VirtIO buses (PCI, MMIO) can provide a generation field so a driver
can ensure either a 64-bit or array read was stable.

MFC after:	2 weeks

Mainly focus on files that use BSD 3-Clause license.

The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.

Special thanks to Wind River for providing access to "The Duke of
Highlander" tool: an older (2014) run over FreeBSD tree was useful as a
starting point.

These defines are applicable to userland too, but virtqueue.h contains
the kernel virtqueue interface, and is therefore not usable in userland.

Note that Linux places these defines in virtio_ring.h, but I don't want
the drivers including this header file to keep the VirtIO ring opaque to
everything but the virtqueue.

MFC after:	1 week

Reorganize the previous contexts of the file as it is in Linux. The
eventual goal is to install the header files and share them between
the kernel and bhyve.

MFC after:	1 week

This commit is primarily a significant cleanup to the interrupt
allocation code that had gotten a bit jumbled from having to
support per-vq MSIX, shared MSIX, MSI, and legacy style interrupts.

Contains projects/virtio commits:

r246064:
    virtio_pci: Rewrite allocation of interrupts
r246065:
    virtio_pci: Remove spaces before a tab
r246066:
    virtio_pci: Dynamically allocate the virtqueue array
r246304:
    virtio_pci: Clean up after failed virtqueue alloc attempt
r246305:
    virtio_pci: Move no interrupt check into the PCI interrupt handlers
r246308:
    virtio_pci: Remove unused variable

MFC after:	1 month

Contains projects/virtio commits:

r245738:
    virtio: Minor man page tweaks
r246060:
    virtio: Cleanup feature description printing
r246306:
    virtio: Remove old debugging flag
r247238:
    virtio: Remove PRIx64 macros from format strings
r247239:
    virtio: Constify some fields
r247240:
    virtio: Minor code simplifications
r249962:
    virtio: Update to my freebsd.org email address

MFC after:	1 month

Submitted by:	Bryan Venteicher  bryanv at daemoninthecloset dot org
Reviewed by:	grehan

Submitted by:	Bryan Venteicher bryanv at daemoninthecloset org

      PCI:
        - Properly handle interrupt fallback from MSIX to MSI to legacy.
          The host may not have sufficient resources to support MSIX,
          so we must be able to fallback to legacy interrupts.
        - Add interface to get the (sub) vendor and device IDs.
        - Rename flags to VTPCI_FLAG_* like other VirtIO drivers.
      Block:
        - No longer allocate vtblk_requests from separate UMA zone.
          malloc(9) from M_DEVBUF is sufficient. Assert segment counts
          at allocation.
        - More verbose error and debug messages.
      Network:
        - Remove stray write once variable.
      Virtqueue:
        - Shuffle code around in preparation of converting the mb()s to
          the appropriate atomic(9) operations.
        - Only walk the descriptor chain when freeing if INVARIANTS is
          defined since the result is only KASSERT()ed.

Submitted by:	Bryan Venteicher (bryanv@daemoninthecloset.org)

them have already included <sys/param.h> before these headers are included.