(I’m putting this up because I couldn’t find anything that described how to set this up and I had to connect different fragments of information, and file an invalid bug report before I figured it out.)

If you want to be able to debug a Linux kernel that’s running as a KVM guest, you need to specify the ‘-s’ parameter for the command line of qemu-kvm. The problem is, there’s no (easy) way to do this when you’re using libvirt and virt-manager to manager your virtual machines, instead of using KVM directly. What you need to do is change the XML configuration of the virtual machine so that the ‘-s’ parameter is passed on to qemu-kvm

$ virsh edit f14-test

Here, f14-test is the name of the VM that is managed via virt-manager. This will bring up the XML configuration of the VM in your editor. The first line of the XML file should be:

<domain type='kvm'>

This has to be changed to

<domain type='kvm' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>

and you also need to add:

<qemu:arg value='-s'/>

under the <domain> level of the XML. After you save and quit the editor, the new configuration will come into effect. When you start the virtual machine, there will be a local TCP port (1234 by default) that can be used as a remote debugging port from gdb. You can connect to this port by using the command

target remote localhost:1234

from gdb running on the host machine.