Re: [RFC v3 0/6] virtio,vhost: Add VIRTIO_F_IN_ORDER support

[Lei Yang]

QE tested this series with packed=on/off, in_order=true and vhost=off
under regression tests, everything are works fine.

Tested-by: Lei Yang <leiyang@redhat.com>

On Mon, Apr 8, 2024 at 11:34 PM Jonah Palmer <jonah.palmer@oracle.com> wrote:
>
> 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 has returned from
> processing and an "unused" element being an element that has not yet
> been processed), we added a boolean '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).
>
> ---
> v3: Add elements to used_elems during virtqueue_split/packed_pop
>     Replace current_seq_idx usage with vq->last_avail_idx
>     Remove used_seq_idx, leverage used_idx and last_avail_idx for
>     searching used_elems
>     Remove seq_idx in VirtQueueElement
>     Add boolean to VirtQueueElement to signal element status
>     Add virtqueue_ordered_fill/flush functions for ordering
>
> v2: Use a VirtQueue's used_elems array as a buffer mechanism
>
> v1: Implement custom GLib GHashTable as a buffer mechanism
>
> 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           | 118 ++++++++++++++++++++++++++++++++++-
>  include/hw/virtio/virtio.h   |   5 +-
>  net/vhost-vdpa.c             |   1 +
>  9 files changed, 127 insertions(+), 4 deletions(-)
>
> --
> 2.39.3
>