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Devices, SMP & networking

ignition wires its devices through a uniform DeviceManager: MMIO-window and SPI allocation, bus registration, FDT-node description, and snapshot enumeration all sit behind the MmioDevice trait.

Console

A 16550 UART provides a fully bidirectional console. TX drains to stdout; RX buffers typed input into the UART’s RX FIFO, sets the LSR data-ready bit, and raises the RX interrupt over the same GIC serial line (INTID 32) that TX uses. A reader thread runs an escape FSM (Ctrl-A x quits) and forwards bytes into the device, so a real interactive root login works: type root, get a shell, run commands, detach with Ctrl-A x.

virtio devices

virtio runs over a generalized virtio-mmio transport: a VirtioDevice trait (device_id/device_features/config_read/queue_count/handle_notify/inject_rx) with per-queue state, hardened feature-select clamping, and a QueueReady invariant. Config space (offset >= 0x100) is byte-addressable at any access width, which Linux needs because it reads multi-byte config fields one byte at a time.

  • virtio-blk carries the real rootfs over a split virtqueue. The device probes, the guest mounts ext4 over the virtqueue, and init runs off the disk. A boot serviced roughly 692 virtqueue requests (about 605 reads, 62 writes) through the QueueNotify -> walk -> file I/O -> used-ring -> SPI path.
  • virtio-rng, virtio-balloon, and virtio-vsock round out the block-era device set.

virtio-net + vmnet

--net (opt-in) brings up a virtio-net NIC backed by vmnet.framework in shared/NAT mode through a C shim. The full data path (TX -> vmnet -> RX -> IRQ on INTID 34 -> guest) is proven on hardware. The --net path needs the vmnet entitlement and must run under sudo for shared mode; without sudo it fails cleanly with a clear message. The rootfs auto-brings-up eth0 and DHCPs on boot, so the guest reaches the internet with no manual steps.

vmnet survives snapshot/restore: on restore the link is bounced and the guest’s carrier-watch re-runs DHCP. Each clone gets a distinct MAC and IP.

virtio-vsock

virtio-vsock carries stream connections between host and guest over the virtio transport. E1 (guest→host) exposes per-port host listeners at {uds}_{port}: a guest process connecting to a vsock port surfaces on the host as a connection to the matching Unix socket path.

vsock host→guest (E2)

A host process opens a connection into a listening guest over the same control socket, using Firecracker’s hybrid protocol:

  1. The host connects to {uds} (the base path of --vsock-uds) and sends CONNECT <guest_port>\n.
  2. ignition allocates an ephemeral host port, signals the guest (REQUEST), and the guest’s listener accepts (RESPONSE).
  3. ignition replies OK <host_port>\n to the host; raw bytes then flow both ways on that same connection. If no guest process is listening, the connection is closed.
# guest init runs e.g.:  socat VSOCK-LISTEN:5000,fork EXEC:cat
socat - UNIX-CONNECT:/tmp/ignition-vsock-e2 <<<'CONNECT 5000'

Guest→host (E1) and host→guest (E2) coexist; per-port paths {uds}_{port} remain the E1 guest→host listeners.

For a full worked example with socat servers and clients on both ends, see the vsock round-trip use case.

SMP

--smp N (default 1, cap 8) boots a real aarch64 Linux with N vCPUs. Secondaries come online via PSCI CPU_ON, schedule work, and stop on SYSTEM_OFF. A VcpuManager owns the linear MPIDR mapping (mpidr_for(index) = index) shared by the FDT, MPIDR_EL1, and the CPU_ON claim guard; lazy bring-up spawns a thread-affine vCPU per core. A restored guest reports nproc == N. The in-kernel hv_gic delivers SGIs/IPIs and per-cpu vtimers natively, so secondaries need no VMM-side interrupt plumbing.

target/debug/boot --smp 4 kimage/out/Image kimage/out/rootfs.ext4
# [    0.010315] SMP: Total of 4 processors activated.
# (none):~# nproc
# 4

Clock

A PL031 RTC plus the EL1 virtual timer keep guest time. The vtimer PPI (INTID 27) is delivered through the in-kernel GIC, and on restore the vtimer offset is set so that CNTVCT resumes continuously across the snapshot rather than jumping forward.

GUI display

--gui provides a software-rendered desktop over virtio-gpu (2D, device id 16) and virtio-input (keyboard + tablet, device id 18) with a cage Wayland compositor. See GUI display for the full details: window threading model, virtio-gpu/input kernel config, cage + foot setup, snapshot/restore/fan-out, and window hotkeys (Ctrl+Alt+R reset, Ctrl+Alt+S snapshot, Ctrl+Alt+X close).