Yesterday, I had to add support for running KVM virtual machines inside an LXC container. More as a reminder to myself, in case I ever have to do this again, here the simple recipe:
LXC Container Config Adjustment
Enable lxc.autodev and execute hook script to be executed after initial /dev creation (updated 20260428: lxc.cgroup2.* instead of lxc.cgroup.*):
[...]
# Auto-create /dev nodes and add native KVM support to the LXC container
lxc.autodev = 1
lxc.hook.autodev = /var/lib/lxc/.hooks/lxc-hook.kvm-support
lxc.cgroup2.devices.allow = c 10:232 rwm
lxc.cgroup2.devices.allow = c 10:238 rwm
lxc.cgroup2.devices.allow = c 10:241 rwm
[...]
[added 20260408] On the internet, you can find a recipe that simply bind-mounts /dev/kvm from the host in to the LXC container. However, this fails if group ID of POSIX group kvm differs between host and container.
LXC Hook Script for KVM Support Enablement
The following script I placed at /var/lib/lxc/.hooks/lxc-hook.kvm-support (on the LXC host!):
#!/bin/sh
# set up native KVM support in LXC container
mknod -m 0660 ${LXC_ROOTFS_MOUNT}/dev/kvm c 10 232
chown :kvm ${LXC_ROOTFS_MOUNT}/dev/kvm
mknod -m 0660 ${LXC_ROOTFS_MOUNT}/dev/vhost-net c 10 238
chown :kvm ${LXC_ROOTFS_MOUNT}/dev/vhost-net
mknod -m 0660 ${LXC_ROOTFS_MOUNT}/dev/vhost-vsock c 10 241
chown :kvm ${LXC_ROOTFS_MOUNT}/dev/vhost-vsock
