[v4,00/20] block: remove aio_disable_external() API

v4:
- Remove external_disable_cnt variable [Philippe]
- Add Patch 1 to fix assertion failure in .drained_end() -> blk_get_aio_context()
v3:
- Resend full patch series. v2 was sent in the middle of a git rebase and was
  missing patches. [Eric]
- Apply Reviewed-by tags.
v2:
- Do not rely on BlockBackend request queuing, implement .drained_begin/end()
  instead in xen-block, virtio-blk, and virtio-scsi [Paolo]
- Add qdev_is_realized() API [Philippe]
- Add patch to avoid AioContext lock around blk_exp_ref/unref() [Paolo]
- Add patch to call .drained_begin/end() from main loop thread to simplify
  callback implementations

The aio_disable_external() API temporarily suspends file descriptor monitoring
in the event loop. The block layer uses this to prevent new I/O requests being
submitted from the guest and elsewhere between bdrv_drained_begin() and
bdrv_drained_end().

While the block layer still needs to prevent new I/O requests in drained
sections, the aio_disable_external() API can be replaced with
.drained_begin/end/poll() callbacks that have been added to BdrvChildClass and
BlockDevOps.

This newer .bdrained_begin/end/poll() approach is attractive because it works
without specifying a specific AioContext. The block layer is moving towards
multi-queue and that means multiple AioContexts may be processing I/O
simultaneously.

The aio_disable_external() was always somewhat hacky. It suspends all file
descriptors that were registered with is_external=true, even if they have
nothing to do with the BlockDriverState graph nodes that are being drained.
It's better to solve a block layer problem in the block layer than to have an
odd event loop API solution.

The approach in this patch series is to implement BlockDevOps
.drained_begin/end() callbacks that temporarily stop file descriptor handlers.
This ensures that new I/O requests are not submitted in drained sections.

The first two virtio-scsi patches were already sent as a separate series. I
included them because they are necessary in order to fully remove
aio_disable_external().

Stefan Hajnoczi (20):
  block-backend: split blk_do_set_aio_context()
  hw/qdev: introduce qdev_is_realized() helper
  virtio-scsi: avoid race between unplug and transport event
  virtio-scsi: stop using aio_disable_external() during unplug
  util/vhost-user-server: rename refcount to in_flight counter
  block/export: wait for vhost-user-blk requests when draining
  block/export: stop using is_external in vhost-user-blk server
  hw/xen: do not use aio_set_fd_handler(is_external=true) in
    xen_xenstore
  block: add blk_in_drain() API
  block: drain from main loop thread in bdrv_co_yield_to_drain()
  xen-block: implement BlockDevOps->drained_begin()
  hw/xen: do not set is_external=true on evtchn fds
  block/export: rewrite vduse-blk drain code
  block/export: don't require AioContext lock around blk_exp_ref/unref()
  block/fuse: do not set is_external=true on FUSE fd
  virtio: make it possible to detach host notifier from any thread
  virtio-blk: implement BlockDevOps->drained_begin()
  virtio-scsi: implement BlockDevOps->drained_begin()
  virtio: do not set is_external=true on host notifiers
  aio: remove aio_disable_external() API

 hw/block/dataplane/xen-block.h              |   2 +
 include/block/aio.h                         |  57 ---------
 include/block/export.h                      |   2 +
 include/hw/qdev-core.h                      |  17 ++-
 include/hw/scsi/scsi.h                      |  14 +++
 include/qemu/vhost-user-server.h            |   8 +-
 include/sysemu/block-backend-common.h       |  25 ++--
 include/sysemu/block-backend-global-state.h |   1 +
 util/aio-posix.h                            |   1 -
 block.c                                     |   7 --
 block/blkio.c                               |  15 +--
 block/block-backend.c                       |  78 ++++++------
 block/curl.c                                |  10 +-
 block/export/export.c                       |  13 +-
 block/export/fuse.c                         |  58 ++++++++-
 block/export/vduse-blk.c                    | 128 ++++++++++++++------
 block/export/vhost-user-blk-server.c        |  70 +++++++----
 block/io.c                                  |   5 +-
 block/io_uring.c                            |   4 +-
 block/iscsi.c                               |   3 +-
 block/linux-aio.c                           |   4 +-
 block/nfs.c                                 |   5 +-
 block/nvme.c                                |   8 +-
 block/ssh.c                                 |   4 +-
 block/win32-aio.c                           |   6 +-
 hw/block/dataplane/virtio-blk.c             |  19 ++-
 hw/block/dataplane/xen-block.c              |  42 +++++--
 hw/block/virtio-blk.c                       |  38 +++++-
 hw/block/xen-block.c                        |  24 +++-
 hw/i386/kvm/xen_xenstore.c                  |   2 +-
 hw/scsi/scsi-bus.c                          |  46 ++++++-
 hw/scsi/scsi-disk.c                         |  28 ++++-
 hw/scsi/virtio-scsi-dataplane.c             |  31 +++--
 hw/scsi/virtio-scsi.c                       |  59 +++++++--
 hw/virtio/virtio.c                          |   6 +-
 hw/xen/xen-bus.c                            |  11 +-
 io/channel-command.c                        |   6 +-
 io/channel-file.c                           |   3 +-
 io/channel-socket.c                         |   3 +-
 migration/rdma.c                            |  16 +--
 tests/unit/test-aio.c                       |  27 +----
 tests/unit/test-bdrv-drain.c                |   1 -
 tests/unit/test-fdmon-epoll.c               |  73 -----------
 util/aio-posix.c                            |  20 +--
 util/aio-win32.c                            |   8 +-
 util/async.c                                |   3 +-
 util/fdmon-epoll.c                          |  18 +--
 util/fdmon-io_uring.c                       |   8 +-
 util/fdmon-poll.c                           |   3 +-
 util/main-loop.c                            |   7 +-
 util/qemu-coroutine-io.c                    |   7 +-
 util/vhost-user-server.c                    |  33 ++---
 hw/scsi/trace-events                        |   2 +
 tests/unit/meson.build                      |   3 -
 54 files changed, 612 insertions(+), 480 deletions(-)
 delete mode 100644 tests/unit/test-fdmon-epoll.c