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Subject: Re: [PATCH] split-ring: document in-order operation
On Thu, Dec 06, 2018 at 02:18:16PM -0500, Michael S. Tsirkin wrote: > 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. > > Suggested-by: Jason Wang <jasowang@redhat.com> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Fixes: https://github.com/oasis-tcs/virtio-spec/issues/30 > --- > split-ring.tex | 28 ++++++++++++++++++++++++++++ > 1 file changed, 28 insertions(+) > > 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 > -- > MST
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