From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: virtio-dev-return-3679-cohuck=redhat.com@lists.oasis-open.org Sender: List-Post: List-Help: List-Unsubscribe: List-Subscribe: From: Lars Ganrot Date: Thu, 29 Mar 2018 18:23:28 +0000 Message-ID: <2552bfc5d77e4b789d08d3479c3baf01@napatech.com> References: <1520629942-36324-1-git-send-email-mst@redhat.com> <1520629942-36324-14-git-send-email-mst@redhat.com> <20180328173142-mutt-send-email-mst@kernel.org> <20180329173105-mutt-send-email-mst@kernel.org> In-Reply-To: <20180329173105-mutt-send-email-mst@kernel.org> Content-Language: en-US Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 Subject: RE: [virtio-dev] [PATCH v10 13/13] split-ring: in order feature To: "Michael S. Tsirkin" Cc: "virtio@lists.oasis-open.org" , "virtio-dev@lists.oasis-open.org" List-ID: > -----Original Message----- > From: Michael S. Tsirkin > Sent: 29. marts 2018 16:42 > To: Lars Ganrot > Cc: virtio@lists.oasis-open.org; virtio-dev@lists.oasis-open.org > Subject: Re: [virtio-dev] [PATCH v10 13/13] split-ring: in order feature >=20 > On Wed, Mar 28, 2018 at 04:12:10PM +0000, Lars Ganrot wrote: > > Missed replying to the lists. Sorry. > > > > > From: Michael S. Tsirkin > > > 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 t= oo. > > > > > This allows devices to skip reading the available ring. > > > > > > > > > > Signed-off-by: Michael S. Tsirkin > > > > > Reviewed-by: Cornelia Huck > > > > > Reviewed-by: Stefan Hajnoczi > > > > > --- > > > > [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 $x =3D queue\_size - 1$) and to $x + 1$ for the rest of t= he > 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]=3D0. 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. >=20 > BTW, for split rings VIRTIO_F_NOTIFICATION_DATA just copies the index > right now. >=20 > Do you have an opinion on whether we should change that for in-order? >=20 Maybe I should think more about this, however adding the last element descr= iptor index, would be useful to accelerate interfaces that frequently use c= haining (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=3D=3Dvirtq_avail.ring[idx] as long as there is no chainin= g. 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[]. >=20 > My point was that device can just assume no chains, and then fall back on > doing extra reads upon encountering a chain. > =20 Yes, you are correct that the HW can speculatively use virtq_avail.idx as t= he direct index to the descriptor table, and if it encounters a chain, reve= rt to using the virtq_avail.ring[] in the traditional way, and this would w= ork without the feature-bit.=20 However the driver would not be able to optimize away the writing of the vi= rtq_avail.ring[] (=3Dcache miss) unless a NO_CHAIN feature has been negotia= ted. 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 cac= he-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 negotiat= ed. >=20 >=20 > > > > > > > ------------------------------------------------------------------ > > > > --- To unsubscribe, e-mail: > > > > virtio-dev-unsubscribe@lists.oasis-open.org > > > > For additional commands, e-mail: > > > > virtio-dev-help@lists.oasis-open.org > > > > --------------------------------------------------------------------- > > To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org > > For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org --------------------------------------------------------------------- To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org