Converting PV to HVM - boot methods

[Stephen Borrill <netbsd%precedence.co.uk@localhost>]

I've set this to port-i386 rather than port-xen as it's more about the x86 
boot process rather than Xen.

I have a (large) number of PV Xen VMs. These were created from a disk 
image built with makefs that the sets were extracted into. This means 
there is no partitioning except for disklabel and the root partition 
begins at sector 0:

16 partitions:
#        size    offset     fstype [fsize bsize cpg/sgs]
 a:  18874368         0     4.2BSD   2048 16384     0  # (Cyl.      0 -  18724*)
 b:   2097152  18874368       swap                     # (Cyl.  18724*-  20805*)
 c:  20971520         0     unused      0     0        # (Cyl.      0 -  20805*)
 d:  20971520         0     unused      0     0        # (Cyl.      0 -  20805*)

Now that XenServer and XCP-ng no longer support PV booting (and PV-on-PVH 
doesn't work either), I now need to convert them to HVM. As there's no 
free sectors at the start, I cannot create a partition table, etc.

My current best fix is to add a 2nd tiny (16MB) virtual disk with 
GPT partitioning and a single MSDOS EFI partition containing:

/efi/boot/bootx64.efi
/efi/NetBSD/boot.cfg

The boot.cfg is altered so that instead of boot netbsd, the lines say boot hd1a:netbsd.

Full set of steps (assuming xbd1 for new disk and no existing wedges):

gpt create xbd1
gpt add -b 64 -t efi -l efiboot xbd1
newfs_msdos dk0
mount NAME=efiboot /mnt
mkdir -p /mnt/efi/boot /mnt/efi/NetBSD
cp /usr/mdec/bootx64.efi /mnt/efi/boot
sed 's/boot netbsd/boot hd1a:netbsd/' /boot.cfg > /mnt/efi/NetBSD/boot.cfg

At the boot prompt:

> dev
disk hd0 mediaId 0 size 16 MB
  NAME=efiboot(EFI System)
disk hd1 mediaId 0 size 10 GB
  hd1a(4.2BSD) hd1b(swap)
net net0 22:6f:a5:97:cc:a0 pci0,4,0
default NAME=efiboot

This all works fine, but I'm interested in others' alternatives 
suggestions.

Observations:
1) According to boot(8) the dev command "Sets the default drive and 
partition for subsequent file system operations", but this doesn't appear 
to work. After entering dev hd1a:, all operations still default to 
NAME=bootconfig (was the same before I labelled the partitions, they 
defaulted to hd0b).
2) Would be nice to be able to chain-load a replacement boot.cfg e.g.
reconfig=hd1a:/boot.cfg

--
Stephen