So to make a Linux initrd:

(1) mkdir .../initrd
(2) mkdir -p .../initrd/boot/defaults
(3) cd src/stand; make install DESTDIR=.../initrd
(4) Copy kernel to .../initrd/boot/kernel
(5) cd .../initrd
(6) cp boot/loader.kboot init
(7) find . | sort | cpio -o -H newc | gzip > /tmp/initrd.cpio
(8) download or build your linux kernel
(9) qemu-system-x86_64 -kernel ~/vmlinuz-5.19.0-051900-generic \
	-initrd /tmp/initrd.cpio \
	-m 256m -nographic \
	-monitor telnet::4444,server,nowait -serial stdio \
	-append "console=ttyS0"
    (though you may need more than 256M of ram to actually boot FreeBSD and do
     anything interesting with it and the serial console to stdio bit hasn't
     been the most stable recipe lately).

Notes:
For #6 you might need to strip loader.kboot if you copy it directly and don't
	use make install.
For #7 the sort is important, and you may need LC_ALL=C for its invocation
For #7 gzip is but one of many methods, but it's the simplest to do.
For #9, this means we can automate it using methods from
	src/tools/boot/rootgen.sh when the time comes.
#9 also likely generalizes to other architectures
For #8, see https://kernel.ubuntu.com/~kernel-ppa/mainline/ to download
	a kernel suitable for testing... For arm, I've been using the
	non 64k page kernels and 5.19 seems to not suck.

aarch64:
qemu-system-aarch64 -m 1024 -cpu cortex-a57 -M virt \
	-kernel ~/linuxboot/arm64/kernel/boot/vmlinuz-5.19.0-051900-generic \
	-initrd ~/linuxboot/arm64/initrd.img -m 256m -nographic \
	-monitor telnet::4444,server,nowait -serial stdio \
	-append "console=ttyAMA0"

General

Add -g -G to have gdb stop and wait for the debugger. This is useful for
debugging the trampoline (hbreak will set a hardware break that's durable across
code changes).  If you set the breakpoint for the trampoline and it never hits,
then there's likely no RAM there and you got the PA to load to wrong. When
debugging the trampiline and up to that, use gdb /boot/loader. When debugging
the kernel, use kernel.full to get all the debugging. hbreak panic() is useful
on the latter since you'll see the original panic, not the panic you get from
there not being an early console.