OVMF: add passthru.mergedFirmware parameter; xen: use the merged OVMF blob

[SigmaSquadron]

Fixes #373416

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[SigmaSquadron]

A question: since OVMF_CODE.fd is indeed the wrong blob to use, why does everything in Nixpkgs use it as the de-facto UEFI firmware blob? Why aren't we using the merged version in things like nixos/qemu-vm?

[adamcstephens]

A question: since OVMF_CODE.fd is indeed the wrong blob to use, why does everything in Nixpkgs use it as the de-facto UEFI firmware blob? Why aren't we using the merged version in things like nixos/qemu-vm?

I couldn't find any official documentation on qemu's site for working with OVMF, but it obviously works as configured. As I understand it the split is to provide the CODE/fw as read-only, but allow for a separate VARS location for read-write state. If you combine them, this could in theory reduce security as the boot code could be modified by the VM in read-write mode.

I wasn't aware until this Xen issue that there was even a combined version. Maybe other hypervisors care as well, but qemu doesn't consider the separated files "wrong".

[philiptaron]

Unrelated to this PR: is there a thought to move the pkgs/build-support/xen into pkgs/by-name/xe/xen in some way? They appear deeply connected.

[SigmaSquadron]

is there a thought to move the pkgs/build-support/xen into pkgs/by-name/xe/xen in some way? They appear deeply connected.

I don't think we can, because we need the generic builder in order to build variants of Xen, like the one in #341429. We would need to have callPackage ../../xe/xen/package.nix in qubes-vmm-xen, or duplicate the entire builder.

[adamcstephens]

It's against by-name rules (and the CI checks) to reference across packages in by-name. You could probably move the builder to by-name/bu/buildXenPackage/package.nix for auto loading though.