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Diffstat (limited to 'split-ring.tex')
| -rw-r--r-- | split-ring.tex | 28 |
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 |