[PATCH 02/12] Kconfig: add QEMU_SYSTEM_UNITS as a new bringup method

From: Daniel Gomez <[email protected]>

The existing NIXOS backend pairs libvirt with a disk-image NixOS
boot. The qsu-driven imageless path shares almost nothing with
it at the lifecycle layer (no libvirtd, no libguestfs, no sudo,
systemd user units instead of virsh, tmpfs-over-virtiofs instead
of qcow2), so folding it behind a flag on NIXOS would collapse
two user-facing choices whose surfaces are distinct.

Introduce QEMU_SYSTEM_UNITS as its own choice entry with a
matching KDEVOPS_ENABLE_QEMU_SYSTEM_UNITS bool, following the
GUESTFS/NIXOS pattern. The QEMU_SYSTEM_UNITS entry also selects
KDEVOPS_ENABLE_NIXOS because the guest is always a NixOS system
(built from the nixos-qemu imageless template), which lets every
NixOS-aware task across kdevops (devconfig, update_etc_hosts,
gen_hosts, gen_nodes) pick the /run/current-system interpreter
paths on qsu guests too. Hoist NIXOS_CONFIG_DIR out of the NIXOS
block and into KDEVOPS_ENABLE_NIXOS so both backends share the
same per-node configuration root.

Note that qsu also supports image-based boot (cloud image, mkosi, qcow2,
raw), but this series wires up only the imageless path.

Generated-by: Claude AI
Signed-off-by: Daniel Gomez <[email protected]>
---
 kconfigs/Kconfig.bringup | 18 ++++++++++++++++++
 kconfigs/Kconfig.nixos   | 18 +++++++++++++-----
 2 files changed, 31 insertions(+), 5 deletions(-)

diff --git a/kconfigs/Kconfig.bringup b/kconfigs/Kconfig.bringup
index 39bd93b8..88c38d49 100644
--- a/kconfigs/Kconfig.bringup
+++ b/kconfigs/Kconfig.bringup
@@ -9,6 +9,10 @@ config KDEVOPS_ENABLE_NIXOS
 	bool
 	output yaml
 
+config KDEVOPS_ENABLE_QEMU_SYSTEM_UNITS
+	bool
+	output yaml
+
 config CLOUD_INITIALIZED
 	bool
 	default $(shell, test -f .cloud.initialized && echo y || echo n) = "y"
@@ -69,6 +73,19 @@ config NIXOS
 	  NixOS will automatically infer the libvirt session type (system vs
 	  user) based on your distribution's defaults, similar to guestfs.
 
+config QEMU_SYSTEM_UNITS
+	bool "qemu-system-units via systemd user units (imageless NixOS)"
+	select KDEVOPS_ENABLE_QEMU_SYSTEM_UNITS
+	select KDEVOPS_ENABLE_NIXOS
+	help
+	  Drive VM lifecycle through systemd user units rendered from
+	  scripts/qemu-system-units/, with nixos-qemu's imageless module as
+	  the guest image (tmpfs root, virtiofs /nix/store, external
+	  kernel). No libvirt daemon, no libguestfs, no sudo on the host.
+	  The guest is NixOS so KDEVOPS_ENABLE_NIXOS is selected too,
+	  letting existing Ansible roles pick the /run/current-system/sw/
+	  bash and python interpreter paths instead of /bin defaults.
+
 config SKIP_BRINGUP
 	bool "Skip bring up - bare metal or existing nodes"
 	select EXTRA_STORAGE_SUPPORTS_512
@@ -95,3 +112,4 @@ source "kconfigs/Kconfig.declared_hosts"
 if LIBVIRT
 source "kconfigs/Kconfig.libvirt"
 endif
+source "kconfigs/Kconfig.qemu_system_units"
diff --git a/kconfigs/Kconfig.nixos b/kconfigs/Kconfig.nixos
index 622b4492..ac4d4360 100644
--- a/kconfigs/Kconfig.nixos
+++ b/kconfigs/Kconfig.nixos
@@ -1,16 +1,24 @@
 # SPDX-License-Identifier: copyleft-next-0.3.1
 
-if NIXOS
+if KDEVOPS_ENABLE_NIXOS
 
-config NIXOS_STORAGE_DIR
+# Host-side path where per-node Nix configurations live (flake +
+# default.nix rendered by the active backend). Shared between the
+# libvirt-backed NIXOS choice and the imageless QEMU_SYSTEM_UNITS
+# choice; both consume nixos-qemu modules from the same subtree.
+config NIXOS_CONFIG_DIR
 	string
 	output yaml
-	default "{{ kdevops_storage_pool_path }}/nixos"
+	default "{{ topdir_path }}/scripts/nixos-qemu/configurations"
 
-config NIXOS_CONFIG_DIR
+endif # KDEVOPS_ENABLE_NIXOS
+
+if NIXOS
+
+config NIXOS_STORAGE_DIR
 	string
 	output yaml
-	default "{{ topdir_path }}/scripts/nixos-qemu/configurations"
+	default "{{ kdevops_storage_pool_path }}/nixos"
 
 config NIXOS_USE_FLAKES
 	bool "Use Nix flakes for configuration"

-- 
2.53.0