The UML management console is a low-level interface to the kernel, somewhat like the i386 SysRq interface. Since there is a full-blown operating system under UML, there is much greater flexibility possible than with the SysRq mechanism.
At this point, there are three things you can do with the mconsole interface:
You need the mconsole client (uml_mconsole) which is present in CVS (/tools/mconsole) in 2.4.5-9um and later, and will be in the RPM in 2.4.6.
You also need CONFIG_MCONSOLE (under 'General Setup') enabled in UML. When you boot UML, you'll see a line like:
mconsole initialized on /home/jdike/.uml/umlNJ32yL/mconsole
umid=debian
mconsole initialized on /home/jdike/.uml/debian/mconsole
uml_mconsole debian
uml_mconsole /home/jdike/.uml/debian/mconsole
You'll get a prompt, at which you can run one of these commands:
This takes no arguments. It prints the UML version.
(mconsole) version
OK Linux usermode 2.4.5-9um #1 Wed Jun 20 22:47:08 EDT 2001 i686
These take no arguments. They shut the machine down immediately, with no syncing of disks and no clean shutdown of userspace. So, they are pretty close to crashing the machine.
(mconsole) halt
OK
"config" adds a new device to the virtual machine. Currently the ubd and network drivers support this. It takes one argument, which is the device to add, with the same syntax as the kernel command line.
(mconsole) config ubd3=/home/jdike/incoming/roots/root_fs_debian22
OK
(mconsole) config eth1=mcast
OK
"remove" deletes a device from the system. Its argument is just the name of the device to be removed. The device must be idle in whatever sense the driver considers necessary. In the case of the ubd driver, the removed block device must not be mounted, swapped on, or otherwise open, and in the case of the network driver, the device must be down.
(mconsole) remove ubd3
OK
(mconsole) remove eth1
OK
This takes one argument, which is a single letter. It calls the generic kernel's SysRq driver, which does whatever is called for by that argument. See the SysRq documentation in Documentation/sysrq.txt in your favorite kernel tree to see what letters are valid and what they do.
"help" returns a string listing the valid commands and what each one does.
This invokes the Ctl-Alt-Del action on init. What exactly this ends up doing is up to /etc/inittab. Normally, it reboots the machine. With UML, this is usually not desired, so if a halt would be better, then find the section of inittab that looks like this
# What to do when CTRL-ALT-DEL is pressed.
ca:12345:ctrlaltdel:/sbin/shutdown -t1 -a -r now