runner: allow passing extra arguments to the VMM via preStart script

Author: nesteroff

lib/runner.nix

@@ -40,11 +40,12 @@ let
   binScripts = microvmConfig.binScripts // {
     microvm-run = ''
       set -eou pipefail
+      unset EXTRA_ARGS
       ${preStart}
       ${createVolumesScript vmHostPackages microvmConfig.volumes}
       ${lib.optionalString (hypervisorConfig.requiresMacvtapAsFds or false) openMacvtapFds}
 
-      exec ${execArg} ${command}
+      exec ${execArg} ${command} ''${EXTRA_ARGS:-}
     '';
   } // lib.optionalAttrs canShutdown {
     microvm-shutdown = shutdownCommand;

nixos-modules/microvm/options.nix

@@ -58,7 +58,11 @@ in
     };
 
     preStart = mkOption {
-      description = "Commands to run before starting the hypervisor";
+      description = ''
+        Commands to run before starting the hypervisor.
+
+        Additional arguments can be passed to the hypervisor using the EXTRA_ARGS variable.
+      '';
       default = "";
       type = types.lines;
     };