to access a network interface of the host system from the
vkernel, you must add the interface to a bridge(4) device which will then
be passed to the -I option:
ifconfig bridge0 create
ifconfig bridge0 addm re0 # assuming re0 is the host's interface
ifconfig bridge0 up
Running the kernel
Finally, the virtual kernel can be run:
./boot/kernel/kernel -m 1g -r rootimg.01 -I auto:bridge0
You can issue the reboot(8), halt(8), or shutdown(8) commands from inside
a virtual kernel. After doing a clean shutdown the reboot(8) command
will re-exec the virtual kernel binary while the other two will cause the
virtual kernel to exit.
Diskless operation (vkernel as a NFS client)
Booting a vkernel with a vknetd(8) network configuration. The line
continuation backslashes have been omitted. For convenience and to
reduce confusion I recommend mounting the server's remote vkernel root
onto the host running the vkernel binary using the same path as the NFS
mount. It is assumed that a full system install has been made to
/var/vkernel/root using a kernel KERNCONF=VKERNEL64 for the kernel build.
-m 1g -n 4 -I /var/run/vknet
In this example vknetd is assumed to have been started as shown below,
before running the vkernel, using an unbridged TAP configuration routed
through the host. IP forwarding must be turned on, and in this example
the server resides on a different network accessible to the host
executing the vkernel but not directly on the vkernel's subnet.
vknetd -t tap0 10.100.0.1/16
You can run multiple vkernels trivially with the same NFS root as long as
you assign each one a different IP on the subnet (2, 3, 4, etc). You
should also be careful with certain directories, particularly /var/run
and possibly also /var/db depending on what your vkernels are going to be
doing. This can complicate matters with /var/db/pkg.
BUILDING THE WORLD UNDER A VKERNEL
The virtual kernel platform does not have all the header files expected
by a world build, so the easiest thing to do right now is to specify a
pc64 (in a 64 bit vkernel) target when building the world under a virtual
kernel, like this:
vkernel# make MACHINE_PLATFORM=pc64 buildworld
vkernel# make MACHINE_PLATFORM=pc64 installworld
vknet(1), bridge(4), ifmedia(4), tap(4), vn(4), sysctl.conf(5), build(7),
config(8), disklabel(8), ifconfig(8), vknetd(8), vnconfig(8)
Aggelos Economopoulos, A Peek at the DragonFly Virtual Kernel, March
Virtual kernels were introduced in DragonFly 1.7.
Matt Dillon thought up and implemented the vkernel architecture and wrote
the vkd device driver. Sepherosa Ziehau wrote the vke device driver.
This manual page was written by Sascha Wildner.
DragonFly 6.3-DEVELOPMENT September 7, 2021 DragonFly 6.3-DEVELOPMENT