4 # The DragonFly virtual kernels
8 ***Obtained from [vkernel(7)](http://leaf.dragonflybsd.org/cgi/web-man?command=vkernel§ion=7) written by Sascha Wildner, added by Matthias Schmidt***
11 The idea behind the development of the vkernel architecture was to find an elegant solution to debugging of the kernel and its components. It eases debugging, as it allows for a virtual kernel being loaded in userland and hence debug it without affecting the real kernel itself. By being able to load it on a running system it also removes the need for reboots between kernel compiles.
13 The vkernel architecture allows for running DragonFly kernels in userland.
19 A number of virtual device drivers exist to supplement the virtual kernel.
21 <!-- XXX: why do they only support 16 devices? is this really true? -->
25 The vkd driver allows for up to 16 [vn(4)](http://leaf.dragonflybsd.org/cgi/web-man?command=vn§ion=4) based disk devices. The root device will be `vkd0`.
29 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.
33 The vke driver supports up to 16 virtual network interfaces which are
35 associated with [tap(4)](http://leaf.dragonflybsd.org/cgi/web-man?command=tap§ion=4) devices on the host. For each `vke` device, the per-interface read only [sysctl(3)](http://leaf.dragonflybsd.org/cgi/web-man?command=sysctl§ion=3) variable `hw.vkeX.tap_unit` holds the unit number of the associated [tap(4)](http://leaf.dragonflybsd.org/cgi/web-man?command=tap§ion=4) device.
42 ## Setup a virtual kernel environment
46 A couple of steps are necessary in order to prepare the system to build and run a virtual kernel.
48 ### Setting up the filesystem
50 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`:
54 % mkdir /home/var.vkernel
55 % ln -s /home/var.vkernel /var/vkernel
57 Next, a filesystem image to be used by the virtual kernel has to be created and populated (assuming world has been built previously):
59 # dd if=/dev/zero of=/var/vkernel/rootimg.01 bs=1m count=2048
60 # vnconfig -c vn0 /var/vkernel/rootimg.01
61 # disklabel -r -w vn0s0 auto
62 # disklabel -e vn0s0 # add 'a' partition with fstype `4.2BSD' size could be '*'
64 # mount /dev/vn0s0a /mnt
66 If instead of using `vn0` you specify `vn` to `vnconfig`, a new `vn` device will be created and a message saying which `vnX` was created will appear. This effectively lifts the limit of 4 vn devices.
68 Assuming that you build your world before, you can populate the image now. If you didn't build your world see [chapter 21](../updating-makeworld.html).
71 # make installworld DESTDIR=/mnt
73 # make distribution DESTDIR=/mnt
76 Create a fstab file to let the vkernel find your image file.
80 # echo '/dev/vkd0s0a / ufs rw 1 1' >/mnt/etc/fstab
81 # echo 'proc /proc procfs rw 0 0' >>/mnt/etc/fstab
84 Edit `/mnt/etc/ttys` and replace the console entry with the following line and turn off all other gettys.
86 # console "/usr/libexec/getty Pc" cons25 on secure
89 Then, unmount the disk.
96 ### Compiling the virtual kernel
98 In order to compile a virtual kernel use the VKERNEL kernel configuration file residing in `/usr/src/sys/config` (or a configuration file derived thereof):
102 # make -DNO_MODULES buildkernel KERNCONF=VKERNEL
103 # make -DNO_MODULES installkernel KERNCONF=VKERNEL DESTDIR=/var/vkernel
107 ### Enabling virtual kernel operation
109 A special sysctl(8), `vm.vkernel_enable`, must be set to enable vkernel operation:
111 # sysctl vm.vkernel_enable=1
114 To make this change permanent, edit `/etc/sysctl.conf`
123 ### Configuring the network on the host system
125 In order to access a network interface of the host system from the vkernel, you must add the interface to a [bridge(4)](http://leaf.dragonflybsd.org/cgi/web-man?command=bridge§ion=4) device which will then be passed to the `-I` option:
129 # kldload if_bridge.ko
131 # ifconfig bridge0 create
132 # ifconfig bridge0 addm re0 # assuming re0 is the host's interface
133 # ifconfig bridge0 up
137 **Note** : You have to change `re0` to the interface of your host machine.
142 ## Run a virtual kernel
146 Finally, the virtual kernel can be run:
149 # ./boot/kernel -m 64m -r /var/vkernel/rootimg.01 -I auto:bridge0
151 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.