[CFT] Paravirtualized KVM clock

[Bryan Venteicher]

For the last few weeks, I've been working on adding support for KVM clock
in the projects/paravirt branch. Currently, a KVM VM guest will end up
selecting either the HPET or ACPI as the timecounter source. Unfortunately,
this is very costly since every timecounter fetch causes a VM exit. KVM
clock allows the guest to use the TSC instead; it is very similar to the
existing Xen timer.

The performance difference between HPET/ACPI and KVMCLOCK can be dramatic:
a simple disk benchmark goes from 10K IOPs to 100K IOPs.

The patch is attached is attached or available at [1]. I'd appreciate any
testing.

Also as a part of this, I've tried to generalized a bit of our existing
hypervisor guest code, with the eventual goal of being able to support more
invasive PV operations. The patch series is viewable in Phabricator.

https://reviews.freebsd.org/D1429 - paravirt: Generalize parts of the XEN
timer code into pvclock
https://reviews.freebsd.org/D1430 - paravirt: Add interface to calculate
the TSC frequency from pvclock
https://reviews.freebsd.org/D1431 - paravirt: Add simple hypervisor
registration and detection interface
https://reviews.freebsd.org/D1432 - paravirt: Add detection of bhyve using
new hypervisor interface
https://reviews.freebsd.org/D1433 - paravirt: Add detection of VMware using
new hypervisor interface
https://reviews.freebsd.org/D1434 - paravirt: Add detection of KVM using
new hypervisor interface
https://reviews.freebsd.org/D1435 - paravirt: Add KVM clock timecounter
support

My current plan is to MFC this series to 10-STABLE, and commit a
self-contained KVM clock to the other stable branches.

[1] - https://people.freebsd.org/~bryanv/patches/kvm_clock-1.patch
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