When a linux computer is "hung" or "frozen" you can use a Magic SysRq key sequence to send various low level requests to the kernel in order to try to recover from or investigate the problem. This is extremely useful when troubleshooting server lockups, but until recently libvirt did not expose this functionality for KVM guests.
In v0.9.3 (and newer) of libvirt you can send a Magic SysRq sequence to a guest by utilizing the
send-keysubcommand provided by
virsh. In other words, sending the 'h' Magic SysRq command is as simple as:
dustymabe@media: ~>virsh send-key guest1 KEY_LEFTALT KEY_SYSRQ KEY_H
After executing the command a help message will be printed to the console of the guest known as 'guest1'. Any other character can be sent as well by substituting KEY_H with KEY_X, where X is the character.