Using qemu-nbd to share /dev/disk## with UTM guest on OSX arm64 laptop

ShareOSXHostBlockDeviceWithGuest.rst

Share OSX Host Block Device with Guest

My daily laptop is a Apple MacBook M4 Pro - it accomplishes everything I need, from API development to OCI containers. My internet gateway runs Dragonfly BSD and has been rock solid for many years now, using my prototype work published as autumnjolitz/cerberus_gateway.

My plan was if the gateway was hosed, I could pull out it's USB drive, connect it to my laptop [1], mount it in a guest virtual machine [2], see what's up, fix the issue or just reimage the drive and then redeploy it.

I completed it enough to put it into production. Then I had other things to do. Along the years that went by, I had switched from an x86-64 laptop to an arm64 one. I naively thought that whatever issues would occur with an architecture switch, they would be solved by someone else in time.

I discovered that options for virtual machines on OSX for running a different architecture (x86-64) than the host (arm64) were few and had documented issues that meant even sharing a USB device was unreliable or broken.

Turns out with qemu-nbd installed, you can share a OSX block device (/dev/disk##). And it works with the App Store release of UTM!

Requirements

• UTM
• qemu-nbd (Suggest you use Homebrew and brew install qemu)

Example

I have a Dragonfly BSD guest named "Rana". It's an x86-64 guest as Dragonfly only supports x86-64.

We will be sharing a user mounted DMG with the guest for purposes of this demo.

1. Create a blank dmg (ASIF preferred but a SPARSEBUNDLE works as well)

$ hdiutil create -layout NONE -size 1024g -type ASIF Blank-1TB
$ echo Blank-1TB*
Blank-1TB.asif

Tip

If you're not using Tahoe, I suggest you create a Tahoe guest via the UTM "Create New Virtual Machine" button. Share a local folder into Tahoe, create the ASIF images in that folder and then you can shutdown the guest and use these blank images

2. Attach (do not mount!) the blank image

$ cp Blank-1TB.asif Dragonfly.asif
$ diskutil image attach --noMount Dragonfly.asif
/dev/disk4

3. Edit UTM guest "Rana" under section Qemu -> Arguments and add the following arguments to the bottom using the "New" button

• -device virtio-blk-pci,drive=driveCAFEBABE,serial=CE2AAFBCA18E4B55AE667359E213AFB6,bootindex=1
• -drive file=nbd:localhost:10809:exportname=rdisk4,if=none,media=disk,id=driveCAFEBABE

4. Share the block device

Note

An actual block device like a physical USB drive will likely require you to prefix your qemu-nbd command with sudo -H.

$ qemu-nbd \
  --bind localhost \
  --port 10809 \
  --image-opts \
    driver=raw,file.driver=host_device,file.filename=/dev/rdisk4 \
  --description 'DragonflyBSD ASIF target volume' \
  --export-name=rdisk4

5. Start the guest and work inside the guest

Footnotes

[1]	The machine that runs the virtual machine is termed the "Host"

[2]	A virtual machine is often referred to as a "Guest"