Import the driver for VT-d DMAR hardware, as specified in the revision
1.3 of Intelб╝ Virtualization Technology for Directed I/O Architecture
Specification.  The Extended Context and PASIDs from the rev. 2.2 are
not supported, but I am not aware of any released hardware which
implements them.  Code does not use queued invalidation, see comments
for the reason, and does not provide interrupt remapping services.

Code implements the management of the guest address space per domain
and allows to establish and tear down arbitrary mappings, but not
partial unmapping.  The superpages are created as needed, but not
promoted.  Faults are recorded, fault records could be obtained
programmatically, and printed on the console.

Implement the busdma(9) using DMARs.  This busdma backend avoids
bouncing and provides security against misbehaving hardware and driver
bad programming, preventing leaks and corruption of the memory by wild
DMA accesses.

By default, the implementation is compiled into amd64 GENERIC kernel
but disabled; to enable, set hw.dmar.enable=1 loader tunable.  Code is
written to work on i386, but testing there was low priority, and
driver is not enabled in GENERIC.  Even with the DMAR turned on,
individual devices could be directed to use the bounce busdma with the
hw.busdma.pci<domain>:<bus>:<device>:<function>.bounce=1 tunable.  If
DMARs are capable of the pass-through translations, it is used,
otherwise, an identity-mapping page table is constructed.

The driver was tested on Xeon 5400/5500 chipset legacy machine,
Haswell desktop and E5 SandyBridge dual-socket boxes, with ahci(4),
ata(4), bce(4), ehci(4), mfi(4), uhci(4), xhci(4) devices.  It also
works with em(4) and igb(4), but there some fixes are needed for
drivers, which are not committed yet.  Intel GPUs do not work with
DMAR (yet).

Many thanks to John Baldwin, who explained me the newbus integration;
Peter Holm, who did all testing and helped me to discover and
understand several incredible bugs; and to Jim Harris for the access
to the EDS and BWG and for listening when I have to explain my
findings to somebody.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 month