From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: virtio-comment-return-1395-cohuck=redhat.com@lists.oasis-open.org Sender: List-Post: List-Help: List-Unsubscribe: List-Subscribe: From: Lars Ganrot Date: Tue, 11 Aug 2020 14:53:39 +0000 Message-ID: <3e1f1c3f6b3a44daabf3cfca1d3e3f66@napatech.com> References: <20200810161501.1572834-1-mst@redhat.com> <20200810185928.2e722231.cohuck@redhat.com> <20200811042047-mutt-send-email-mst@kernel.org> In-Reply-To: <20200811042047-mutt-send-email-mst@kernel.org> Content-Language: en-US MIME-Version: 1.0 Subject: [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: "Michael S. Tsirkin" , Cornelia Huck Cc: "virtio-comment@lists.oasis-open.org" , "virtio-dev@lists.oasis-open.org" , "virtio@lists.oasis-open.org" List-ID: Hi Michael, Short comment below. BR, -Lars > From: virtio-dev@lists.oasis-open.org O= n > Behalf Of Michael S. Tsirkin > Sent: 11. august 2020 10:23 >=20 > 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 order. > > > > > > 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 fault 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? >=20 > Good point. I think so, yes. Will document. Isn't it more natural to think of VIRTIO_F_IN_ORDER as the simple case whic= h 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 bi= t 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. 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/