Implement Sandro's suggestion from the PR review

Author: Ramblurr

lib/default.nix

@@ -107,7 +107,7 @@ rec {
       => { values = ["linux"]; args = ["-vnc" ":0" "-usb"]; }
 
       # Extract multiple occurrences:
-      extractOptValues "-platform" ["-a" "a" "-b" "b" "-c" "c" "-b" "b2"]
+      extractOptValues "-b" ["-a" "a" "-b" "b" "-c" "c" "-b" "b2"]
       => { values = ["b" "b2"]; args = ["-b" "b" "-c" "c"]; }
 
       # Extract with multiple flag aliases:

lib/runner.nix

@@ -8,13 +8,13 @@ let
 
   inherit (microvmConfig) hostName;
 
-  inherit (import ./. { inherit lib; }) createVolumesScript makeMacvtap withDriveLetters;
+  inherit (import ./. { inherit lib; }) createVolumesScript makeMacvtap withDriveLetters extractOptValues;
   inherit (makeMacvtap {
     inherit microvmConfig hypervisorConfig;
   }) openMacvtapFds macvtapFds;
 
   hypervisorConfig = import (./runners + "/${microvmConfig.hypervisor}.nix") {
-    inherit pkgs microvmConfig macvtapFds withDriveLetters;
+    inherit pkgs microvmConfig macvtapFds withDriveLetters extractOptValues;
   };
 
   inherit (hypervisorConfig) command canShutdown shutdownCommand;

lib/runners/cloud-hypervisor.nix

@@ -1,12 +1,12 @@
 { pkgs
 , microvmConfig
 , macvtapFds
+, extractOptValues
 , ...
 }:
 
 let
   inherit (pkgs) lib;
-  inherit (import ../. { inherit (pkgs) lib; }) extractOptValues;
   inherit (microvmConfig) vcpu mem balloon initialBalloonMem deflateOnOOM hotplugMem hotpluggedMem user interfaces volumes shares socket devices hugepageMem graphics storeDisk storeOnDisk kernel initrdPath;
   inherit (microvmConfig.cloud-hypervisor) platformOEMStrings extraArgs;
 
@@ -95,10 +95,11 @@ let
 
   supportsNotifySocket = true;
 
-  oemStringValues = (lib.optionals supportsNotifySocket ["io.systemd.credential:vmm.notify_socket=vsock-stream:2:8888"]) ++ platformOEMStrings;
-  oemStringOptions = lib.optionals  (oemStringValues != [])  ["oem_strings=[${lib.concatStringsSep "," oemStringValues}]"];
-  extraArgsWithoutPlatform = (extractOptValues "--platform" extraArgs).args;
-  userPlatformOpts = (extractOptValues "--platform" extraArgs).values;
+  oemStringValues = platformOEMStrings ++ lib.optional supportsNotifySocket "io.systemd.credential:vmm.notify_socket=vsock-stream:2:8888";
+  oemStringOptions = lib.optional (oemStringValues != []) "oem_strings=[${lib.concatStringsSep "," oemStringValues}]";
+  platformExtracted = extractOptValues "--platform" extraArgs;
+  extraArgsWithoutPlatform = platformExtracted.args;
+  userPlatformOpts = platformExtracted.values;
   platformOps = lib.concatStringsSep "," (oemStringOptions ++ userPlatformOpts);
 in {
   inherit tapMultiQueue;

nixos-modules/microvm/asserts.nix

@@ -105,8 +105,17 @@ lib.mkIf config.microvm.guest.enable {
       message = ''
         MicroVM ${hostName}: `config.microvm.forwardPorts` works only with qemu and one network interface with `type = "user"`
       '';
+    } ]
+    ++
+    # cloud-hypervisor specific asserts
+    lib.optionals (config.microvm.hypervisor == "cloud-hypervisor") [ {
+      assertion = ! (lib.any (str: lib.hasInfix "oem_strings" str) config.microvm.cloud-hypervisor.platformOEMStrings);
+      message = ''
+        MicroVM ${hostName}: `config.microvm.cloud-hypervisor.platformOEMStrings` items must not contain `oem_strings`
+      '';
     } ];
 
+
   warnings =
     # 32 MB is just an optimistic guess, not based on experience
     lib.optional (config.microvm.mem < 32) ''

nixos-modules/microvm/options.nix

@@ -519,17 +519,17 @@ in
       type = with types; listOf str;
       default = [];
       description = ''
-        Extra arguments to pass to cloud-hypervisor's --platform oem_strings= argument.
+        Extra arguments to pass to cloud-hypervisor's --platform oem_strings=[] argument.
 
         All the oem strings will be concatenated with a comma (,) and wrapped in oem_string=[].
 
+        Do not include oem_string= or the [] brackets in the value.
+
         The resulting string will be combined with any --platform options in
         `config.microvm.cloud-hypervisor.extraArgs` and passed as a single
         --platform option to cloud-hypervisor
       '';
-      example = lib.literalExpression /* nix */ ''
-          [ "io.systemd.credential:APIKEY=supersecret" ]
-        '';
+      example = lib.literalExpression /* nix */ ''[ "io.systemd.credential:APIKEY=supersecret" ]'';
     };
 
     cloud-hypervisor.extraArgs = mkOption {