diff options
| author | Michael S. Tsirkin <mst@redhat.com> | 2018-03-09 23:23:40 +0200 |
|---|---|---|
| committer | Michael S. Tsirkin <mst@redhat.com> | 2018-03-20 02:29:10 +0200 |
| commit | ad89d36f241c9f1583bf625e6be3fb5226dec655 (patch) | |
| tree | df8cb38053f45692aa90d47695105aa2a2f9148e /split-ring.tex | |
| parent | 911127631f58da756a109121a62a75894b38480b (diff) | |
split-ring: in order feature
For a split ring, require that drivers use descriptors in order too.
This allows devices to skip reading the available ring.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Approved-by: https://www.oasis-open.org/apps/org/workgroup/virtio/ballot.php?id=3177
Fixes: https://github.com/oasis-tcs/virtio-spec/issues/3
Diffstat (limited to 'split-ring.tex')
| -rw-r--r-- | split-ring.tex | 18 |
1 files changed, 18 insertions, 0 deletions
diff --git a/split-ring.tex b/split-ring.tex index 87ecee2..df278fe 100644 --- a/split-ring.tex +++ b/split-ring.tex @@ -203,6 +203,10 @@ struct virtq_desc { The number of descriptors in the table is defined by the queue size for this virtqueue: this is the maximum possible descriptor chain length. +If VIRTIO_F_IN_ORDER has been negotiated, driver uses +descriptors in ring order: starting from offset 0 in the table, +and wrapping around at the end of the table. + \begin{note} The legacy \hyperref[intro:Virtio PCI Draft]{[Virtio PCI Draft]} referred to this structure as vring_desc, and the constants as @@ -218,6 +222,12 @@ purposes). Drivers MUST NOT add a descriptor chain over than $2^{32}$ bytes long in total; this implies that loops in the descriptor chain are forbidden! +If VIRTIO_F_IN_ORDER has been negotiated, and when making a +descriptor with VRING_DESC_F_NEXT set in \field{flags} at offset +$x$ in the table available to the device, driver MUST set +\field{next} to $0$ for the last descriptor in the table +(where $x = queue\_size - 1$) and to $x + 1$ for the rest of the descriptors. + \subsubsection{Indirect Descriptors}\label{sec:Basic Facilities of a Virtio Device / Virtqueues / The Virtqueue Descriptor Table / Indirect Descriptors} Some devices benefit by concurrently dispatching a large number @@ -247,6 +257,10 @@ chained by \field{next}. An indirect descriptor without a valid \field{next} A single indirect descriptor table can include both device-readable and device-writable descriptors. +If VIRTIO_F_IN_ORDER has been negotiated, indirect descriptors +use sequential indices, in-order: index 0 followed by index 1 +followed by index 2, etc. + \drivernormative{\paragraph}{Indirect Descriptors}{Basic Facilities of a Virtio Device / Virtqueues / The Virtqueue Descriptor Table / Indirect Descriptors} The driver MUST NOT set the VIRTQ_DESC_F_INDIRECT flag unless the VIRTIO_F_INDIRECT_DESC feature was negotiated. The driver MUST NOT @@ -259,6 +273,10 @@ the device. A driver MUST NOT set both VIRTQ_DESC_F_INDIRECT and VIRTQ_DESC_F_NEXT in \field{flags}. +If VIRTIO_F_IN_ORDER has been negotiated, indirect descriptors +MUST appear sequentially, with \field{next} taking the value +of 1 for the 1st descriptor, 2 for the 2nd one, etc. + \devicenormative{\paragraph}{Indirect Descriptors}{Basic Facilities of a Virtio Device / Virtqueues / The Virtqueue Descriptor Table / Indirect Descriptors} The device MUST ignore the write-only flag (\field{flags}\&VIRTQ_DESC_F_WRITE) in the descriptor that refers to an indirect table. |