Need direct callbacks from socket upcalls via ksocket
From the original Joyent bug:
We'd like to be able receive direct callbacks from a ksocket as opposed to having to read and write.
Work by Robert Mustacchi
This provides an alternative way for a ksocket to directly receive data when it arrives in sockfs, rather than having it queued in a socket buffer that it must separately poll; the result being that no data ends up being queued in sockfs.
The initial consumer for this will be the #13500 (network overlays) feature which is being upstreamed from Joyent SmartOS via https://github.com/illumos/ipd/blob/master/ipd/0018/README.md
The current proposed changes for #13500 are available to view on Gerrit at https://code.illumos.org/c/illumos-gate/+/1972 - this has not been sent out for wider review pending completion of testing (in particular interoperability with SmartOS and OmniOS, which side-pulled this in 2019).