As I kept watching some DotA videos online, Cherry (cherry@) was hard at work
trying to integrate the uvm_hotplug(9) changes into the branch that was at the
time the NetBSD-CURRENT to merge his changes into the code upstream so that,
both uvm_hotplug(9) and the legacy memory segment handler co-exist side by side.

The uvm_hotplug(9) being a fundamental change in the way NetBSD’s virtual memory
works, we needed to make sure that the existing system works as is, while
hot-plug be enabled in the kernel separately when it is build with that option
enabled. This means that the uvm_hotplug(9) is mutually exclusive with the
current static array implementation.

Once Cherry was done with most of the merging with strategically paritioning the
code with UVM_HOTPLUG macros and conditionally defining mutually exclusive
parts of the code. Cherry’s greatest challenge was making sure that the new
changes were written in such a way that the MD part of the code used up all the
new API so that UVM_HOTPLUG is not needed to separation of code within the MD
parts of NetBSD, because that would involve changing code in 79 different ports
(the number of ports that existed for NetBSD at the time of writing this post),
and he had already spend considerable amount of time on the MD parts rewriting
sections that referenced the old vm_physseg[] as well as struct vm_physseg*,
which he tells me is the experience equivalent of “yak shaving”.

My job was to make sure that I could conditionally compile these mutually
exclusive parts in the ATF tests making sure the ATF compiles are not broken
depending on which implementation that is being tested.

During the test development we made sure that the tests itself were
implementation agnostic and this was very important so that the same tests could
be run on either of the implementations to see what failed and why. Except for
some minor pre-conditions for the tests which I had to segregate using the
UVM_HOTPLUG macro, most of the tests worked out of the box as expected after
the merge.

By the end of the merge we had the options enabled in the sys/arch/<arch>/conf
file by adding in one single line to the GENERIC file. Me being lazy as usual
made a GENERIC_HOTPLUG for ease of building and put it in my conf folder.

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# $NetBSD$

include "arch/i386/conf/GENERIC"

options         UVM_HOTPLUG     # Experimental uvm_hotplug(9) api.

After this merge our work mostly focused on this merged branch making our code
much closer to the release into NetBSD-CURRENT. But all was not done, we still
had to benchmark how well our API performed with respect to the old one.