QEMU/Linux guest

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This article describes the setup of a Gentoo Linux guest in QEMU using Gentoo bootable media.

Installation

Kernel

If you use genkernel do not build the VirtIO drivers as modules, compile them into the kernel.

KERNEL
Processor type and features  --->
    [*] Linux guest support --->
        [*] Enable Paravirtualization code
        [*] KVM Guest support (including kvmclock)
Device Drivers  --->
    [*] Virtio drivers  --->
        <*> PCI driver for virtio devices
    [*] Block devices  --->
        <*> Virtio block driver
    SCSI device support  --->
        [*] SCSI low-level drivers  --->
            [*] virtio-scsi support
    [*] Network device support  --->
        [*] Network core driver support
            <*> Virtio network driver
    Graphics support  --->
        <*> Virtio GPU driver
    Character devices ---> 
       <*>   Hardware Random Number Generator Core support --->
           <*>   VirtIO Random Number Generator support

As an alternative, use these commands after emerging the kernel sources:

(chroot) livecd /usr/src/linux #make defconfig
(chroot) livecd /usr/src/linux #make kvm_guest.config

Additional software

Guest Linux OS requires sys-power/acpid for proper shutdown handling by libvirt.


Configuration

Files

GRUB

For a minimal grub BIOS install:

(chroot) livecd / #echo 'GRUB_PLATFORMS="pc"' >> /etc/portage/make.conf
(chroot) livecd / #echo 'sys-boot/grub -fonts -nls -themes' > /etc/portage/package.use/grub
(chroot) livecd / #emerge --ask sys-boot/grub:2

Optional: to make the guest work in the headless mode, add these lines:

FILE /etc/default/grub
GRUB_CMDLINE_LINUX="console=tty0 console=ttyS0"
GRUB_TERMINAL=console

and uncomment the following:

FILE /etc/inittab
# SERIAL CONSOLES
s0:12345:respawn:/sbin/agetty -L 115200 ttyS0 vt100

Install grub on the guest disk:

(chroot) livecd / #grub-install /dev/vda
Installing for i386-pc platform.
Installation finished. No error reported.

Configure grub for the kernel build earlier:

(chroot) livecd / #grub-mkconfig -o /boot/grub/grub.cfg
Generating grub.cfg ...
Found linux image: /boot/vmlinuz-4.9.16-gentoo
done

Host

To create a disk image for the virtual machine, run:

user $qemu-img create -f qcow2 Gentoo-VM.img 15G

Download a minimal Gentoo LiveCD from here.

Since QEMU requires a lot of options, it would be a good idea to put them into a shell script, e.g.:

FILE start_Gentoo_VM.sh
#!/bin/bash
exec qemu-system-x86_64 -enable-kvm \
        -cpu host \
        -drive file=Gentoo-VM.img,if=virtio \
        -netdev user,id=vmnic,hostname=Gentoo-VM \
        -device virtio-net,netdev=vmnic \
        -device virtio-rng-pci \
        -m 512M \
        -smp 2 \
        -monitor stdio \
        -name "Gentoo VM" \
        $@

Change the path to your disk image Gentoo-VM.img in the script. You can add more options when calling the script. To boot the disk image, run:

user $./start_Gentoo_VM.sh -boot d -cdrom install-amd64-minimal-20120621.iso

Install the guest per the Gentoo Handbook. See the guest section for optimum support. After the installation start the script without the additional options.

Headless server

If running on a headless server, you will need to tweak the settings a bit

FILE start_Gentoo_VM.sh
#!/bin/bash
exec qemu-system-x86_64 -enable-kvm \
        -cpu host \
        -drive file=Gentoo-VM.img,if=virtio \
        -netdev user,id=vmnic,hostname=Gentoo-VM \
        -device virtio-net,netdev=vmnic \
        -device virtio-rng-pci \
        -m 512M \
        -smp 2 \
        -nographic \
        -name "Gentoo VM" \
        $@

and when prompted at boot time to select the kernel, you should input

FILE start_Gentoo_VM.sh
boot: gentoo console=ttyS0

Guest

Hard drive

The VirtIO hard drive is mapped to /dev/vda. Where the handbook refers to /dev/sdaX, always use /dev/vdaX when configuring the guest.


