amd64 kernel crash introduced between 20180329 & 20180408

[Dan Allen]

> On 17 Apr 2018, at 8:49 AM, Kyle Evans <kevans at freebsd.org> wrote:
> 
> As "the guy most likely to have broken boot code in stable," may I ask
> what leads you specifically to amd64 boot code? Mostly curious if
> there's something beyond "i386 works well" that lead you to this
> conclusion.

It is partly just a hunch.

I installed 11.0 for use with qemu a while ago.  I did binary upgrades for patches using
freebsd-update.  When 11.1 came out, it would not work correctly, again with the same
kind of behavior.  Then, I got some later snapshots that worked again, notably the 20180329
build.  When the next snapshot came out, things broke.  I also tried my own builds, same story.

I even got both source trees together - 20180329 and 20180408 - and did a diff on the entire
trees, and I noticed activity in the boot & kernel code.  It could just as likely be something in the
kernel as well, but none of this happens with the i386 build.

> When you say it crashes and does a kernel dump- you're landing at a
> ddb prompt, yeah? What does executing bt at that prompt look like?

No, I am not ever given a prompt.  I get to watch a mini-dump happen and then an automatic
reboot.  It is a kernel panic.  Here is what I see: