Adopt bind-once microVM image slots

[Enrique Saurez (esaurez)]

Summary

Pins OpenVMM to nanvix/openvmm#116 (6aaf6fb7), which adds bind-once read-only virtio-blk image slots to the microVM profile, and adopts them in NVX. The repin contains only these commits on top of the current pin 4355c010:

• 368b9d2a virtio_blk: add bind-once image slots
• 6c12f620 openvmm_entry: add microVM image slots
• 10106b13 microvm: admit declared image-slot transports in machine validation
• bf98f65f microvm: advertise and gate image-slot restore targets
• 6aaf6fb7 microvm: end host-control sessions quietly on a clean close

No other fork changes are included.

• ABI: VMs that declare slots use microVM ABI 3; the source manifest's microvm_abi_version is 3. VMs without slots keep ABI 2 unchanged.
• CLI and harness: --image-slot-boot-count for managed sandboxes, --restore-image-slots, query-image-slots, and bind-image-slot. Slot launches may omit the distro layer; ABI-2 launches keep the existing layer requirement.
• Host control: authenticated protocol-v1 client, using the existing 32-byte capability and the owner-restricted local endpoint.
• Guest:
  • kernel shared-status mapping for the four slot windows;
  • OPENVMM_ENTROPY_V4 parsing, with portb status bit 6 routing image-slot targets to restore repair;
  • restore-time prefix activation before acknowledgement;
  • capacity/config-change discovery, partition rescan, and stable read-only /dev/nvx-imageN links. Placeholders stay unbound.
• CI: cross-backend image-slot correctness covers:
  • boot with one active slot, then bind and read;
  • idempotent bind, and rejection of a different identity;
  • restore to four active slots, then bind a restored slot.
• Benchmarks: benchmark --suite image-slot-boot compares ABI 2, ABI 3 with one active slot, and ABI 3 with four active slots.
• Docs: machine and device ABI, limits, remaining work, snapshot and restore packet and status bit, run/usage/setup/build guides, implementation status, and benchmark methodology.

Base

This PR is intentionally based on db5138e3, before the CPU-profile repin on dev (#394), and targets dev-pre-cpu-profile, created at that commit. CI workflows run only for pull requests into dev, so I validated this PR manually on an isolated MSHV host.

Validation

Local (Windows):

• At 1cbf5c7d: python scripts/nvx.py verify, compileall, pyright (Linux and Windows), ruff check, ruff format --check, and git diff --check passed. Focused unit tests: 63 passed. Supporting suites: 108 passed, 17 skipped.
• Later commits: ruff check, ruff format --check, and git diff --check passed. The benchmark tests passed (83, with 1 skipped), and the new console-ordering test fails without its fix.
• shellcheck --shell=sh and shfmt -d -ln posix -i 4 -ci on the changed guest scripts: clean.
• nvx-port-io.c compiles for x86_64-linux-musl with -static -Os -Wall -Wextra -Werror.

Linux/MSHV, head 402c01f2 with OpenVMM bf98f65f:

• Host: AMD EPYC 9V74, Linux 6.6.148-200.mshv-b1f02da.azl3.

• python3 scripts/nvx.py build-guest (Docker) and build-openvmm --backend mshv (musl, Rust 1.95.0). Both built from clean sources.

• The CI CLI validation (compileall, the seven unittest modules, test_adversarial.py, verify) passed: 540 tests passed (29 skipped), and 70 adversarial tests passed.

• test-microvm --backend mshv --scenario image-slots: passed.

  • Boot B = 1, then bind, idempotent rebind, rejection of a different identity, and a guest read of the bound media.
  • Capture an empty-slot snapshot, restore with --restore-image-slots 4, then bind and read slot 3.

• test-microvm --backend mshv (default suite, 36 scenarios): all passed. This covers boot, console, lifecycle, sandbox blocks, scratch/SMP/network/filesystem snapshots, restore processors/memory/TSC, snapshot tiers, network policy, and image slots.

• benchmark --suite image-slot-boot --backend mshv --processors 1 --warmups 5 --runs 51, measuring cold boot to ALPINE-MICROVM-BOOT-OK:

  Configuration	p50	p95	Δ p50 vs ABI 2
  ABI 2	342.3 ms	392.6 ms	—
  ABI 3, 1 active slot	345.6 ms	396.2 ms	+3.3 ms
  ABI 3, 4 active slots	353.0 ms	405.3 ms	+10.7 ms

  Configurations ran in consecutive blocks, not interleaved, on a host that also had an unrelated VM running.

Linux/MSHV, head 6da62e93 with OpenVMM 6aaf6fb7:

• 6da62e93 changes only the OpenVMM pin and the design doc. OpenVMM 6aaf6fb7 stops reporting a client's normal close of the host-control connection as a failed connection.
• Clean rebuild: build-guest (initramfs a04b57e4..., unchanged from 402c01f2) and build-openvmm --backend mshv (bf0ec482..., source revision 6aaf6fb7).
• test-microvm --backend mshv --scenario image-slots: passed.

Live validation found four defects, all fixed in this PR:

• OpenVMM rejected the slot transports during machine validation (fixed in 10106b13).
• Restore-time activation had no status bit, so the guest never repaired the slots or released the gate (bf98f65f plus 46c6f595).
• The benchmark closed its console before the guest consumed the exit command (402c01f2).
• OpenVMM reported every normal host-control client close as a failed connection (6aaf6fb7, pinned by 6da62e93).

Not yet validated

KVM and WHP live runs.