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Diffstat (limited to 'packed-ring.tex')
| -rw-r--r-- | packed-ring.tex | 24 |
1 files changed, 24 insertions, 0 deletions
diff --git a/packed-ring.tex b/packed-ring.tex index ebdba09..4b3d9d9 100644 --- a/packed-ring.tex +++ b/packed-ring.tex @@ -272,6 +272,30 @@ Buffer ID is also reserved and is ignored by the device. In Descriptors with VIRTQ_DESC_F_INDIRECT set VIRTQ_DESC_F_WRITE is reserved and is ignored by the device. +\subsection{In-order use of descriptors} +\label{sec:Packed 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 descriptor with the Buffer ID +corresponding to the last descriptor in the batch. + +Device then skips forward in the ring according to the size of +the batch. Driver needs to look up the used Buffer ID and +calculate the batch size to be able to advance to where the next +used descriptor will be written by the device. + +This will result in the used descriptor overwriting the first +available descriptor in the batch, the used descriptor for the +next batch overwriting the first available descriptor in the next +batch, etc. + +The skipped buffers (for which no used descriptor was written) +are assumed to have been used (read or written) by the +device completely. + \subsection{Multi-buffer requests} \label{sec:Packed Virtqueues / Multi-buffer requests} Some devices combine multiple buffers as part of processing of a |