Quick and simple VMs with qemu

I don’t use VMs very often, so there’s no chance I can remember all the dozens of command line flags for qemu.

I end up using virt-manager most of the time. It’s a GUI for managing QEMU (and other VM backends) and has dozens of checkboxes and buttons, which come in handy for really complex virtual hardware configurations where I’d need to know a dozen obscure command line flags to replicate the same results. virt-manager is also very handy for VMs with a graphical interface (e.g.: to test something on Fedora or Ubuntu), but not so much when the only need is a simple console.

After taking less than half an hour of reading, it turns out qemu is far easier than I suspected. Should have done this years ago.

Basic VM

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The most basic VM simply starts a console with the alpine ISO:

qemu-system-x86_64 -nographic -drive file=alpine-virt-3.15.4-x86_64.iso,format=raw

By default qemu shows output in a separate window (this is the VM’s “monitor). This is rather annoying because it hijacks mouse, and doesn’t have the terminal’s regular copy-paste support. -nographic prevents qemu from showing a separate window for graphic output and uses the current terminal console instead (it simply connects the current terminal to the VM’s serial port).

CPU and Memory

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By default, the VM has a single CPU core and 128MB of RAM. This can be changed very easily:

qemu-system-x86_64 -m 1G -smp cores=2 -drive file=alpine-virt-3.15.4-x86_64.iso,format=raw

Network

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The internet is a kinda useful thing, and it’s nice to be able to use it inside the VM. There’s the -net and -netdev flags, but those are really complicated – mostly useful for more advanced configurations. For the most basic setup (userspace NAT), -nic is the “new” flag which makes things dead simple¹:

qemu-system-x86_64 -nic user -nographic -drive file=alpine-virt-3.15.4-x86_64.iso,format=raw

After booting, network itself needs to be configured in the guest. For alpine, use:

ip link set eth0 up
udhcpc eth0

Putting it all together

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The following example defines CPU, memory and network:

qemu-system-x86_64 \
    -nic user \
    -nographic \
    -m 1G \
    -smp cores=2 \
    -drive file=alpine-virt-3.15.4-x86_64.iso,format=raw

Architecture

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QEMU supports A LOT of architectures. For ARM64, use qemu-system-aarch64. Make sure the ISO image’s architecture matches the VM’s architecture.