NetBSD supports UEFI firmware (BIOS replacement) and work very well even under Hyper-V

Worked pretty well without any issue.

Tested Host:

• HW: MSI-7250 board, AMD Opteron X2 dual core, 8GB RAM
• OS Hypervisor: Windows Server 2012R2 with Hyper-V Role and Hyper-V Manager

Tested Guest:

• NetBSD 10 RC1
• UEFI Mode (knowns as "Generation 2" under Hyper-V)

How I installed it:

curl -fLO https://cdn.netbsd.org/pub/NetBSD/NetBSD-10.0_RC1/images/NetBSD-10.0_RC1-amd64-install.img.gz
gzip -dkv NetBSD-10.0_RC1-amd64-install.img.gz
# WARNING! Conversion must be done at NON-NTFS filesystem (FAT32 or exFAT), probably
# due broken sparese files support in QEMU
qemu-img convert -p -f raw -O vhdx NetBSD-10.0_RC1-amd64-install.img nbsd10rc1-install.vhdx

• Now copy nbsd10rc1-install.vhdx to target Hyper-V disk where it will be used. IMPORTANT! Target must be on NTFS (Hyper-V depends on specific NTFS features to run VM!)
• Also create new empty VHDX for installation Target (where we will install NetBSD). Here is Windows PowerShell example:

$vhd="C:\Hyper-V\Virtual Disks\nbsd10rc1-uefi.vhdx"
new-vhd -Path $vhd -Dynamic -SizeBytes 30GB -BlockSizeBytes 1MB `
   -LogicalSectorSizeBytes 512 -PhysicalSectorSizeBytes 4096

Important: I use block-size 1MB (default is 32MB) to avoid excessive VHDX size (because with 32MB blocks, there has to be 32MB of all zeroes to reclaim such block as unused - which is very unlikely).

New in Hyper-V create VM and specify:

• 1 or more CPU
• Generation 2 (UEFI)
• 2GB RAM (or more)
• attach existing disk - and specify full path to target disk, in my case C:\Hyper-V\Virtual Disks\nbsd10rc1-uefi.vhdx (there we will install NetBSD)
• after Finish select "SCSCI controller" and Add existing disk - this time nbsd10rc1-install.vhdx (this contains NetBSD installation files)
• now go to Firmware
  • disable "Secure" Boot (!)
  • ensure that boot order is
    1. nbsd10rc1-uefi.vhdx (empty disk where will be NetBSD installed)
    2. nbsd10rc1-install.vhdx NetBSD installation disk
    3. Network (not used, but I don't know how to remove it)

And start VM - the installation should proceed fine without any issues...

Proxmox VM:

• OS Type: Linux 6.x - 2.6 kernel (intentional - to avoid emulating too old hardware)
• Machine: Default (i440fx)
• BIOS: OVMF (UEFI)
• 1xCPU, host passtrough
• 2GB RAM
• Disk: Virtio BLK, 32GB

And here is GPT partition layout:

$ sysctl hw.disknames
hw.disknames = ld0 dk0 dk1 dk2 cd0

$ fdisk ld0

...

Partition table:
0: GPT Protective MBR (sysid 238)
    start 1, size 67108863 (32768 MB, Cyls 0-4177/85/4)
        PBR is not bootable: Bad magic number (0x0000)
1: <UNUSED>
2: <UNUSED>
3: <UNUSED>
No active partition.
Drive serial number: 0 (0x00000000)

$ gpt show ld0

     start      size  index  contents
         0         1         PMBR
         1         1         Pri GPT header
         2        32         Pri GPT table
        34        30         Unused
        64    262144      1  GPT part - EFI System
    262208  62664576      2  GPT part - NetBSD FFSv1/FFSv2
  62926784   4182016      3  GPT part - NetBSD swap
  67108800        31         Unused
  67108831        32         Sec GPT table
  67108863         1         Sec GPT header

Now using Wedges to access GPT partitions:

$ dkctl ld0 listwedges

/dev/rld0: 3 wedges:
dk0: 1369ba6d-3be2-49f6-afc6-0e5faf5a10b9, 262144 blocks at 64, type: msdos
dk1: 9a447701-b2bf-49cd-b21a-86b066eff3d9, 62664576 blocks at 262208, type: ffs
dk2: 05fde297-57bc-4a4c-8622-18c4ab5985a8, 4182016 blocks at 62926784, type: swap

Example mounting read-only ESP partition with UEFI loader:

mount -r -t msdos /dev/dk0 /mnt

• Original boot loader is /usr/mdec/bootx64.efi
• bootx64.efi source in under /usr/src/sys/arch/i386/stand/efiboot/bootx64