[PATCH v2 0/5] 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 create a device with config->features different than 0 without
  fetch the api version (so API == 0) and with API_VERSION set to 1
  (-EINVAL returned from VDUSE_CREATE_DEV ioctl).
* Test regular initialization of single queue devices with V2 with and
  without VDUSE_F_QUEUE_READY set in device->config.
* 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").

This series depends on
https://lore.kernel.org/lkml/20260119143306.1818855-1-eperezma@redhat.com/

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 (5):
  vduse: store control device pointer
  vduse: Add API v2 definition
  vduse: add VDUSE_GET_FEATURES ioctl
  vduse: advertise API V2 support
  vduse: add F_QUEUE_READY feature

 drivers/vdpa/vdpa_user/vduse_dev.c | 66 +++++++++++++++++++++++++++---
 include/uapi/linux/vduse.h         | 29 ++++++++++++-
 2 files changed, 88 insertions(+), 7 deletions(-)

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2.53.0