spin-machine

A virtual machine: QEMU, the guest kernel, the base image, and the definition of the machine they make.

The four are one thing, and the reason is not tidiness. A VM resumed from a checkpoint loads device and CPU state into a machine that has to be the same shape as the one the checkpoint was saved on, and nothing checks that at run time. So the machine's identity is computed from the things that decide its shape — machine.Spec.Fingerprint hashes the QEMU binary, the kernel, the initrd and the firmware the guest runs (the BIOS, qboot.bin and pvh.bin) by content, together with the five arguments that decide what a guest sees. Two machines with the same fingerprint can take each other's checkpoints. Two with different fingerprints cannot, and a release in which any of those files moved has a different fingerprint by construction.

One repository, one version, one machine a checkpoint resumes onto.

What is deliberately not in a release: software that owns a guest. This repository builds a machine, and a release is not bootable on its own by design — whoever runs guests brings the initrd. Tests and performance probes may use caller-supplied diagnostic initrds or disposable guest helpers, but those are not release artifacts and must stay isolated from the published machine.

What it publishes

One tarball:

path under usr/share/spin-stack/
bin/qemu-system-x86_64	static; KVM only — it refuses to emulate, on purpose
bin/qemu-system-x86_64-tcg	for CI, which has no /dev/kvm
bin/qemu-img
bin/mkfs.ext4	static e2fsprogs; read e2fsprogs/mke2fs.conf through MKE2FS_CONFIG
bin/debugfs, bin/e2fsck, bin/dumpe2fs	the same e2fsprogs build; the boot/ probes edit a copy of the base image with this debugfs
e2fsprogs/mke2fs.conf	the defaults an ext4 made for this kernel is made with
qemu/{qboot.bin,bios.bin,bios-256k.bin,pvh.bin,kvmvapic.bin,efi-virtio.rom}	qboot.bin is the BIOS a machine boots with
qemu/patches/, qemu/qboot-*.patch, qemu/qboot-COPYING	the changes this tree makes to QEMU and qboot, shipped with the binaries built from them
kernel/vmlinux	plus kernel-config
image/rootfs.qcow2	read-only, 0444
machine.env	the version, the commit it was built from, and the checksums that decide whether a checkpoint resumes
SOURCES	every upstream source by version, URL and SHA-256, and the written offer
packages.txt	every package and exact version in the base image

LICENSE and NOTICE sit at the root of the tarball.

task build writes that same tree into _output/, and machine.OpenRelease reads either. What a release has beyond a build directory is what hack/release creates or copies when it names one: machine.env and SOURCES it writes, and the QEMU and qboot patches under qemu/ it takes from this repository, not from the build. A tree without a manifest is still a release for every purpose except being named — the fingerprint decides whether checkpoints resume, and it is computed from the files, not from machine.env. _output/bin/ holds one thing more: spin-machine, which task tools builds for working in this repository and hack/release does not ship — a release is the machine, not the tool that boots it. There is one layout: nothing rearranges the files on the way out of a build, into a tarball or into a consumer. Let the three differ and what falls out of the translation between them is a path that exists and holds the previous release's kernel.

Using it

rel, err := machine.OpenRelease("/usr/share/spin-stack")  // says which file is missing, if one is
spec := rel.Spec()                                        // QEMU, Kernel, Firmware
// … the caller's initrd, memory, CPUs, disks, monitors
args, err := spec.Args()          // the QEMU command line
fp, err := spec.Fingerprint()     // which checkpoints this machine may resume

By hand, spin-machine (task tools) boots one, prints its command line or its fingerprint, attaches, detaches or saves on a running one, and restores a saved one; spin-machine <command> -h lists each command's flags.

To run a command inside a throwaway guest and read what it printed, boot it non-interactively. The guest's console is on stdin/stdout, so a script on stdin is what the guest's shell runs; poweroff -f ends the boot.

task build                                   # once; a boot reads _output/
printf '\n\n\n\n\n\n\n\n\n\nmount -t proc proc /proc\necho hello from $(uname -r)\npoweroff -f\n' |
  _output/bin/spin-machine boot --release _output \
    --init /bin/sh --console stdio --memory 1024 --cpus 2

--init /bin/sh skips systemd and runs the commands as PID 1 with nothing mounted, so the script mounts /proc itself — poweroff refuses without it. The leading blank lines are load-bearing: the guest's shell is not reading its console when QEMU starts feeding it, so the first bytes of the first line are lost, and the blanks absorb that instead of the first real command. How far that window reaches is host timing, not a line count — a quiet host may lose nothing — so treat the number of blanks as insurance, not a figure to trim: a blank costs one prompt. Nothing touches the base image — without --disk, QEMU boots it under a throwaway overlay (-snapshot).

Read the output by grepping a marker, since the guest's prompt and the kernel's boot chatter share stdout: | grep -a MARKER. The exit status is QEMU's, so a script that ends in poweroff -f returns 0.

What the guest answers is a property of the machine, not of the host that ran it. uname -r reports kernel/vmlinux's version, not the host kernel's. grep -c -E 'vmx|svm' /proc/cpuinfo is 0 because the machine removes both from the CPU it shows (-cpu host,migratable=on,-vmx,-svm: nothing inside a guest may nest, and exposing the flags would hand a guest's root the host's nested-virtualisation code), and there is no /dev/kvm to open because this kernel is built without KVM and without loadable modules. So a check run inside a guest can prove things about the guest and can never establish anything about its host's KVM — read those on the host.

Building

task build         # everything, into _output/
task shell         # boot the machine and look around inside it
task lint          # gofmt, vet, and whether the scripts and Taskfiles parse
task test          # the machine definition
task verify:args   # and whether the QEMU in _output/ accepts what it produces
task fingerprint   # this machine's identity
task release       # one tarball, one version

task is taskfile.dev, not GNU task. The part builds run in containers (qemu/Dockerfile, kernel/Dockerfile, e2fsprogs/Dockerfile, image/Dockerfile), so a container engine is needed to build; a boot is not, and neither is task build — a consumer points --release at an unpacked release tree instead. A boot needs /dev/kvm readable and writable by whoever runs it, the same requirement docs/releasing.md states for the lab runner; a host without it boots the release's TCG build with --accel tcg, at perhaps fifty times the cost, which is why it is stated and never silently fallen back to.

Each part's targets live beside what they build — task qemu:build is next to qemu/Dockerfile — and the root Taskfile.yml holds the vars every part reads and the targets that cross all of them. Every version, digest and commit a build takes from outside is in versions.yaml: task versions says what is behind, task bump NAME=... moves one.

Where to read next

docs/machine.md	the definition: fixed slots, vmgenid, and what the fingerprint hashes
docs/migration.md	one VM moved to another host, and why the CPU model decides it
docs/memory-and-disk.md	what a guest's page cache, freed memory and disk cost its host: the defaults, what was measured to choose them, and what was turned down
docs/releasing.md	CI, versions, and what a release owes its upstreams
qemu/, kernel/, image/, e2fsprogs/	why each part is built the way it is
boot/	what a boot costs, measured from the host
CLAUDE.md	how to work in here: the release invariants, and the rules for changing them

Licence

Apache-2.0, matching the rest of this stack — and not only for consistency: three scripts under image/mkosi.extra/ came from another Apache-2.0 project here, so a different licence would make this a mixed-licence tree for nothing. NOTICE names them and states which was modified, as section 4(b) requires.

That licence covers the recipes, not what they build: a release is mostly GPL and LGPL binaries, and docs/releasing.md says how a release answers for them.