[PATCH V16 00/13] iothread: Support tracking and querying IOThread holder

[Zhang Chen <zhangckid@gmail.com>]

### Motivation
Currently, the relationship between IOThreads and the resources using them is
opaque. From a management perspective, it is difficult to determine which
devices or block exports are currently associated with a specific IOThread
via QMP or HMP.

This series introduces a "holder" tracking mechanism based on QOM paths or
block exports to significantly improve IOThread observability.
In high-performance virtualization environments, users often pre-allocate
a set of IOThreads to serve as a persistent thread pool. During device
hotplug/unplug cycles, while IOThread objects remain persistent,
their attachment state changes dynamically.

By tracking these "holders," administrators can use `query-iothreads` to
monitor the real-time load distribution of the thread pool and identify which
specific devices (e.g., virtio-blk, BlockExport) are attached to which thread.

### Technical Overview
* Holder tracking

  Add a QAPI holder list to each IOThread.  A holder identifies either
  a QOM object by its canonical path or a block export by its unique
  BlockExportOptions ID.

* Explicit reference lifecycle

  Add iothread_ref_and_get_aio_context() and the matching
  iothread_unref_and_put_aio_context().  Acquiring an AioContext registers a
  holder and keeps the IOThread alive until the matching put.

  Legacy callers without a matching lifecycle use the explicitly named
  iothread_unsafe_get_aio_context().  The ambiguous legacy
  iothread_get_aio_context() is removed after all production callers
  are migrated.

* Subsystem integration

  Track holders for block exports, monitors, virtio-blk, virtio-scsi,
  virtio queue mappings, virtio balloon thread pinning, COLO compare,
  vfio-user proxy, and xen-block.

* Introspection

  Extend IOThreadInfo with a holders array returned by
  query-iothreads.  Display the same information with info iothreads.



### V16 -> V15 Changelog
  - Unified per-monitor IOThread checks and accesses through
    mon->iothread after initialization.
  - Simplified PATCH 13's commit message and clarified why blockdev
    cannot use the holder-aware get/put APIs.
  - Dropped the v15 Patch 1 in this series, It will be taken away
    independently by trivial-patches tree.

### V15 -> V14 Changelog
  - Moved monitor IOThread cleanup from unparent() to monitor_finalize(),
    pairing complete() with instance finalization.
  - Squashed V14 PATCH 04 and PATCH 14 into PATCH 13 and clarified the
    blockdev lifetime assumption. Since the additional changes are minimal,
    PATCH 13 retains Stefan's Reviewed-by from V14 PATCH 04.
  - Rebased and adapted info iothreads to the MonitorHMP interface.

### V14 -> V13 Changelog
  - Renamed patch 3 and clarified that it introduces holder-tracking
    infrastructure, with users converted in subsequent patches.
  - Documented that monitor holders are not visible to management
    applications, while retaining the tracking for a consistent holder
    and IOThread reference lifecycle.
  - Rebased on the latest upstream.

### V13 -> V12 Changelog
  - Refactored block export IOThread reference handling with shared
    init/cleanup helpers and preserved the existing single-IOThread
    driver arguments.
  - Moved monitor IOThread cleanup to the QOM unparent callback,
    where the canonical path is still available.
  - Kept internal IOThreads hidden from query-iothreads.
  - Made virtio queue-mapping holder arguments const.
  - Improved some detail comments.

### V12 -> V11 Changelog
  - Change the function name from the "iothread_put_aio_context()"
    to "iothread_unref_and_put_aio_context()" match the
    "iothread_unref_and_put_aio_context".
  - Fix unnecessary char * cast since the holder variable is
    declared const.
  - Add detailed commit msg in patch 06/15.
  - Optimize the HMP print in patch 13/15.
  - Rebase code on upstream.

### V11 -> V10 Changelog
  - Keep iothread_ref_and_get_aio_context() as an explicit acquiring API
    instead of merging it into iothread_get_aio_context().
  - Retain iothread_get_aio_context() during the caller migration so
    every intermediate patch builds, then remove it in patch 14.
  - Update patch 15 to use the holder-aware get/put API in unit-test
    stubs and callers.
  - Rework patch 2 documentation to describe holders as QOM objects or
    block exports, simplify the ref/unref descriptions, and use
    imperative function comments.
  - Identify block export holders with BlockExportOptions member id and
    rename the QAPI field to export-id.
  - Fix monitor cleanup so it only releases an AioContext acquired by
    that monitor, preventing an iothread_unref() assertion at shutdown.
  - Rebase the series onto the current upstream tree.


Zhang Chen (13):
  iothread: introduce holder tracking
  iothread: add holder-aware AioContext accessors
  block/export: track IOThread references
  monitor: track IOThread users with QOM paths
  virtio-vq-mapping: track iothread-vq-mapping references using device
    path
  virtio: track IOThread references for thread pinning
  net/colo: track IOThread references using path-based holder
  virtio-balloon: Update tracking iothread users with holder
  vfio-user/proxy: Update tracking iothread users with holder name
  xen-block: Update tracking iothread users with holder name
  monitor/hmp: display IOThread holders
  iothread: make legacy AioContext getter explicitly unsafe
  tests/unit/iothread: update AioContext ref/put helpers

 block/export/export.c                   | 62 +++++++++++++---
 blockdev.c                              |  9 ++-
 hw/block/dataplane/xen-block.c          | 19 ++++-
 hw/block/virtio-blk.c                   | 21 ++++--
 hw/scsi/virtio-scsi-dataplane.c         | 22 ++++--
 hw/vfio-user/proxy.c                    | 16 ++++-
 hw/virtio/iothread-vq-mapping.c         | 20 ++++--
 hw/virtio/virtio-balloon.c              | 22 ++++--
 include/block/export.h                  |  5 ++
 include/hw/virtio/iothread-vq-mapping.h |  6 +-
 include/system/iothread.h               | 38 +++++++++-
 iothread.c                              | 89 ++++++++++++++++++++++-
 monitor/hmp-cmds.c                      | 20 ++++++
 monitor/monitor-internal.h              |  7 +-
 monitor/monitor.c                       | 36 +++++++---
 monitor/qmp.c                           | 14 ++--
 net/colo-compare.c                      | 29 +++++---
 qapi/misc.json                          | 94 +++++++++++++++++++++----
 tests/unit/iothread.c                   | 18 +++--
 tests/unit/iothread.h                   |  7 +-
 tests/unit/test-aio-multithread.c       |  5 +-
 tests/unit/test-bdrv-drain.c            | 18 +++--
 tests/unit/test-block-iothread.c        | 21 ++++--
 23 files changed, 496 insertions(+), 102 deletions(-)

-- 
2.43.0