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(-) -- 2.39.3