Metadata and signalling channels for Zephyr virtio-backends on Xen

[Alex Bennée <alex.bennee@xxxxxxxxxx>]

Hi Stefano,

Vincent gave an update on his virtio-scmi work at the last Stratos sync
call and the discussion moved onto next steps. Currently the demo setup
is intermediated by a double-ended vhost-user daemon running on the
devbox acting as a go between a number of QEMU instances representing
the front and back-ends. You can view the architecture with Vincents
diagram here:

  
https://docs.google.com/drawings/d/1YSuJUSjEdTi2oEUq4oG4A9pBKSEJTAp6hhcHKKhmYHs/edit?usp=sharing

The key virtq handling is done over the special carve outs of shared
memory between the front end and guest. However the signalling is
currently over a virtio device on the backend. This is useful for the
PoC but obviously in a real system we don't have a hidden POSIX system
acting as a go between not to mention the additional latency it causes
with all those context switches.

I was hoping we could get some more of the Xen experts to the next
Stratos sync (17th Feb) to go over approaches for a properly hosted on
Xen approach. From my recollection (Vincent please correct me if I'm
wrong) of last week the issues that need solving are:

 * How to handle configuration steps as FE guests come up

The SCMI server will be a long running persistent backend because it is
managing real HW resources. However the guests may be ephemeral (or just
restarted) so we can't just hard-code everything in a DTB. While the
virtio-negotiation in the config space covers most things we still need
information like where in the guests address space the shared memory
lives and at what offset into that the queues are created. As far as I'm
aware the canonical source of domain information is XenStore
(https://wiki.xenproject.org/wiki/XenStore) but this relies on a Dom0
type approach. Is there an alternative for dom0less systems or do we
need a dom0-light approach, for example using STR-21 (Ensure Zephyr can
run cleanly as a Dom0 guest) providing just enough services for FE's to
register metadata and BE's to read it?

 * How to handle mapping of memory

AIUI the Xen model is the FE guest explicitly makes grant table requests
to expose portions of it's memory to other domains. Can the BE query the
hypervisor itself to discover the available grants or does it require
coordination with Dom0/XenStore for that information to be available to
the BE domain?

 * How to handle signalling

I guess this requires a minimal implementation of the IOREQ calls for
Zephyr so we can register the handler in the backend? Does the IOREQ API
allow for a IPI style notifications using the global GIC IRQs?

Forgive the incomplete notes from the Stratos sync, I was trying to type
while participating in the discussion so hopefully this email captures
what was missed:

  
https://linaro.atlassian.net/wiki/spaces/STR/pages/28682518685/2022-02-03+Project+Stratos+Sync+Meeting+Notes

Vincent, anything to add?

-- 
Alex Bennée