[cross-project PATCH v2] NBD 64-bit extensions

[Eric Blake]

This is a cover letter for a set of multi-project patch series all
designed to implement 64-bit operations in NBD, and demonstrate
interoperability of the new extension between projects.

v1 of the project was attempted nearly a year ago:
https://lists.nongnu.org/archive/html/qemu-devel/2021-12/msg00453.html

Since then, I've addressed a lot of preliminary cleanups in libnbd to
make it easier to test things, and incorporated review comments issued
on v1, including:

- no orthogonality between simple/structured replies and 64-bit mode:
  once extended headers are negotiated, all transmission traffic (both
  from client to server and server to client) uses just one header
  size

- add support for the client to pass a payload on commands to the
  server, and demonstrate it by further implementing a way to pass a
  flag with NBD_CMD_BLOCK_STATUS that says the client is passing a
  payload to request status of just a subset of the negotiated
  contexts, rather than all possible contexts that were earlier
  negotiated during NBD_OPT_SET_META_CONTEXT

- tweaks to the header layouts: tweak block status to provide 64-bit
  flags values (although "base:allocation" will remain usable with
  just 32-bit flags); reply with offset rather than padding and with
  fields rearranged for maximal sharing between client and server
  layouts

- word-smithing of the NBD proposed protocol; split things into
  several smaller patches, where we can choose how much to take

- more unit tests added in qemu and libnbd

- the series end with RFC patches on whether to support 64-bit hole
  responses to NBD_CMD_READ, even though our current limitations say
  servers don't have to accept more than a 32M read request and
  therefore will never have that big of a hole to read)

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org