FreeBSD POWER9 status

[Justin Hibbits]

Hello fellow PowerPC'ers,

Over the last 4 weeks I've been working on the FreeBSD port to POWER9,
3 weeks of which were on my physical hardware (Raptor Talos II).  As
of last night, FreeBSD can boot to multiuser mode, with the addition
of two patches not yet committed for enabling PCI, and I plan to
commit those in the next few days.  The patches can be found at:

https://reviews.freebsd.org/D15461
https://reviews.freebsd.org/D15566

What works (with patches):
* Boots to a USB stick
* OPAL sensors for temperature, voltage, power, etc
* OPAL IPMI (tested only for attachment)
* Interrupts via XIVE XICS emulation
* Everything already done for powernv by Semihalf (NVMe, OPAL UART)

What's left:
* No display via AST2500 BMC VGA
* All threads currently spin-idle, rather than halting for idle, this
wastes CPU cycles, and reduces performance for other threads.  I hope
to get this fixed in the next few days.
* Power management and cpufreq
* hwpmc
* Performance optimizations as we find them
* Full PCI handling.  There are a lot of capabilities not currently handled.
* Interrupts via XIVE exploitation.  This should improve stability and
performance of the XIVE controller.

Special thanks to our newest committers, Leandro Lupori and Breno
Leitao, for their help with the bring-up.  Special thanks also to
Benjamin Herrenschmidt from IBM for providing the key insights to
fixing PCI and interrupts.

- Justin