[PATCH 0/9] block: BLOCK_IO_DELAY event

[Hanna Czenczek <hreitz@redhat.com>]

Based-on: <20260724152315.234183-1-hreitz@redhat.com>
          [PATCH 0/6] hw/[block]: Fix missing accounting
          Fri, 24 Jul 2026 17:23:09 +0200

Hi,

I’m told there are installations where storage is very slow, and some
would like the VM stack to report this proactively.  To do so, we should
(via QAPI events) report on extremely slow I/O requests.

We already have the latency histogram, but this is not deemed sufficient
because it is not proactively reporting and would require repeated
querying.  Therefore, this series introduces the event still.

Now, from a user's perspective, it would be nice if this event could be
raised exactly when an I/O request crosses the user-defined threshold,
but this would require keeping all active requests in a list and
checking it periodically.  Now, if we used latency cookies for this
(which makes sense), then that would require that every cookie set up is
also finalized when the request is done, because if we don't, results
could well be catastrophic:
- Either we use cookies as-is, which are often allocated on the stack or
  in some other structure managed by the device; then this would result
  in use-after-free,
- Or we allocate something specifically for this checking, so lingering
  requests would at most create spurious latency events and memory
  leaks, but this would require an additional heap allocation per
  request that we would probably want to avoid.

So ideally we could use latency cookies and could statically verify that
they are always finalized when the request is done, but doing this in C
may well be impossible.


So, because it is basically impossible (or at least it would be very
hard, and presumably require a large refactoring) to guarantee, without
additional heap allocations, that a list of active requests won’t run
into catastrophic use-after-frees, this series does the much simpler
version first, which is to just raise an event when a request *finishes*
and took more than a user-defined latency threshold.


(PS: The nice thing about throwing an alert while the request is still
going on would be that it could allow us to also stop the VM in case of
excessive latency, before the request completes, so the guest would be
shielded from such excessive latency.  This might be useful for Windows
guests that just have a maximum request lantency before throwing a
BSOD.)


Hanna Czenczek (9):
  block/accounting: Add offset to BlockAcctCookie
  qapi/block: Add IoAccountingOperation enum
  qapi/block: Add BLOCK_IO_DELAY event
  block-backend: Public blk_get_attached_dev_path()
  block/accounting: Add BB field to latency checker
  block/accounting: Emit BLOCK_IO_DELAY event
  block: Add delay-alert-ms property
  block/accounting: Move latency_ns override down
  iotests: Add delay-alert test

 qapi/block.json                          |  52 +++++++++
 include/block/accounting.h               |  20 +++-
 include/hw/block/block.h                 |   5 +-
 include/system/block-backend-io.h        |   9 ++
 include/system/dma.h                     |   2 +-
 block/accounting.c                       |  60 ++++++++--
 block/block-backend.c                    |   8 +-
 blockdev.c                               |  16 ++-
 hw/block/block.c                         |   4 +-
 hw/block/dataplane/xen-block.c           |   4 +-
 hw/block/virtio-blk.c                    |  15 +--
 hw/ide/ahci.c                            |   6 +-
 hw/ide/atapi.c                           |   9 +-
 hw/ide/core.c                            |   9 +-
 hw/ide/macio.c                           |  15 ++-
 hw/nvme/ctrl.c                           |  43 ++++---
 hw/nvme/dif.c                            |   8 +-
 hw/scsi/scsi-disk.c                      |  28 +++--
 qemu-io-cmds.c                           |  14 +--
 system/dma-helpers.c                     |   4 +-
 tests/unit/test-block-accounting.c       |   2 +-
 tests/qemu-iotests/172.out               |  38 +++++++
 tests/qemu-iotests/tests/delay-alert     | 136 +++++++++++++++++++++++
 tests/qemu-iotests/tests/delay-alert.out |  46 ++++++++
 24 files changed, 472 insertions(+), 81 deletions(-)
 create mode 100755 tests/qemu-iotests/tests/delay-alert
 create mode 100644 tests/qemu-iotests/tests/delay-alert.out

-- 
2.55.0