[PATCH V11 00/15] iothread: Support tracking and querying IOThread holders

[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_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,
  while omitting the holders line when the list is empty.


### 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.

### V10 -> V9 Changelog
  - Rebase this series on the Daniel P. Berrangé's series:
    [PATCH v7 00/35] monitor: turn QMP and HMP into QOM objects
    https://lore.kernel.org/qemu-devel/20260706135824.2623960-1-berrange@redhat.com/
  - Remove the monitor type of IOThreadHolder, make the monitor as a QOM object.
  - Remove GList related code and stays within QAPI instead of mixing GList and QAPI.
  - Fix missed comments in the patch 2.
  - Rewrite parts of commit log generated by AI.
  - Keep Stefan's reviewed-by in patch 2 and addressed Markus's comments.
  - Rewrite parts of code comments.
  - Drop original patch 6/7/8 for monitor parts.

### V9 -> V8 Changelog
  - Drop to assign unique default ID for monitor, and keep the NULL ID.
  - Rewrite and split the monitor patch 7 and 8.
  - Add the [PATCH 17/17]tests/unit/iothread: Update the iothread_get_aio_context
    to keep the API same in the unit tests.
  - Refactorcode according comments.
  - Update the example.
  - Fix other comments issues.

### V7 -> V8 Changelog
  - Drop the assumption of the Monitor QOM, add the new type
    IO_THREAD_HOLDER_KIND_MONITOR_NAME.
  - Drop the assumption of the string '/' as QOM path.
  - Introduce new patch for Monitor:
    [PATCH 06/15] assign unique default ID to anonymous monitors
  - Fixed Monitor ID for the IOThread.
  - Refactorcode according comments.
  - Update QMP example.
  - Rebased patches on upstream code.
  - Fixed comments issues.
  - Removed redundant code.

### V6 -> V7 Changelog
  - Fixed comments issues in patch 02/14 (Thanks Markus).
  - Removed redundant code.
  - Rebased patches on upstream code.
  - Fixed code conflict with latest patches.
  - Added detailed description in git log and code.
  - Added comments for patch 06/14 assuming the monitor is a QOM object,
    Depends on Daniel Berrange's RFC patch:
    [PATCH RFC 00/17] monitor: turn QMP and HMP into QOM objects

### V5 -> V6 Changelog
* API Refactoring (Breaking Change):
  - Implement the 'union': 'IoThreadHolder' for combine the QOM objects
    and the block-node.
  - Completely merged iothread_get_aio_context variants.
  - The iothread_get_aio_context function now strictly requires a 'holder'
    argument to ensure every context retrieval is accounted for.
* Expanded Module Coverage:
  - Added holder tracking for the monitor subsystem (tracking threads used
    by QMP/HMP).
  - Added support for the virtio-scsi dataplane.
* Code Cleanup:
  - Removed redundant and obsolete API declarations.
  - Standardized the use of object_get_canonical_path() across all callers
    to provide consistent and unique holder names.
* Bug Fixes:
  - Updated qapi/misc.json documentation to fix missing descriptions in
    query-iothreads items.
* Build System:
  - Improved header inclusion logic to avoid redundant declaration errors
    in unit tests.
* QAPI part:
  - Sorry to drop the "Acked-by: Markus Armbruster <armbru@redhat.com>",
    because this version have lots of changes in the:
    [PATCH V6 13/14]qapi: examine IOThread attachment status via query-iothreads



Zhang Chen (15):
  qapi/misc: Fix missed query-iothreads items
  iothread: introduce holder tracking
  iothread: track users with holder name
  iothread: introduce iothread_unsafe_get_aio_context()
  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: remove legacy iothread_get_aio_context()
  tests/unit/iothread: update AioContext ref/put helpers

 block/export/export.c                   | 79 +++++++++++++++++++---
 blockdev.c                              |  2 +-
 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                  |  6 ++
 include/hw/virtio/iothread-vq-mapping.h |  6 +-
 include/system/iothread.h               | 37 ++++++++++-
 iothread.c                              | 88 ++++++++++++++++++++++++-
 monitor/hmp-cmds.c                      | 22 +++++++
 monitor/monitor-internal.h              |  3 +
 monitor/monitor.c                       | 26 ++++++--
 monitor/qmp.c                           | 16 ++++-
 net/colo-compare.c                      | 29 +++++---
 qapi/misc.json                          | 86 ++++++++++++++++++++++--
 tests/unit/iothread.c                   | 17 +++--
 tests/unit/iothread.h                   |  6 +-
 tests/unit/test-aio-multithread.c       |  5 +-
 tests/unit/test-bdrv-drain.c            | 18 +++--
 tests/unit/test-block-iothread.c        | 21 ++++--
 23 files changed, 505 insertions(+), 82 deletions(-)

-- 
2.53.0