From: William Lee Irwin III <wli@holomorphy.com>

Contributions from:
	Jan Dittmer <jdittmer@sfhq.hn.org>
	Arnd Bergmann <arnd@arndb.de>
	"Bryan O'Sullivan" <bos@serpentine.com>
	"David S. Miller" <davem@redhat.com>
	Badari Pulavarty <pbadari@us.ibm.com>
	"Martin J. Bligh" <mbligh@aracnet.com>
	Zwane Mwaikambo <zwane@linuxpower.ca>

It has ben tested on x86, sparc64, x86_64, ia64 (I think), ppc and ppc64.

cpumask_t enables systems with NR_CPUS > BITS_PER_LONG to utilize all their
cpus by creating an abstract data type dedicated to representing cpu
bitmasks, similar to fd sets from userspace, and sweeping the appropriate
code to update callers to the access API.  The fd set-like structure is
according to Linus' own suggestion; the macro calling convention to ambiguate
representations with minimal code impact is my own invention.

Specifically, a new set of inline functions for manipulating arbitrary-width
bitmaps is introduced with a relatively simple implementation, in tandem with
a new data type representing bitmaps of width NR_CPUS, cpumask_t, whose
accessor functions are defined in terms of the bitmap manipulation inlines.
This bitmap ADT found an additional use in i386 arch code handling sparse
physical APIC ID's, which was convenient to use in this case as the
accounting structure was required to be wider to accommodate the physids
consumed by larger numbers of cpus.

For the sake of simplicity and low code impact, these cpu bitmasks are passed
primarily by value; however, an additional set of accessors along with an
auxiliary data type with const call-by-reference semantics is provided to
address performance concerns raised in connection with very large systems,
such as SGI's larger models, where copying and call-by-value overhead would
be prohibitive.  Few (if any) users of the call-by-reference API are
immediately introduced.

Also, in order to avoid calling convention overhead on architectures where
structures are required to be passed by value, NR_CPUS <= BITS_PER_LONG is
special-cased so that cpumask_t falls back to an unsigned long and the
accessors perform the usual bit twiddling on unsigned longs as opposed to
arrays thereof.  Audits were done with the structure overhead in-place,
restoring this special-casing only afterward so as to ensure a more complete
API conversion while undergoing the majority of its end-user exposure in -mm.
 More -mm's were shipped after its restoration to be sure that was tested,
too.

The immediate users of this functionality are Sun sparc64 systems, SGI mips64
and ia64 systems, and IBM ia32, ppc64, and s390 systems.  Of these, only the
ppc64 machines needing the functionality have yet to be released; all others
have had systems requiring it for full functionality for at least 6 months,
and in some cases, since the initial Linux port to the affected architecture.