kvm guest without libvirt

requirements

    which kvm
    man kvm

    mkdir -p /etc/qemu/
    vi /etc/qemu/bridge.conf

    allow guestbr0

disk skeleton

    guest=bookworm1

mkdir -p /data/guests/$guest/
cd /data/guests/$guest/

–either– skeleton sparse-file

    dd if=/dev/zero of=$guest.ext4 bs=1G count=0 seek=10

    mkfs.ext4 $guest.ext4

    mkdir lala/ 
    mount -o loop $guest lala/

bootstrap

    # use same mirror as from host system
    cat /etc/apt/sources.list
    mirror=`cat /etc/apt/sources.list | awk '{print $2}'`

    apt install lsb-release debootstrap

    # use same release as host system
    lsb_release -cs
    time debootstrap --arch=amd64 `lsb_release -cs 2>/dev/null` lala/ $mirror

post-bootstrap

    cat > lala/etc/fstab <<-EOF # override
    /dev/vda1 / ext4 defaults 0 0
    proc /proc proc defaults 0 0
    tmpfs /tmp tmpfs rw,nodev,nosuid,noatime,relatime 0 0
    devpts /dev/pts devpts gid=5,mode=620 0 0
    EOF

setup the network

vi lala/etc/network/interfaces

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
    address x.x.x.x/xx
    gateway x.x.x.x

unlock the root account

chroot lala/

passwd root
^D

is host kernel suitable for serial output?

    ls -lhF /boot/config*
    grep CONFIG_VIRTIO_CONSOLE /boot/config-6.1.0-32-cloud-amd64
    grep -i virtio /boot/config-6.1.0-32-cloud-amd64

get guest kernel modules ready

mkdir lala/lib/modules/
cp -R /lib/modules/6.1.0-32-cloud-amd64/ lala/lib/modules/

fortunately here we’re using same kernel on both sides – anyhow that would be executed at boot-time

chroot lala/ depmod -a

and you’re good to go

umount -R lala/
rmdir lala/

    kvm -nic model=help

    screen -S consoles

    vdisk=/root/guest1.ext4
    vdisk=/root/guest2.ext4
    vdisk=/root/guest3.ext4

    bridge=guestbr0

    # we are lucking we're doing a poc using nested virtualization here...
    ls -lhF /boot/vmlinuz*
    ls -lhF /boot/initrd*
    kernel=/boot/vmlinuz-6.1.0-32-cloud-amd64
    initrd=/boot/initrd.img-6.1.0-32-cloud-amd64

    # beware the path here needs to be /dev/vda not /dev/vda1
    kvm --enable-kvm -m 256 \
            -display curses -serial pty \
            -drive file=$vdisk,media=disk,if=virtio,format=raw \
            -kernel $kernel -initrd $initrd -append "ro root=/dev/vda net.ifnames=0 biosdevname=0 mitigations=off" \
            -nic bridge,br=guestbr0,model=virtio-net-pci

    # init=/bin/bash
    # ,mac=xx:xx:xx:xx:xx:xx
            # -overcommit mem-lock=off \
            # -display none -nographic -serial pty \
            # -device virtio-serial-pci,id=serial0 \