From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: virtio-comment-return-1398-cohuck=redhat.com@lists.oasis-open.org Sender: List-Post: List-Help: List-Unsubscribe: List-Subscribe: From: Lars Ganrot Date: Wed, 12 Aug 2020 15:55:18 +0000 Message-ID: <71876de5ce464fbda5607af7c508d0d4@napatech.com> References: <20200810161501.1572834-1-mst@redhat.com> <20200810185928.2e722231.cohuck@redhat.com> <20200811042047-mutt-send-email-mst@kernel.org> <3e1f1c3f6b3a44daabf3cfca1d3e3f66@napatech.com> <49f4f4c248a844d4a51f97308adf19b3@napatech.com> <20200812145051.32922356.cohuck@redhat.com> In-Reply-To: <20200812145051.32922356.cohuck@redhat.com> Content-Language: en-US MIME-Version: 1.0 Subject: RE: [virtio-comment] Re: [virtio-dev] Re: [virtio] [PATCH RFC] VIRTIO_F_PARTIAL_ORDER for page fault handling Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable To: Cornelia Huck Cc: "Michael S. Tsirkin" , "virtio-comment@lists.oasis-open.org" , "virtio-dev@lists.oasis-open.org" , "virtio@lists.oasis-open.org" List-ID: > From: virtio-comment@lists.oasis-open.org open.org> On Behalf Of Cornelia Huck > Sent: 12. august 2020 14:51 >=20 > On Tue, 11 Aug 2020 15:43:44 +0000 > Lars Ganrot wrote: >=20 > > > From: virtio-comment@lists.oasis-open.org > > > On Behalf Of Lars Ganrot > > > Sent: 11. august 2020 16:54 > > > > > > > From: virtio-dev@lists.oasis-open.org > > > > On Behalf Of Michael S. Tsirkin > > > > Sent: 11. august 2020 10:23 > > > > > > > > On Mon, Aug 10, 2020 at 06:59:28PM +0200, Cornelia Huck wrote: > > > > > On Mon, 10 Aug 2020 12:15:15 -0400 "Michael S. Tsirkin" > > > > > wrote: > > > > > > > > > > > Devices that normally use buffers in order can benefit from > > > > > > ability to temporarily switch to handle some buffers out of ord= er. > > > > > > > > > > > > As a case in point, a networking device might handle RX > > > > > > buffers in order normally. However, should an access to an RX > > > > > > buffer cause a page fault (e.g. when using PRI), the device > > > > > > could benefit from ability to temporarily keep using following > > > > > > buffers in the ring (possibly with higher overhead) until the f= ault has > been resolved. > > > > > > > > > > > > Page faults allow more features such as THP, auto-NUMA, live > > > > > > migration. > > > > > > > > > > > > Out of order is of course already possible, however, IN_ORDER > > > > > > is currently required for descriptor batching where device > > > > > > marks a whole batch of buffers used in one go. > > > > > > > > > > > > The idea behind this proposal is to relax that requirement, > > > > > > allowing batching without asking device to be in orde rat all > > > > > > times, as > > > > > > follows: > > > > > > > > > > > > Device uses buffers in any order. Eventually when device > > > > > > detects that it has used all previously outstanding buffers, > > > > > > it sets a FLUSH flag on the last buffer used. If it set this > > > > > > flag on the last buffer used previously, and now uses a batch > > > > > > of descriptors in-order, it can now signal the last buffer > > > > > > used again setting the FLUSH > > > flag. > > > > > > > > > > > > Driver can detect in-order when it sees two FLUSH flags one > > > > > > after another. In other respects the feature is similar to > > > > > > IN_ORDER from the driver implementation POV. > > > > > > > > > > > > Signed-off-by: Michael S. Tsirkin > > > > > > --- > > > > > > content.tex | 9 ++++++++- > > > > > > packed-ring.tex | 23 +++++++++++++++++++++++ split-ring.tex > > > > > > | > > > > > > 26 > > > > > > ++++++++++++++++++++++++-- > > > > > > 3 files changed, 55 insertions(+), 3 deletions(-) > > > > > > > > > > > > diff --git a/content.tex b/content.tex index 91735e3..8494eb6 > > > > > > 100644 > > > > > > --- a/content.tex > > > > > > +++ b/content.tex > > > > > > @@ -296,7 +296,11 @@ \section{Virtqueues}\label{sec:Basic > > > > > > Facilities of a Virtio Device / Virtqueues} > > > > > > > > > > > > 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 > > > > > > +VIRTIO_F_IN_ORDER feature. Other devices sometimes use > > > > > > +descriptors in the same order in which they have been made > > > > > > +available. These devices can offer the VIRTIO_F_PARTIAL_ORDER > > > > > > +feature. If one of the features VIRTIO_F_IN_ORDER or > > > > > > +VIRTIO_F_PARTIAL_ORDER is > > > > negotiated, > > > > > > +this knowledge > > > > > > > > > > Do these two features