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Stage K-6: riscv64 vblk_io timeouts root-caused and fixed -- time bound, exact used-ring accounting (FABRIC-3.10.8 §4.7)

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- Investigation (riscv64): completions are slow, not lost -- up to 15 ms,
  while the ~1M-iteration bound was ~14 ms of guest time. Each timeout
  left the request in the device while the driver forgot it; the next
  request took its used entry as its own and returned before its own I/O
  finished, and so did every later one (394 in one boot), rewriting
  descriptors the device was still reading.
- Fix: VBLK_TIMEOUT_MS (2 s) on the raw hardware counter; each request
  waits for exactly one more used entry; a timed-out request stays
  outstanding and its completion is drained before anything is reused,
  or the next request is refused untouched. Counters for timeouts, late
  drains and busy refusals.
- After the fix, all three ISAs: 0 timeouts, 0 unaccounted completions
  over 1,912 I/Os; riscv64 slowest completion 7.7 ms (was 15.1).
- Standard sequence on amd64/aarch64/riscv64: hash, G, ledger unchanged;
  0 timeouts in the riscv64 log.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
This commit is contained in:
Robert Allan James
2026-09-26 08:33:25 -04:00
co-authored by Claude Opus 5.5
parent 0c1fa2e510
commit acea36ea55
19 changed files with 73459 additions and 39 deletions
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