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

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