Replaces the old nvbandwidth-all-gpu-power-watch scenario (dropped: the
power/IPMI-sampler framing was a dead-end — the blackbox shows every other
load test, incl. NCCL all-reduce across all 6 GPUs, passes with power fully
connected; only nvbandwidth on cross-socket GPU P2P reboots the box).
nvbandwidth-acs-ab is a two-phase reproduction for the MSI CG480-S6053 case.
Phase 1 disables PCIe ACS redirect at runtime via setpci (saving each
bridge's original ACS control word to /run/bee-acs-orig) then runs
nvbandwidth across all 6 GPUs — expected to survive. Phase 2 restores the
saved ACS values and repeats — expected to reproduce the reboot. Safe phase
first so its full blackbox reaches the USB stick before the risky phase can
reset the box (sync-bracket before the final job flushes phase-1 evidence).
ACS mutation runs via `bash -c` since scenario cmd is exec'd directly with no
shell. Both shipped copies (repo scenarios/ and image overlay) identical;
verified both parse via ParseScenarioJSON (4 jobs).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
