vfkit: Add vfkit.log (#21096)

Like krunkit and vmnet-helper, we redirect vfkit logs to

    $MINIKUBE_HOME/.minikube/machines/name/vfkit.log

This will be helpful to debug case when vfkit exits with an error.
Currently this error goes to /dev/null which makes debugging impossible.
It will be also useful if we need to get help from vfkit folks.

Example log:

    time="2025-07-20T02:48:43+03:00" level=info msg="&{2 6144    {[efi variable-store=/Users/nir/.minikube/machines/minikube/vfkit.efivars create] true}  [virtio-net,nat,mac=1e:ac:f2:43:84:b3 virtio-rng virtio-blk,path=/Users/nir/.minikube/machines/minikube/boot2docker.iso virtio-blk,path=/Users/nir/.minikube/machines/minikube/disk.img virtio-serial,logFilePath=/Users/nir/.minikube/machines/minikube/serial.log] unix:///Users/nir/.minikube/machines/minikube/vfkit.sock debug false  {[] false}}"
    time="2025-07-20T02:48:43+03:00" level=info msg="boot parameters: &{EFIVariableStorePath:/Users/nir/.minikube/machines/minikube/vfkit.efivars CreateVariableStore:true}"
    time="2025-07-20T02:48:43+03:00" level=info
    time="2025-07-20T02:48:43+03:00" level=info msg="virtual machine parameters:"
    time="2025-07-20T02:48:43+03:00" level=info msg="\tvCPUs: 2"
    time="2025-07-20T02:48:43+03:00" level=info msg="\tmemory: 6144 MiB"
    time="2025-07-20T02:48:43+03:00" level=info
    time="2025-07-20T02:48:43+03:00" level=info msg="Adding virtio-net device (nat: true macAddress: [1e:ac:f2:43:84:b3])"
    time="2025-07-20T02:48:43+03:00" level=info msg="Adding virtio-rng device"
    time="2025-07-20T02:48:43+03:00" level=info msg="Adding virtio-blk device (imagePath: /Users/nir/.minikube/machines/minikube/boot2docker.iso)"
    time="2025-07-20T02:48:43+03:00" level=info msg="Adding virtio-blk device (imagePath: /Users/nir/.minikube/machines/minikube/disk.img)"
    time="2025-07-20T02:48:43+03:00" level=info msg="Adding virtio-serial device (logFile: /Users/nir/.minikube/machines/minikube/serial.log)"
    time="2025-07-20T02:48:43+03:00" level=info msg="virtual machine is running"
    time="2025-07-20T02:48:43+03:00" level=info msg="waiting for VM to stop"

Author: nirs

pkg/drivers/vfkit/vfkit.go

@@ -57,6 +57,7 @@ const (
 	isoFilename     = "boot2docker.iso"
 	pidFileName     = "vfkit.pid"
 	sockFilename    = "vfkit.sock"
+	logFileName     = "vfkit.log"
 	serialFileName  = "serial.log"
 	efiVarsFileName = "vfkit.efivars"
 	defaultSSHUser  = "docker"
@@ -251,7 +252,8 @@ func (d *Driver) startVfkit(socketPath string) error {
 	startCmd = append(startCmd,
 		"--memory", fmt.Sprintf("%d", d.Memory),
 		"--cpus", fmt.Sprintf("%d", d.CPU),
-		"--restful-uri", fmt.Sprintf("unix://%s", d.sockfilePath()))
+		"--restful-uri", fmt.Sprintf("unix://%s", d.sockfilePath()),
+		"--log-level", "debug")
 
 	efiVarsPath := d.ResolveStorePath(efiVarsFileName)
 	startCmd = append(startCmd,
@@ -294,6 +296,13 @@ func (d *Driver) startVfkit(socketPath string) error {
 	// to terminate the entire process group without harming the vfkit process.
 	cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
 
+	logfile, err := d.openLogfile()
+	if err != nil {
+		return fmt.Errorf("failed to open vfkit logfile: %w", err)
+	}
+	defer logfile.Close()
+	cmd.Stderr = logfile
+
 	if err := cmd.Start(); err != nil {
 		return err
 	}
@@ -342,6 +351,11 @@ func isBootpdError(err error) bool {
 	return strings.Contains(err.Error(), "could not find an IP address")
 }
 
+func (d *Driver) openLogfile() (*os.File, error) {
+	logfile := d.ResolveStorePath(logFileName)
+	return os.OpenFile(logfile, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0o600)
+}
+
 func (d *Driver) stopVfkit() error {
 	if err := d.SetVFKitState("Stop"); err != nil {
 		// vfkit may be already stopped, shutting down, or not listening.