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CPU.md

Lookaside notes for u-root/cpu (cpu + cpud) as it relates to this repo and the v9fs/docker initrd.

Per AGENTS.md, treat source code as the authority.

What cpud is

cpud is the “daemon side” of a Plan9-inspired cpu session. It can run:

  • as PID 1 (init + daemon)
  • as a daemon (listening server that forks per-session cpuds)
  • as a per-session “remote” process

Source of truth:

  • u-root/cpu cmds/cpud/main_linux.go and cmds/cpud/cpuddoc.go

cpud flags (server mode)

From cmds/cpud/main_linux.go and cmds/cpud/cpuddoc.go:

  • -hk: host key file (host key)
  • -pk: public key file (default key.pub)
  • -sp: listen port (default 17010)
  • -net: network (default tcp)
  • -d: debug prints
  • -klog: log to kernel log instead of stdout
  • -register / -registerTO: optional controller registration
  • -sleepBeforeServing: delay before serving (useful when running as init)

cpud “remote” mode and argument rules

In remote mode (invoked as cpud -remote ...) the accepted flag set is intentionally smaller, and the program expects remaining args to be the command to run.

Source of truth:

  • cmds/cpud/main_linux.go (if len(os.Args)>1 && os.Args[1] == "-remote"... and the custom FlagSet)

Relevance to v9fs/docker

The v9fs/docker base initrd’s /init is a symlink to bbin/cpud (and bbin/cpud is an applet of the bb binary).

For this repo’s harness, we are not depending on cpu/cpud features to run tests yet; we only rely on the u-root init sequence and the availability of /bbin/bb applets to implement /bin/uinit and mount 9p.