packed ring layout spec

This is a proposal to implement an alternative ring layout. The idea is to have a r/w descriptor in a ring structure, replacing the used and available ring, index and descriptor buffer.

This is more efficient and easier for devices to implement than the 1.0 layout.

Additionally, a new feature flag is proposed that makes devices promise to process descriptors in-order. With this feature drivers can also be made simpler and more efficient.

Discussion and performance analysis of this is in Michael Tsirkin's kvm forum 2016 and 2017 presentations.

---

This revision addresses review comments on v9.

Thanks a lot to all reviewers of earlier revisions!

I plan to start voting on this shortly.

A compiled version can
be found under https://github.com/oasis-tcs/virtio-docs.git

See

virtio-v1.1-packed-wd10-diff.pdf virtio-v1.1-packed-wd10.pdf

for redline and clean versions, respectively.

If you are interested in changes from v9, that's in

virtio-v1.1-packed-w09-to-wd10-diff.pdf

in the same directory.

Note: please do not try to edit the pdf and post comments in the edited file. Please post comments in a text format, as pdfs are not archived with the list.

TODO: support for actual passthrough devices will likely require more new features, such as requirement for stronger memory barriers.

Changes since v9:
- corrected pseudo-code to work correctly without IN_ORDER (since that's what the accompanying text says).
- new bit-field notation
- update format for the event suppression structure
- prefix packed ring structures with pvirtq_ for consistency and to avoid confusion with the split ring structures
- deferred NOTIFICATION_DATA patches - will post separately, they need more review by s390 editor, and proof of concept code is not ready yet (needs host kernel support).

Note: should this proposal be accepted and approved, one or more
claims disclosed to the TC admin and listed on the Virtio TC IPR page https://github.com/oasis-tcs/virtio-admin/blob/master/IPR.md
might become Essential Claims.

Michael S. Tsirkin (13):
introduction: document bitfield notation
content: move 1.0 queue format out to a separate section
content: move ring text out to a separate file
content: move virtqueue operation description
content: len -> used length, used ring -> vq
content: generalize transport ring part naming
content: generalize rest of text
split-ring: generalize text
split-ring: typo: aligment
packed virtqueues: more efficient virtqueue layout
content: in-order buffer use
packed-ring: add in order support
split-ring: in order feature

 conformance.tex  |   5 +-
 content.tex      | 808 ++++++++-----------------------------------------------
 introduction.tex |  41 +++
 packed-ring.tex  | 714 ++++++++++++++++++++++++++++++++++++++++++++++++
 split-ring.tex   | 689 +++++++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 1563 insertions(+), 694 deletions(-)
 create mode 100644 packed-ring.tex
 create mode 100644 split-ring.tex

MST


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