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Subject: RE: [virtio-dev] [PATCH v10 13/13] split-ring: in order feature
Missed replying to the lists. Sorry. > From: Michael S. Tsirkin <mst@redhat.com> > Sent: 28. marts 2018 16:39 > > On Wed, Mar 28, 2018 at 08:23:38AM +0000, Lars Ganrot wrote: > > Hi Michael et al > > > > > Behalf Of Michael S. Tsirkin > > > Sent: 9. marts 2018 22:24 > > > > > > 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> > > > --- > > [snip] > > > > > > +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. > > > > > > > The use of VIRTIO_F_IN_ORDER for split-ring can eliminate some accesses > to the virtq_avail.ring and virtq_used.ring. However I'm wondering if the > proposed descriptor ordering for multi-element buffers couldn't be tweaked > to be more HW friendly. Currently even with the VIRTIO_F_IN_ORDER > negotiated, there is no way of knowing if, or how many chained descriptors > follow the descriptor pointed to by the virtq_avail.idx. A chain has to be > inspected one descriptor at a time until > virtq_desc.flags[VIRTQ_DESC_F_NEXT]=0. This is awkward for HW offload, > where you want to DMA all available descriptors in one shot, instead of > iterating based on the contents of received DMA data. As currently defined, > HW would have to find a compromise between likely chain length, and cost > of additional DMA transfers. This leads to a performance penalty for all > chained descriptors, and in case the length assumption is wrong the impact > can be significant. > > > > Now, what if the VIRTIO_F_IN_ORDER instead required chained buffers to > place the last element at the lowest index, and the head-element (to which > virtq_avail.idx points) at the highest index? Then all the chained element > descriptors would be included in a DMA of the descriptor table from the > previous virtq_avail.idx+1 to the current virtq_avail.idx. The "backward" > order of the chained descriptors shouldn't pose an issue as such (at least not > in HW). > > > > Best Regards, > > > > -Lars > > virtq_avail.idx is still an index into the available ring. > > I don't really see how you can use virtq_avail.idx to guess the placement of a > descriptor. > > I suspect the best way to optimize this is to include the relevant data with the > VIRTIO_F_NOTIFICATION_DATA feature. > Argh, naturally. For HW offload I'd want to avoid notifications for buffer transfer from host to device, and hoped to just poll virtq_avail.idx directly. A split virtqueue with VITRIO_F_IN_ORDER will maintain virtq_avail.idx==virtq_avail.ring[idx] as long as there is no chaining. It would be nice to allow negotiating away chaining, i.e add a VIRTIO_F_NO_CHAIN. If negotiated, the driver agrees not to use chaining, and as a result (of IN_ORDER and NO_CHAIN) both device and driver can ignore the virtq_avail.ring[]. > > > --------------------------------------------------------------------- > > To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org > > For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org
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