conflict with each other? I.e., at most > > > > > one of them may be negotiated (or offered?) at a time? > > > > > > > > Good point. I think so, yes. Will document. > > > > > > Isn't it more natural to think of VIRTIO_F_IN_ORDER as the simple > > > case which always maintains ordered access, while the new feature > > > flag allows active control of when descriptors are ordered and when > > > not? To make it backward compatible let VIRTIO_F_IN_ORDER imply the > > > new bit is set, while the new bit set by itself without > > > VIRTIO_F_IN_ORDER set means only active control is offered. I guess > > > a name like VIRTIO_F_CTRL_ORDER would be more appropriate with this > interpretation. > > > > > > > On second thought that might be a bit backwards - how about: > > > > Legacy case: VIRTIO_F_IN_ORDER=3D=3D0/1 + VIRTIO_F_ORDER_RELAX=3D=3D0 T= his > > proposal: VIRTIO_F_IN_ORDER=3D=3D1 + VIRTIO_F_ORDER_RELAX=3D=3D1 Potent= ial > > future use: VIRTIO_F_???_ORDER=3D=3D1 + VIRTIO_F_ORDER_RELAX=3D=3D0/1 >=20 > What happens in the new device/old driver case? > - device offers IN_ORDER and PARTIAL_ORDER > - driver does not know PARTIAL_ORDER, accepts IN_ORDER > - device now only can do complete ordering >=20 > Maybe I don't understand the purpose of the new feature correctly, but I > thought it was for those devices that don't do full in-order, but can do = it for a > subset of buffers? As such, the two features can't really imply each othe= r: a > device offering IN_ORDER might not know about the new feature and its > mechanism, and a device offering the new feature, but not IN_ORDER > probably does so because it cannot support full IN_ORDER. >=20 > I think it makes the most sense if the device can offer both flags, but t= he > driver must only accept at most one of them? >=20 Yes, you are absolutely right. Keeping them as two mutually exclusive optio= ns is probably best. Another aspect of the proposal is that the functionality is only described = for the packed ring layout, but the concept should be equally applicable to= the split ring. The split ring does not have flags for each used ring elem= ent, so the FLUSH flag functionality needs to be a bit different. One way i= s to add an IN_ORDER-flag to the virtq_used.flags, that allows the device t= o signal when it starts to adhere to IN_ORDER buffer use. The driver acknow= ledges this when it is ready to fulfil its part of the IN_ORDER restriction= s by setting a new virtq_avail.flag: IN_ORDER_ACK. At this point both devic= e and driver can assume IN_ORDER optimization until the device clears IN_OR= DER. After clearing IN_ORDER, the device must wait until IN_ORDER_ACK is cl= eared before IN_ORDER can be asserted it again. This is a little less fine-grained than the packed-ring solution with the F= LUSH flag, but for infrequent scenarios like migration it should be OK. >=20 > This publicly archived list offers a means to provide input to the OASIS = Virtual > I/O Device (VIRTIO) TC. >=20 > In order to verify user consent to the Feedback License terms and to mini= mize > spam in the list archive, subscription is required before posting. >=20 > Subscribe: virtio-comment-subscribe@lists.oasis-open.org > Unsubscribe: virtio-comment-unsubscribe@lists.oasis-open.org > List help: virtio-comment-help@lists.oasis-open.org > List archive: https://lists.oasis-open.org/archives/virtio-comment/ > Feedback License: https://www.oasis- > open.org/who/ipr/feedback_license.pdf > List Guidelines: https://www.oasis-open.org/policies-guidelines/mailing-l= ists > Committee: https://www.oasis-open.org/committees/virtio/ > Join OASIS: https://www.oasis-open.org/join/ This publicly archived list offers a means to provide input to the=0D OASIS Virtual I/O Device (VIRTIO) TC.=0D =0D In order to verify user consent to the Feedback License terms and=0D to minimize spam in the list archive, subscription is required=0D before posting.=0D =0D Subscribe: virtio-comment-subscribe@lists.oasis-open.org=0D Unsubscribe: virtio-comment-unsubscribe@lists.oasis-open.org=0D List help: virtio-comment-help@lists.oasis-open.org=0D List archive: https://lists.oasis-open.org/archives/virtio-comment/=0D Feedback License: https://www.oasis-open.org/who/ipr/feedback_license.pdf= =0D List Guidelines: https://www.oasis-open.org/policies-guidelines/mailing-lis= ts=0D Committee: https://www.oasis-open.org/committees/virtio/=0D Join OASIS: https://www.oasis-open.org/join/