1 # The DragonFly virtual kernels
4 ***Obtained from [vkernel(7)](http://leaf.dragonflybsd.org/cgi/web-man?command=vkernel§ion=7) written by Sascha Wildner, added by Matthias Schmidt***
6 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.
8 The vkernel architecture allows for running DragonFly kernels in userland.
12 A number of virtual device drivers exist to supplement the virtual kernel.
14 <!-- XXX: why do they only support 16 devices? is this really true? -->
18 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`.
22 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.
26 The vke driver supports up to 16 virtual network interfaces which are
28 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.
30 ## Setup a virtual kernel environment
32 A couple of steps are necessary in order to prepare the system to build and run a virtual kernel.
34 ### Setting up the filesystem
36 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`:
40 % mkdir /home/var.vkernel
41 % ln -s /home/var.vkernel /var/vkernel
43 Next, a filesystem image to be used by the virtual kernel has to be created and populated (assuming world has been built previously):
45 # dd if=/dev/zero of=/var/vkernel/rootimg.01 bs=1m count=2048
46 # vnconfig -c vn0 /var/vkernel/rootimg.01
47 # disklabel -r -w vn0s0 auto
48 # disklabel -e vn0s0 # add 'a' partition with fstype `4.2BSD' size could be '*'
50 # mount /dev/vn0s0a /mnt
52 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.
54 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).
57 # make installworld DESTDIR=/mnt
59 # make distribution DESTDIR=/mnt
61 Create a fstab file to let the vkernel find your image file.
65 # echo '/dev/vkd0s0a / ufs rw 1 1' >/mnt/etc/fstab
66 # echo 'proc /proc procfs rw 0 0' >>/mnt/etc/fstab
68 Edit `/mnt/etc/ttys` and replace the console entry with the following line and turn off all other gettys.
70 # console "/usr/libexec/getty Pc" cons25 on secure
72 Then, unmount the disk.
77 ### Compiling the virtual kernel
79 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):
83 # make -DNO_MODULES buildkernel KERNCONF=VKERNEL
84 # make -DNO_MODULES installkernel KERNCONF=VKERNEL DESTDIR=/var/vkernel
86 ### Enabling virtual kernel operation
88 A special sysctl(8), `vm.vkernel_enable`, must be set to enable vkernel operation:
90 # sysctl vm.vkernel_enable=1
92 To make this change permanent, edit `/etc/sysctl.conf`
96 ### Configuring the network on the host system
98 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:
102 # kldload if_bridge.ko
104 # ifconfig bridge0 create
105 # ifconfig bridge0 addm re0 # assuming re0 is the host's interface
106 # ifconfig bridge0 up
108 **Note** : You have to change `re0` to the interface of your host machine.
113 ## Run a virtual kernel
115 Finally, the virtual kernel can be run:
118 # ./boot/kernel/kernel -m 64m -r /var/vkernel/rootimg.01 -I auto:bridge0
120 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.