From 6e8248c8b24948fbf513998f96950dd1ee9c6f7f Mon Sep 17 00:00:00 2001
From: Raito Bezarius <masterancpp@gmail.com>
Date: Wed, 26 Apr 2023 18:28:31 +0200
Subject: [PATCH] nixos/qemu-vm: make it possible to use UEFI without
 bootloaders

`useEFIBoot` is somewhat misleading, but we should make it possible to
enable UEFI environment / firmware without buying into a bootloader.

This makes it possible.
---
 nixos/modules/virtualisation/qemu-vm.nix | 28 +++++++++++++++---------
 1 file changed, 18 insertions(+), 10 deletions(-)

diff --git a/nixos/modules/virtualisation/qemu-vm.nix b/nixos/modules/virtualisation/qemu-vm.nix
index 5b515a29ae6..f9f88b7340a 100644
--- a/nixos/modules/virtualisation/qemu-vm.nix
+++ b/nixos/modules/virtualisation/qemu-vm.nix
@@ -169,18 +169,26 @@ let
       # Create a directory for exchanging data with the VM.
       mkdir -p "$TMPDIR/xchg"
 
-      ${lib.optionalString cfg.useBootLoader
+      ${lib.optionalString cfg.useEFIBoot
       ''
+        # Expose EFI variables, it's useful even when we are not using a bootloader (!).
+        # We might be interested in having EFI variable storage present even if we aren't booting via UEFI, hence
+        # no guard against `useBootLoader`.  Examples:
+        # - testing PXE boot or other EFI applications
+        # - directbooting LinuxBoot, which `kexec()s` into a UEFI environment that can boot e.g. Windows
         NIX_EFI_VARS=$(readlink -f "''${NIX_EFI_VARS:-${config.system.name}-efi-vars.fd}")
-
-        ${lib.optionalString cfg.useEFIBoot
-        ''
-          # VM needs writable EFI vars
-          if ! test -e "$NIX_EFI_VARS"; then
-            cp ${systemImage}/efi-vars.fd "$NIX_EFI_VARS"
-            chmod 0644 "$NIX_EFI_VARS"
-          fi
-        ''}
+        # VM needs writable EFI vars
+        if ! test -e "$NIX_EFI_VARS"; then
+        ${if cfg.useBootLoader then
+            # We still need the EFI var from the make-disk-image derivation
+            # because our "switch-to-configuration" process might
+            # write into it and we want to keep this data.
+            ''cp ${systemImage}/efi-vars.fd "$NIX_EFI_VARS"''
+            else
+            ''cp ${cfg.efi.variables} "$NIX_EFI_VARS"''
+          }
+          chmod 0644 "$NIX_EFI_VARS"
+        fi
       ''}
 
       cd "$TMPDIR"