Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Introduce the sysclock_getsnapshot() and sysclock_snap2bintime() KPIs. The
sysclock_getsnapshot() function allows the caller to obtain a snapshot of all
the system clock and timecounter state required to create time stamps at a later
point. The sysclock_snap2bintime() function converts a previously obtained
snapshot into a bintime time stamp according to the specified flags e.g. which
system clock, uptime vs absolute time, etc.

These KPIs enable useful functionality, including direct comparison of the
feedback and feed-forward system clocks and generation of multiple time stamps
with different formats from a single timecounter read.

Committed on behalf of Julien Ridoux and Darryl Veitch from the University of
Melbourne, Australia, as part of the FreeBSD Foundation funded "Feed-Forward
Clock Synchronization Algorithms" project.

For more information, see http://www.synclab.org/radclock/

In collaboration with: Julien Ridoux (jridoux at unimelb edu au)

Revise the sysctl handling code and restructure the hierarchy of sysctls
introduced when feed-forward clock support is enabled in the kernel:

- Rename the "choice" variable to "available".

- Streamline the implementation of the "active" variable's sysctl handler
function.

- Create a kern.sysclock sysctl node for general sysclock related configuration
options. Place the "available" and "active" variables under this node.

- Create a kern.sysclock.ffclock sysctl node for feed-forward clock specific
configuration options. Place the "version" and "ffcounter_bypass" variables
under this node.

- Tweak some of the description strings.

Discussed with: Julien Ridoux (jridoux at unimelb edu au)

Make sysclock_active publicly available to external consumers.

Committed on behalf of Julien Ridoux and Darryl Veitch from the University of
Melbourne, Australia, as part of the FreeBSD Foundation funded "Feed-Forward
Clock Synchronization Algorithms" project.

For more information, see http://www.synclab.org/radclock/

Discussed with: Julien Ridoux (jridoux at unimelb edu au)
Submitted by: Julien Ridoux (jridoux at unimelb edu au)

Introduce the new "fromclock" public wrapper API which allows consumers to
select which system clock to obtain time from, independent of the current
default system clock. In the brave new multi sysclock world, both feedback and
feed-forward system clocks can be maintained and used concurrently, so this API
provides a minimalist first step for interested consumers to exercise control
over their choice of system clock.

Committed on behalf of Julien Ridoux and Darryl Veitch from the University of
Melbourne, Australia, as part of the FreeBSD Foundation funded "Feed-Forward
Clock Synchronization Algorithms" project.

For more information, see http://www.synclab.org/radclock/

Discussed with: Julien Ridoux (jridoux at unimelb edu au)
Submitted by: Julien Ridoux (jridoux at unimelb edu au)

Make the fbclock_[get]{bin,nano,micro}[up]time() function prototypes public so
that new APIs with some performance sensitivity can be built on top of them.
These functions should not be called directly except in special circumstances.

Committed on behalf of Julien Ridoux and Darryl Veitch from the University of
Melbourne, Australia, as part of the FreeBSD Foundation funded "Feed-Forward
Clock Synchronization Algorithms" project.

For more information, see http://www.synclab.org/radclock/

Discussed with: Julien Ridoux (jridoux at unimelb edu au)
Submitted by: Julien Ridoux (jridoux at unimelb edu au)

- Add the ffclock_getcounter(), ffclock_getestimate() and ffclock_setestimate()
system calls to provide feed-forward clock management capabilities to
userspace processes. ffclock_getcounter() returns the current value of the
kernel's feed-forward clock counter. ffclock_getestimate() returns the current
feed-forward clock parameter estimates and ffclock_setestimate() updates the
feed-forward clock parameter estimates.

- Document the syscalls in the ffclock.2 man page.

- Regenerate the script-derived syscall related files.

Committed on behalf of Julien Ridoux and Darryl Veitch from the University of
Melbourne, Australia, as part of the FreeBSD Foundation funded "Feed-Forward
Clock Synchronization Algorithms" project.

For more information, see http://www.synclab.org/radclock/

Submitted by: Julien Ridoux (jridoux at unimelb edu au)

- Provide a sysctl interface to change the active system clock at runtime.

- Wrap [get]{bin,nano,micro}[up]time() functions of sys/time.h to allow
requesting time from either the feedback or the feed-forward clock. If a
feedback (e.g. ntpd) and feed-forward (e.g. radclock) daemon are both running
on the system, both kernel clocks are updated but only one serves time.

- Add similar wrappers for the feed-forward difference clock.

Committed on behalf of Julien Ridoux and Darryl Veitch from the University of
Melbourne, Australia, as part of the FreeBSD Foundation funded "Feed-Forward
Clock Synchronization Algorithms" project.

For more information, see http://www.synclab.org/radclock/

Submitted by: Julien Ridoux (jridoux at unimelb edu au)

Fix a whitespace nit.

Submitted by: Julien Ridoux (jridoux at unimelb edu au)

Provide high-level functions to access the feed-forward absolute and difference
clocks. Each routine can output an upper bound on the absolute time or time
interval requested. Different flavours of absolute time can be requested, for
example with or without leap seconds, monotonic or not, etc.

Committed on behalf of Julien Ridoux and Darryl Veitch from the University of
Melbourne, Australia, as part of the FreeBSD Foundation funded "Feed-Forward
Clock Synchronization Algorithms" project.

For more information, see http://www.synclab.org/radclock/

Submitted by: Julien Ridoux (jridoux at unimelb edu au)

Core structure and functions to support a feed-forward clock within the kernel.
Implement ffcounter, a monotonically increasing cumulative counter on top of the
active timecounter. Provide low-level functions to read the ffcounter and
convert it to absolute time or a time interval in seconds using the current
ffclock estimates, which track the drift of the oscillator. Add a ring of
fftimehands to track passing of time on each kernel tick and pick up updates of
ffclock estimates.

Committed on behalf of Julien Ridoux and Darryl Veitch from the University of
Melbourne, Australia, as part of the FreeBSD Foundation funded "Feed-Forward
Clock Synchronization Algorithms" project.

For more information, see http://www.synclab.org/radclock/

Submitted by: Julien Ridoux (jridoux at unimelb edu au)