[PATCH V9 00/17] 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,
block nodes or monitor 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).


### 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 (17):
  qapi/misc: Fix missed query-iothreads items
  iothread: introduce iothread_ref/unref to track attached devices
  iothread: tracking iothread users with holder name
  iothread: introduce iothread_unsafe_get_aio_context()
  block/export: track IOThread reference in BlockExport
  monitor: refactor monitor_data_init() to pass ID
  monitor: support iothread ref/unref for anonymous monitors
  monitor: switch to iothread_unsafe_get_aio_context()
  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
  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
  tests/unit/iothread: Update the iothread_get_aio_context

 block/export/export.c                   |  62 +++++++++--
 blockdev.c                              |   2 +-
 chardev/char.c                          |   2 +-
 gdbstub/system.c                        |   3 +-
 hw/block/dataplane/xen-block.c          |  19 +++-
 hw/block/virtio-blk.c                   |  22 ++--
 hw/scsi/virtio-scsi-dataplane.c         |  22 ++--
 hw/vfio-user/proxy.c                    |  15 ++-
 hw/virtio/iothread-vq-mapping.c         |  19 +++-
 hw/virtio/virtio-balloon.c              |  22 +++-
 include/block/export.h                  |   6 ++
 include/hw/virtio/iothread-vq-mapping.h |   6 +-
 include/monitor/monitor.h               |   5 +-
 include/system/iothread.h               |  17 ++-
 iothread.c                              | 136 +++++++++++++++++++++++-
 monitor/hmp-cmds.c                      |  26 +++++
 monitor/hmp.c                           |   5 +-
 monitor/monitor-internal.h              |   4 +-
 monitor/monitor.c                       |  37 +++++--
 monitor/qmp-cmds.c                      |   2 +-
 monitor/qmp.c                           |   7 +-
 net/colo-compare.c                      |  29 +++--
 qapi/misc.json                          |  82 +++++++++++++-
 stubs/monitor-internal.c                |   3 +-
 tests/unit/iothread.c                   |  16 +--
 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 ++--
 29 files changed, 526 insertions(+), 93 deletions(-)

-- 
2.49.0