Cross-Domain Debug: Races, Stuck Sync, Phase Trace

Symptom catalog

Simulation hangs at sync barrier

1. One domain never reaches the sync target phase — objection leak in lagging domain.

2. Circular sync graph — domain A waits for B, B waits for A.

3. Target phase typo — syncing at wrong phase name.

Race-like functional failures

1. Traffic sent before synced reset completes on peer domain.

2. Missing sync at configure — register programming races.

3. Assuming global reset_phase completion when only common domain finished.

simv +UVM_PHASE_TRACE +UVM_OBJECTION_TRACE +UVM_VERBOSITY=UVM_MEDIUM

Phase trace interpretation

[PHASE][UVM] reading multi-domain trace

look for interleaved entries:
  [PHASE] axi_agent.reset_phase START  (domain=common)
  [PHASE] pmu_agent.reset_phase START   (domain=pmu_domain)
  ...
  [SYNC]  waiting at reset_phase for pmu_domain
  [PHASE] pmu_agent.reset_phase END
  [SYNC]  barrier released

• Tag log messages with domain name in custom components.

• Compare phase START/END pairs per domain for leaks.

• If sync never releases, find the domain that never reaches target.

Key takeaways

• +UVM_PHASE_TRACE is essential for multi-domain debug.

• Stuck sync almost always means one domain did not reach the barrier.

• Race bugs mean missing or misplaced sync, not random simulator behavior.

Common pitfalls

• Removing all sync to 'fix' hang without fixing root dependency.

• Debugging with waveforms only — phase trace shows domain interleaving.

Isolation procedure

1. Run each domain's subtree in single-domain mode (sanity baseline).

2. Add sync one barrier at a time.

3. After each sync, rerun phase trace and confirm release.

4. Re-enable cross-domain traffic only after reset sync passes.