cloud-hypervisor: allow overriding --serial and --console arguments

Check if --serial or --console are provided in extraArgs before adding
defaults. This allows users to specify custom values like file output
or socket connections instead of being forced to use the hardcoded
defaults. We skip the default kernel serial parameter when serial is not
set to tty.

Fixes microvm-nix#376

Author: Ramblurr

lib/runners/cloud-hypervisor.nix

@@ -9,18 +9,26 @@ let
   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;
 
+  hasUserConsole = (extractOptValues "--console" extraArgs).values != [];
+  hasUserSerial = (extractOptValues "--serial" extraArgs).values != [];
+  userSerial = if hasUserSerial then (extractOptValues "--serial" extraArgs).values else "";
+
   kernelPath = {
     x86_64-linux = "${kernel.dev}/vmlinux";
     aarch64-linux = "${kernel.out}/${pkgs.stdenv.hostPlatform.linux-kernel.target}";
   }.${pkgs.stdenv.system};
 
-  kernelConsole =
+  kernelConsoleDefault =
     if pkgs.stdenv.system == "x86_64-linux"
     then "earlyprintk=ttyS0 console=ttyS0"
     else if pkgs.stdenv.system == "aarch64-linux"
     then "console=ttyAMA0"
     else "";
 
+  kernelConsole = if (hasUserSerial && userSerial == "tty") then kernelConsoleDefault else "";
+
+  kernelCmdLine = "${kernelConsole} reboot=t panic=-1 ${builtins.unsafeDiscardStringContext (toString microvmConfig.kernelParams)}";
+
   useHotPlugMemory = hotplugMem > 0;
 
   useVirtiofs = builtins.any ({ proto, ... }: proto == "virtiofs") shares;
@@ -145,16 +153,18 @@ in {
         )
         "--cpus" "boot=${toString vcpu}"
         "--watchdog"
-        "--console" "null"
-        "--serial" "tty"
         "--kernel" kernelPath
         "--initramfs" initrdPath
-        "--cmdline" "${kernelConsole} reboot=t panic=-1 ${builtins.unsafeDiscardStringContext (toString microvmConfig.kernelParams)}"
+        "--cmdline" kernelCmdLine
         "--seccomp" "true"
         "--memory" memOps
         "--platform" platformOps
       ]
       ++
+      lib.optionals (!hasUserConsole) ["--console" "null"]
+      ++
+      lib.optionals (!hasUserSerial) ["--serial" "tty"]
+      ++
       lib.optionals supportsNotifySocket [
         "--vsock" "cid=3,socket=notify.vsock"
       ]