Support for ramdisks in PVH boot

[Pierre Pronchery <khorben%defora.org@localhost>]

			Hi tech-kern@,

This post is related to iMil's recent work on PVH support for NetBSD/amd64.
I was unable to use his work to boot on ramdisks directly with QEMU's -
initrd flag, when using -kernel.

Well after a deep dive into it, I think I am almost there:
https://git.edgebsd.org/gitweb/?
p=src.git;a=commitdiff;h=629621f41089af50584214a4d32b50ae8ee414f2

This patch:
- extends sys/arch/amd64/amd64/genassym.cf for additional knowledge of
  Xen's hvm_start_info (notably nr_modules and modlist_paddr)
- extends .start_genpvh in locore.S to copy the module entries, and their
  respective command lines and contents
- teaches x86_machdep.c to load Xen modules when a VM_GUEST_GENPVH guest

The code is not working yet unfortunately.
On an Intel-macOS host:

$ qemu-system-x86_64 -m 512 -accel hvf -cpu host -display none -serial \
  stdio -kernel netbsd-GENERIC -initrd ramdisk-cgdroot.fs -append \
  "console=com rw -v" -action reboot=shutdown -D qemu.log -d \
  cpu_reset,in_asm,guest_errors,unimp -drive \
  file=ld0.img,format=raw,if=virtio
[...]
[   4.5370781] boot device: ld0
[   4.5370781] md0: internal 5000 KB image area
[   4.5370781] root on md0a dumps on md0b
[   4.5370781] uvm_fault(0xffffffff81912da0, 0xffffffff9faf8000, 1) -> e
[   4.5370781] fatal page fault in supervisor mode
[   4.5370781] trap type 6 code 0 rip 0xffffffff80ff3274 cs 0x8 rflags 
0x10206 cr2 0xffffffff9faf8ff8 ilevel 0 rsp 0xffffffff81a07c38
[   4.5484101] curlwp 0xffffffff8188bfc0 pid 0.0 lowest kstack 
0xffffffff81a022c0
kernel: page fault trap, code=0
Stopped in pid 0.0 (system) at  ffffffff80ff3274:       movq    
fffffffffffffff8
(%rsi,%rdx,1),%r10

It seems to fail in bdev_strategy(); as of right now I do not know why.
I suspect my assembly code in locore.S to need review in particular.
Therefore: your help in completing this is very welcome.

Cheers & HTH,
-- 
khorben