On Tue, Apr 03, 2018 at 07:19:47AM +0000, Lars Ganrot wrote: > > From: virtio-dev@lists.oasis-open.org <virtio-dev@lists.oasis-open.org> On > > Behalf Of Michael S. Tsirkin > > Sent: 29. marts 2018 21:13 > > > > On Thu, Mar 29, 2018 at 06:23:28PM +0000, Lars Ganrot wrote: > > > > > > > > > > From: Michael S. Tsirkin <m...@redhat.com> > > > > Sent: 29. marts 2018 16:42 > > > > > > > > On Wed, Mar 28, 2018 at 04:12:10PM +0000, Lars Ganrot wrote: > > > > > Missed replying to the lists. Sorry. > > > > > > > > > > > From: Michael S. Tsirkin <m...@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 <m...@redhat.com> > > > > > > > > Reviewed-by: Cornelia Huck <coh...@redhat.com> > > > > > > > > Reviewed-by: Stefan Hajnoczi <stefa...@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. > > > > > > > > BTW, for split rings VIRTIO_F_NOTIFICATION_DATA just copies the > > > > index right now. > > > > > > > > Do you have an opinion on whether we should change that for in-order? > > > > > > > > > > Maybe I should think more about this, however adding the last element > > descriptor index, would be useful to accelerate interfaces that frequently > > use chaining (from a HW DMA perspective at least). > > > > > > > > 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[]. > > > > > > > > My point was that device can just assume no chains, and then fall > > > > back on doing extra reads upon encountering a chain. > > > > > > > > > > Yes, you are correct that the HW can speculatively use virtq_avail.idx as > > > the > > >direct index to the descriptor table, and if it encounters a chain, revert > > >to > > >using the virtq_avail.ring[] in the traditional way, and this would work > > >without the feature-bit. > > > > Sorry that was not my idea. > > > > Device should not need to read the ring at all. > > It reads the descriptor table and counts the descriptors without the next > > bit. > > Once the count reaches the available index, it stops. > > > > Agreed, that would work as well, with the benefit of keeping the ring out of > the loop. > > > > > > However the driver would not be able to optimize away the writing of > > > the virtq_avail.ring[] (=cache miss) > > > > > > BTW writing is a separate question (there is no provision in the spec to > > skip > > writes) but device does not have to read the ring. > > > > Yes, I understand the spec currently does not allow writes to be skipped, but > I'm wondering if that ought to be reconsidered for optimization features such > as IN_ORDER and NO_CHAIN?
Why not just use the packed ring then? > By opting for such features, both driver and > device acknowledge their willingness to accept reduced flexibility for > improved performance. Why not then make sure they get the biggest bang for > their buck? I would expect up to 20% improvement over PCIe (virtio-net, > single 64B packet), if the device does not have to write to virtq_used.ring[] > on > transmit, and bandwidth over PCI is a very precious resource in e.g. virtual > switch offload with east-west acceleration (for a discussion see Intel's > white- > paper 335625-001). Haven't looked at it yet but we also need to consider the complexity, see below. > > Without device accesses ring will not be invaliated in cache so no misses > > hopefully. > > > > > unless a NO_CHAIN feature has > > > been negotiated. > > > The IN_ORDER by itself has already eliminated the need to maintain the > > > TX virtq_used.ring[], since the buffer order is always known by the > > > driver. > > > With a NO_CHAIN feature-bit both RX and TX virtq_avail.ring[] related > > > cache-misses could be eliminated. I.e. > > > looping a packet over a split virtqueue would just experience 7 driver > > > cache misses, down from 10 in Virtio v1.0. Multi-element buffers would > > > still be possible provided INDIRECT is negotiated. > > > > > > NO_CHAIN might be a valid optimization, it is just unfortunately somewhat > > narrow in that devices that need to mix write and read descriptors in the > > same ring (e.g. storage) can not use this feature. > > > > Yes, if there was a way of making indirect buffers support it, that would be > ideal. However I don't see how that can be done without inline headers in > elements to hold their written length. Kind of like it's done with with packed ring? > At the same time storage would not be hurt by it even if they are unable to > benefit from this particular optimization, It will be hurt if it uses shared code paths which potentially take up more cache, or if bugs are introduced. > and as long as there is a substantial > use case/space that benefit from an optimization, it ought to be considered. > I believe virtual switching offload with virtio-net devices over PCIe is such > a > key use-case. It looks like the packed ring addresses the need nicely, while being device-independent. > > > > > > > > > > > > > > > > > > > > > > > -------------------------------------------------------------- > > > > > > > ---- > > > > > > > --- To unsubscribe, e-mail: > > > > > > > virtio-dev-unsubscr...@lists.oasis-open.org > > > > > > > For additional commands, e-mail: > > > > > > > virtio-dev-h...@lists.oasis-open.org > > > > > > > > > > ------------------------------------------------------------------ > > > > > --- To unsubscribe, e-mail: > > > > > virtio-dev-unsubscr...@lists.oasis-open.org > > > > > For additional commands, e-mail: > > > > > virtio-dev-h...@lists.oasis-open.org > > > Disclaimer: This email and any files transmitted with it may contain > > confidential information intended for the addressee(s) only. The information > > is not to be surrendered or copied to unauthorized persons. If you have > > received this communication in error, please notify the sender immediately > > and delete this e-mail from your system. > > > > --------------------------------------------------------------------- > > To unsubscribe, e-mail: virtio-dev-unsubscr...@lists.oasis-open.org > > For additional commands, e-mail: virtio-dev-h...@lists.oasis-open.org > > Disclaimer: This email and any files transmitted with it may contain > confidential information intended for the addressee(s) only. The information > is not to be surrendered or copied to unauthorized persons. If you have > received this communication in error, please notify the sender immediately > and delete this e-mail from your system. --------------------------------------------------------------------- To unsubscribe, e-mail: virtio-dev-unsubscr...@lists.oasis-open.org For additional commands, e-mail: virtio-dev-h...@lists.oasis-open.org