Services

The main CLI interface to QEMU is virsh.

Defining a domain service

See virt-manager QEMU guest (or Libvirt/QEMU_guest) for (un)defining a domain.

Starting a domain service

root #virsh start my_vm_domain_name

Stopping a domain service

root #virsh destroy my_vm_domain_name

This virsh destroy command is like pulling the power-cord on the computer's OS: very abrupt and curt.

Suspending a domain service

root #virsh shutdown my_vm_domain_name

Tweaking VM settings

If you have made a change to the XML configuration file, KVM needs to reload before restarting its VM:

root #virsh define /etc/libvirt/qemu/my_vm_domain_name.xml
Domain my_vm_domain_name defined from /etc/libvirt/qemu/my_vm_domain_name.xml

Then restart the VM:

root #virsh start my_vm_domain_name
Domain my_vm_domain_name started

One Gentoo user provides a bash script that is a front-end to the qemu-system-(arch)to directly which consolidates all the virsh subcommands which conveniently configure, start and stop a Linux (or any other) guest; check out this QEMU init script.

Advanced

Expose images to LAN

Sometimes it is required that the image should get a proper IP address on the LAN network to allow other peers to access it.
Such a configuration is possible by using an existing network bridge and telling the machine to use it.

Assuming that there exists a bridge called br0 on the machine, the following configuration exposes the image to the LAN.

FILE start_Gentoo_VM.sh
#!/bin/bash
exec qemu-system-x86_64 -enable-kvm \
        -cpu host \
        -drive file=Gentoo-VM.img,if=virtio \
        -netdev bridge,id=net0,br=br0 \
        -device virtio-net-pci,netdev=net0 \
        -device virtio-rng-pci \
        -m 512M \
        -smp 2 \
        -nographic \
        -name "Gentoo VM" \
        $@
root #./start_Gentoo_VM.sh -boot d -cdrom install-amd64-minimal-20120621.iso

Optional post install guest IPv6 setup

For IPv6 networking see the IPv6 subarticle.

Mount guest image

To access the guest disk from the host (and e.g. chroot into the guest), use a "Network Block Device":

root # modprobe nbd max_part=16
root # qemu-nbd -c /dev/nbd0 Gentoo-VM.img
root # mount /dev/nbd0p4 /mnt/gentoo

Make any changes required and clean up:

root # umount /mnt/gentoo
root # qemu-nbd -d /dev/nbd0

Troubleshooting

Boot hangs at syslog-ng

If the guest boots slow, or if the boot hangs on * Checking your configfile (/etc/syslog-ng/syslog-ng.conf) or there are syslog messages like [ 1.264763] random: dbus-deamon: uninitialized urandom read (12 bytes read) or [ 12.667558] random: crng init done (12 seconds after booting), this is likely due to the lack of entropy. A way to fix this is to enable the "VirtIO Random Number Generator support" (HW_RANDOM_VIRTIO=y) in the guest kernel and boot with the QEMU virtio-rng-pci device.

Another way to solve this is to enable "Trust the CPU manufacturer to initialize Linux's CRNG" (RANDOM_TRUST_CPU=y) in the guest kernel. However, there are security concerns with this approach.

VM shutdown problems

Host control scripts may send a system_powerdown message to the virtual machine in order to shut it down. For this to work properly, ACPI functionality on the guest is necessary. Also, ACPI daemon sys-power/acpid should be installed and running on the guest.

See also

  • Virtualization — the concept and technique that permits running software in an environment separate from a computer operating system.
  • QEMU — a generic, open source hardware emulator and virtualization suite.
  • QEMU/QEMU front-ends — user interface application to the QEMU/libvirt API/library.