The goal of these patches is to add support to a variety of virtio and
vhost devices for the VIRTIO_F_IN_ORDER transport feature. This feature
indicates that all buffers are used by the device in the same order in
which they were made available by the driver.

These patches attempt to implement a generalized, non-device-specific
solution to support this feature.

The core feature behind this solution is a buffer mechanism in the form
of a VirtQueue's used_elems VirtQueueElement array. This allows devices
who always use buffers in-order by default to have a minimal overhead
impact. Devices that may not always use buffers in-order likely will
experience a performance hit. How large that performance hit is will
depend on how frequently elements are completed out-of-order.

A VirtQueue whose device uses this feature will use its used_elems
VirtQueueElement array to hold used VirtQueueElements. The index that
used elements are placed in used_elems is the same index on the
used/descriptor ring that would satisfy the in-order requirement. In
other words, used elements are placed in their in-order locations on
used_elems and are only written to the used/descriptor ring once the
elements on used_elems are able to continue their expected order.

To differentiate between a "used" and "unused" element on the used_elems
array (a "used" element being an element that has returned from
processing and an "unused" element being an element that has not yet
been processed), we added a boolean 'in_order_filled' member to the
VirtQueueElement struct. This flag is set to true when the element comes
back from processing (virtqueue_ordered_fill) and then set back to false
once it's been written to the used/descriptor ring
(virtqueue_ordered_flush).

---
v2: Make 'in_order_filled' more descriptive.
    Change 'j' to more descriptive var name in virtqueue_split_pop.
    Use more definitive search conditional in virtqueue_ordered_fill.
    Avoid code duplication in virtqueue_ordered_flush.

v1: Move series from RFC to PATCH for submission.

Jonah Palmer (6):
  virtio: Add bool to VirtQueueElement
  virtio: virtqueue_pop - VIRTIO_F_IN_ORDER support
  virtio: virtqueue_ordered_fill - VIRTIO_F_IN_ORDER support
  virtio: virtqueue_ordered_flush - VIRTIO_F_IN_ORDER support
  vhost,vhost-user: Add VIRTIO_F_IN_ORDER to vhost feature bits
  virtio: Add VIRTIO_F_IN_ORDER property definition

 hw/block/vhost-user-blk.c    |   1 +
 hw/net/vhost_net.c           |   2 +
 hw/scsi/vhost-scsi.c         |   1 +
 hw/scsi/vhost-user-scsi.c    |   1 +
 hw/virtio/vhost-user-fs.c    |   1 +
 hw/virtio/vhost-user-vsock.c |   1 +
 hw/virtio/virtio.c           | 119 ++++++++++++++++++++++++++++++++++-
 include/hw/virtio/virtio.h   |   6 +-
 net/vhost-vdpa.c             |   1 +
 9 files changed, 129 insertions(+), 4 deletions(-)

-- 
2.39.3