- 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>
