split-ring: document in-order operation

The point of in-order for the device is to be able to skip
writing out some used descriptors, but the documentation
that we have was put in the packed ring section and
written in a way specific to the packed ring.

Writing it in a generic way is tricky, for now this patch just copies
the text from the packed ring to the split ring section, with minor
tweaks.

Fixes: https://github.com/oasis-tcs/virtio-spec/issues/30
Suggested-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

1 files changed, 28 insertions, 0 deletions

diff --git a/split-ring.tex b/split-ring.tex
index 6f4a0a6..4e373d6 100644
--- a/split-ring.tex
+++ b/split-ring.tex
@@ -466,6 +466,34 @@ that uninitialized memory has been overwritten when it has not.
 The driver MUST NOT make assumptions about data in device-writable buffers
 beyond the first \field{len} bytes, and SHOULD ignore this data.
 
+\subsection{In-order use of descriptors}
+\label{sec:Basic Facilities of a Virtio Device / Virtqueues / In-order use of descriptors}
+
+Some devices always use descriptors in the same order in which
+they have been made available. These devices can offer the
+VIRTIO_F_IN_ORDER feature. If negotiated, this knowledge allows
+devices to notify the use of a batch of buffers to the driver by
+only writing out a single used ring entry with the \field{id}
+corresponding to the head entry of the
+descriptor chain describing the last buffer in the batch.
+
+The device then skips forward in the ring according to the size of
+the batch. Accordingly, it increments the used \field{idx} by the
+size of the batch.
+
+The driver needs to look up the used \field{id} and
+calculate the batch size to be able to advance to where the next
+used ring entry will be written by the device.
+
+This will result in the used ring entry at an offset matching the
+first available ring entry in the batch, the used ring entry for
+the next batch at an offset matching the first available ring
+entry in the next batch, etc.
+
+The skipped buffers (for which no used ring entry was written)
+are assumed to have been used (read or written) by the
+device completely.
+
 \subsection{Available Buffer Notification Suppression}\label{sec:Basic Facilities of a Virtio Device / Virtqueues / Virtqueue Notification Suppression}
 
 The device can suppress available buffer notifications in a manner