### 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 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
* Atomic API Design: Merged iothread_get_aio_context and 
  iothread_ref_and_get_aio_context into a single unified function. The new 
  API enforces a 'holder' parameter, making "retrieval" and "tracking" an 
  atomic operation to prevent reference tracking leaks.
* Subsystem Integration: Fully migrated and adapted core modules including 
  Virtio (blk, vq-mapping, balloon, scsi), Block Layer (export, xen), 
  Network (COLO), VFIO, and the Monitor subsystem.
* Introspection: Expanded the IOThreadInfo QAPI structure to include a 
  'holders' array (list of QOM paths), accessible via both 
  query-iothreads (QMP) and info iothreads (HMP).

### V5 -> V6 Changelog
* - First of all, I apologize for spending so much time dealing with
    the background details of the code implementation.
* 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 (14):
  qapi/misc: Fix missed query-iothreads items
  iothread: introduce iothread_ref/unref to track attached devices
  iothread: tracking iothread users with holder name
  blockdev: Update tracking iothread users with holder name
  block/export: track IOThread reference in BlockExport
  monitor: Update tracking iothread users with holder name
  virtio-vq-mapping: track iothread-vq-mapping references using device
    path
  virtio: use iothread_get/put_aio_context for thread pinning
  net/colo: track IOThread references using path-based holder
  virtio-balloon: Update tracking iothread users with holder name
  vfio-user/proxy: Update tracking iothread users with holder name
  xen-block: Update tracking iothread users with holder name
  qapi: examine IOThread attachment status via query-iothreads
  iothread: simplify API by merging iothread_get_aio_context variants

 block/export/export.c                   |  44 +++++++---
 blockdev.c                              |   6 +-
 hw/block/dataplane/xen-block.c          |  10 ++-
 hw/block/virtio-blk.c                   |  13 ++-
 hw/scsi/virtio-scsi-dataplane.c         |  13 ++-
 hw/vfio-user/proxy.c                    |   7 +-
 hw/virtio/iothread-vq-mapping.c         |  11 ++-
 hw/virtio/virtio-balloon.c              |  14 ++--
 include/block/block_int-common.h        |   5 ++
 include/block/export.h                  |   6 ++
 include/hw/virtio/iothread-vq-mapping.h |   6 +-
 include/system/iothread.h               |   5 +-
 iothread.c                              | 102 +++++++++++++++++++++++-
 monitor/hmp-cmds.c                      |  22 +++++
 monitor/monitor-internal.h              |   3 +
 monitor/monitor.c                       |  24 +++++-
 monitor/qmp.c                           |   3 +-
 net/colo-compare.c                      |  18 +++--
 qapi/misc.json                          |  72 ++++++++++++++++-
 19 files changed, 328 insertions(+), 56 deletions(-)

-- 
2.49.0