Bootloader-style automatic firmware updates are incompatible with make-disk-image.nix

[hacker1024]

Nixpkgs's make-disk-image.nix function, used for generating NixOS disk images, attempts to install the bootloader while running in a VM.

When hardware.nvidia-jetpack.firmware.autoUpdate is enabled, it adds the firmware update script to the bootloader installation command list. This isn't suitable for a VM, though, for the following reasons:

• /sys/devices/virtual/dmi/id/bios_version doesn't exist, causing the update script (and transitively, the bootloader installation and disk image build) to exit 1
  • If the QEMU/UEFI setup in the VM changes in the future, it may start existing too, but its value will not be useful
• NVRAM variables to configure the capsule update don't actually get set on the Jetson

To resolve these issues, there are at least three different solutions:

1. exit 0 instead of exit 1 if /sys/devices/virtual/dmi/id/bios_version doesn't exist during bootloader installation. I don't think a failed firmware update attempt should necessarily indicate that the bootloader didn't get installed.
2. Work out somehow if a VM is in use, and don't attempt to update at all.
3. Work out somehow if a VM is in use, and update unconditionally. This will work as long as touchEFIVars is used to set the NVRAM on the Jetson.