[PATCH v4 0/4] Add queue ready message to VDUSE

["Eugenio Pérez" <eperezma@redhat.com>]

This series introduces a new VDUSE message for VDUSE userland instance
to detect when a VirtQueue (VQ) is enabled, replacing the polling.

VirtIO net devices' dataplane is started after the control virtqueue so
QEMU can apply the configuration in the destination of a Live Migration.
Without this feature, the VDUSE instance must poll the VQs to check when
(and if) a VQ has been enabled.

This series also implements VDUSE feature flags allowing the VDUSE
devices to opt-in to the VQ ready message.  Devices that opt-in to this
feature will receive explicit notifications when a VQ is ready.  Devices
that do not set this flag remain unaffected, ensuring backward
compatibility without indefinitely incrementing API versions.

The VDUSE features is a 64 bit bitmap for simplicity, the same way as
vhost and vhost-net started.  It can be extended as a flexible array of
bits when we reach so many features, but it seems unlikely at this
point.

Error cases tested:
* Call VDUSE_GET_FEATURES without get the API VERSION (so API == 0) and
  with API VERSION set to 1 with VDUSE_SET_API_VERSION (-EINVAL returned
  from VDUSE_GET_FEATURES ioctl).
* Try to set invalid features.
* Test regular initialization of single queue devices with and without
  VDUSE_F_QUEUE_READY set.
* Test expected behavior when VDUSE userland instance returns
  VDUSE_REQ_RESULT_FAILED from VDUSE_SET_VQ_READY message.
* Repeat all the tests with multiqueue devices, by reverting
  56e71885b0349 ("vduse: Temporarily fail if control queue feature requested").

v4:
* Create its own spinlock for vq->ready instead of relying on kick and
  call spinlocks.
* Add VDUSE_SET_FEATURES ioctl and the corresponding validation paths instead
  of using device config.

v3:
* Remove API_VERSION bump to 2
* Add comment about struct vduse_dev_config:vduse_features is only valid
  if VDUSE_GET_FEATURES success.

v2:
* Fix comment of vduse_dev_request.vq_ready
* Set vq_ready before sending the message to the VDUSE userland
  instance, avoiding the need for SMP sync after receiving the message.
* Return -EINVAL if control ioctl called with version < 2, so userland
  visible reply is kept (Jason).

Eugenio Pérez (4):
  vduse: store control device pointer
  vduse: add VDUSE_GET_FEATURES ioctl
  vduse: add VDUSE_SET_FEATURES ioctl
  vduse: add F_QUEUE_READY feature

 drivers/vdpa/vdpa_user/vduse_dev.c | 111 +++++++++++++++++++++++------
 include/uapi/linux/vduse.h         |  24 +++++++
 2 files changed, 115 insertions(+), 20 deletions(-)

-- 
2.55.0