[RFC v2 0/5] virtio,vhost: Add VIRTIO_F_IN_ORDER support

[Jonah Palmer]

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 frequent elements are completed out-of-order.

A VirtQueue whose device who 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 was already written to the
used/descriptor ring and an "unused" element being an element that
wasn't), we use an element's in_num and out_num values. If the sum of
these two values is greater than 0, the element is considered unused. If
the sum is 0, then the element is considered used and invalid. When we
find an order and write the element to the used/descriptor ring, we set
these two values to 0 to indicate that it's been used.

---
v2: Use a VirtQueue's used_elems array as a buffer mechanism

v1: Implement custom GLib GHashTable as a buffer mechanism

Jonah Palmer (5):
  virtio: Initialize sequence variables
  virtio: In-order support for split VQs
  virtio: In-order support for packed VQs
  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           | 118 +++++++++++++++++++++++++++++++----
 include/hw/virtio/virtio.h   |   5 +-
 net/vhost-vdpa.c             |   1 +
 9 files changed, 119 insertions(+), 12 deletions(-)

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2.39.3