Boot from /dev/vda by default

[jserv]

Boot Linux directly from a virtio-blk ext4 image instead of unpacking an initramfs cpio archive. The new path skips the unpack_to_rootfs RCU stall and shortens guest boot. Add ENABLE_EXTERNAL_ROOT=0 to fall back to the legacy initramfs path; auto-fall back when fakeroot is unavailable.

Also:

• Add -H/--headless so make check and benchmark scripts can run without an SDL window.
• Wire an atexit msync hook in virtio-blk so MAP_SHARED pages reach disk on graceful shutdown.
• Source prebuilt Image and rootfs.cpio from the GitHub releases endpoint and detect stale artifacts via PREBUILT_INPUTS_SHA1. Cache key in main.yml hashes every input that defines the prebuilt content so CI does not silently restore stale artifacts.
• New prebuilt.yml workflow republishes the release whenever any PREBUILT_INPUTS file changes.
• Move clang-format and prebuilt manifest logic out of inline shell into .ci/suggest-format.sh and .ci/publish-prebuilt.sh.
• Cap -c hart count to [1,32] (matches plic_state_t.ie[] dimension) with strtol-based validation.
• Check vm->hart calloc result, free wake_fd[] on realloc failure, call coro_cleanup() on normal SMP exit.
• Install SIGINT/SIGTERM handlers via sigaction (no SA_RESTART) so poll() returns EINTR; signal handler also writes to wake_fd[1] so the threaded VIRTIOINPUT path wakes up and runs atexit hooks.

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Summary by cubic

Boot Linux from /dev/vda (virtio‑blk ext4) by default to skip initramfs unpack, avoid the RCU stall, and speed up boot. Adds a headless mode and safer shutdown so disk writes flush on exit.

• New Features

  • Default root on /dev/vda; ext4.img is built from rootfs.cpio via scripts/rootfs_ext4.sh. Auto‑fallback to the legacy initramfs path when fakeroot is unavailable or unusable. -i now requires ENABLE_EXTERNAL_ROOT=0 or a DTB that advertises linux,initrd-*.
  • -H/--headless runs without an SDL window (used by make check and CI).
  • Safer shutdown: virtio‑blk msyncs MAP_SHARED pages at exit; SIGINT/SIGTERM handled via sigaction and a wake pipe so loops break and atexit hooks run.
  • CLI hardening: -c hart count validated to 1–32; better allocation cleanup; coro_cleanup() on normal SMP exit; DTB bootargs updated for vda boot and quieter logs.
  • Prebuilts: fetched from the repo’s blob branch; mk/external.mk compares PREBUILT_INPUTS_SHA1 to detect stale artifacts. prebuilt.yml and .ci/publish-prebuilt.sh publish a rolling prerelease and CI cache keys hash all inputs. A CI job keeps the legacy initramfs path tested.

• Bug Fixes

  • Pin PREBUILT_INPUTS_SHA1 to the current inputs so stale-prebuilt detection is active.
  • Guard /dev/vda bootargs on both EXTERNAL_ROOT and VIRTIOBLK.
  • virtio-blk: check fstat() return and reject empty/invalid disk sizes.
  • scripts/bench-login.expect: use Tcl list expansion (spawn {*}$argv) to pass args safely.
  • Makefile: detect non‑functional fakeroot via a real exec (fakeroot /bin/sh -c :) so macOS arm64e falls back to initramfs automatically.

^{Written for commit eb6705c. Summary will update on new commits. Review in cubic}