[Qemu-devel] [RFC PATCH 0/7] Fix transactional snapshot with virtio-blk dataplane

[Fam Zheng <famz@redhat.com>]

Reported by Paolo.

Unlike the iohandler in main loop, iothreads currently process the event
notifier used as virtio-blk ioeventfd in all nested aio_poll. This is dangerous
without proper protection, because guest requests could sneak to block layer
where they mustn't.

For example, a QMP transaction may involve multiple bdrv_drain_all() in
handling the list of AioContext it works on. If an aio_poll in one of the
bdrv_drain_all() happens to process a guest VQ kick by dispatching the
ioeventfd event, a new guest write is then submitted, and voila, the
transaction semantics is violated.

This series avoids this problem by disabling virtio-blk handlers during
bdrv_drain_all() and transactions.

Notes:

If the general approach is right, other transaction types could get the
blockers similarly, in next revision. And some related bdrv_drain_all() could
also be changed to bdrv_drain().

virtio-scsi-dataplane will be a bit more complicated, but still doable.  It
would probably need one more interface abstraction between scsi-disk, scsi-bus
and virtio-scsi.

Although other devices don't have a pause/resume callback yet, the
blk_check_request, which returns -EBUSY if "device io" op blocker is set, could
hopefully cover most cases already.

Timers and block jobs also generate IO, but it should be fine as long as they
don't change guest visible data, which is true AFAICT.


Fam Zheng (7):
  block: Add op blocker type "device IO"
  block: Block "device IO" during bdrv_drain and bdrv_drain_all
  block: Add op blocker notifier list
  block-backend: Add blk_op_blocker_add_notifier
  virtio-blk: Move complete_request to 'ops' structure
  virtio-blk: Don't handle output when there is "device IO" op blocker
  blockdev: Add "device IO" op blocker during snapshot transaction

 block.c                         | 20 ++++++++++++
 block/block-backend.c           | 10 ++++++
 block/io.c                      | 12 +++++++
 blockdev.c                      |  7 +++++
 hw/block/dataplane/virtio-blk.c | 36 ++++++++++++++++++---
 hw/block/virtio-blk.c           | 69 +++++++++++++++++++++++++++++++++++++++--
 include/block/block.h           |  9 ++++++
 include/block/block_int.h       |  3 ++
 include/hw/virtio/virtio-blk.h  | 17 ++++++++--
 include/sysemu/block-backend.h  |  2 ++
 10 files changed, 174 insertions(+), 11 deletions(-)

-- 
1.9.3