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base image: serial-getty@ttyS0 hangs ~90s on every boot (dev-ttyS0.device timeout)

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
55/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Quiet
Tech stack
linux, ubuntu
Domain
infrastructure, operating-systems

Research direction

Start with the images/base guest and inspect its udev and systemd behavior around dev-ttyS0.device and [email protected]. Reproduce a fresh boot using the dev/vagrant/ real-KVM harness, then verify the console log and service state. Done means no device timeout, an intentional working or removed serial getty with rationale, and verification in the harness.

Written by the indexing model from the issue text.

Description

nanofuse

Summary

Every microVM booted from images/base (ghcr.io/daax-dev/nanofuse/base:latest) hangs for ~90 seconds during guest boot waiting on dev-ttyS0.device, then times out and fails [email protected]:

[ TIME ] Timed out waiting for device dev-ttyS0.device - /dev/ttyS0.
[DEPEND] Dependency failed for [email protected] - Serial Getty on ttyS0.
[  OK  ] Reached target multi-user.target - Multi-User System.

multi-user.target is still reached and SSH/network work normally, so VMs are usable — but the serial console (Firecracker's primary out-of-band access path) is dead for the VM's entire lifetime, and every boot pays a ~90s console-readiness penalty.

Scope / reproducibility

  • 100% reproducible on every boot, fresh or resumed.
  • Independent of snapshot/resume: a fresh, never-snapshotted VM shows the identical timeout. (Discovered while validating issue #227; initially misattributed to the snapshot-resume path, then disproven with a fresh-boot control.)
  • Independent of Firecracker version: identical on FC 1.7.0 and 1.16.1.

Environment

  • Closed-loop KVM harness (dev/vagrant/, libvirt + nested KVM), Ubuntu 24.04 guest.
  • Kernel cmdline includes console=ttyS0; Firecracker provides the 8250 UART.
  • base image kernel 6.1.90 + Ubuntu 24.04 rootfs.

Likely cause (unconfirmed)

[email protected] BindsTo=/After= dev-ttyS0.device, and that device unit never activates — udev appears not to emit/process the add uevent that tags /dev/ttyS0 with SYSTEMD_WANTS in this minimal microVM kernel/udev setup. Candidate fixes to investigate (base-image side): ensure the serial UART is coldplugged (udev trigger), or decouple the getty from the device unit, or drop the enabled serial-getty@ttyS0 if serial-console login is not a product requirement.

Impact

  • Does not block SSH-based use (daax-devtools coding-in-microVM works).
  • Does break serial-console login and adds ~90s to console readiness on every boot.

Acceptance criteria

  • A fresh microVM from images/base reaches multi-user.target with no dev-ttyS0.device timeout in the console log.
  • [email protected] is either active (console login works) or intentionally removed, with the rationale recorded.
  • Verified on the dev/vagrant/ real-KVM harness.

Filed from real-KVM closed-loop testing. Root-cause diagnosis is a hypothesis; validate against the guest udev/systemd behavior before fixing.

Dominant language
Go
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