drivers: Add VirtioFS shares for vfkit and krunkit

Add --virtiofs-share flag for specifying one or more shared directories
on the host to share with the guest using Virtiofs.

Testing with krunkit shows that virtiofs mount is 10 times faster than
9p mount.

Usage:

   --virtiofs-share SHARED_DIR:MOUNT_TAG[,SHARED_DIR:MOUNT_TAG...]

Example:

    minikube start --virtiofs-share /Users/joe/models:models

In krunkit and vfkit, every virtiofs shared directory will be added as:

    --device virtio-fs,sharedDir=/shared-dir,mountTag=mount-tag

In the guest the shared directory will be mounted using:

    sudo mount -t virtiofs models /mnt/models

This change only adds the options and configure vfkit and krunkit. The
next step is connecting to the guest and mounting the shared
directories.

More work is needed to add VirtioFS to qemu. I'm starting with fkit and
krunkit since they have identical interface (krunkit was designed as
drop-in replacement for vfkit).

Author: nirs

cmd/minikube/cmd/start_flags.go

@@ -56,6 +56,7 @@ const (
 	humanReadableDiskSize   = "disk-size"
 	nfsSharesRoot           = "nfs-shares-root"
 	nfsShare                = "nfs-share"
+	virtiofsShare           = "virtiofs-share"
 	kubernetesVersion       = "kubernetes-version"
 	noKubernetes            = "no-kubernetes"
 	hostOnlyCIDR            = "host-only-cidr"
@@ -283,6 +284,9 @@ func initDriverFlags() {
 
 	// qemu
 	startCmd.Flags().String(qemuFirmwarePath, "", "Path to the qemu firmware file. Defaults: For Linux, the default firmware location. For macOS, the brew installation location. For Windows, C:\\Program Files\\qemu\\share")
+
+	// vfkit, krunkit
+	startCmd.Flags().StringSlice(virtiofsShare, []string{}, "Local folders to share with guest via Virtiofs mounts (vfkit and krunkit drivers only)")
 }
 
 // initNetworkingFlags inits the commandline flags for connectivity related flags for start
@@ -583,6 +587,7 @@ func generateNewConfigFromFlags(cmd *cobra.Command, k8sVersion string, rtime str
 		HyperkitVSockPorts:      viper.GetStringSlice(vsockPorts),
 		NFSShare:                viper.GetStringSlice(nfsShare),
 		NFSSharesRoot:           viper.GetString(nfsSharesRoot),
+		VirtiofsShare:           viper.GetStringSlice(virtiofsShare),
 		DockerEnv:               config.DockerEnv,
 		DockerOpt:               config.DockerOpt,
 		InsecureRegistry:        insecureRegistry,
@@ -832,6 +837,7 @@ func updateExistingConfigFromFlags(cmd *cobra.Command, existing *config.ClusterC
 	updateStringSliceFromFlag(cmd, &cc.HyperkitVSockPorts, vsockPorts)
 	updateStringSliceFromFlag(cmd, &cc.NFSShare, nfsShare)
 	updateStringFromFlag(cmd, &cc.NFSSharesRoot, nfsSharesRoot)
+	updateStringSliceFromFlag(cmd, &cc.VirtiofsShare, virtiofsShare)
 	updateStringFromFlag(cmd, &cc.HostOnlyCIDR, hostOnlyCIDR)
 	updateStringFromFlag(cmd, &cc.HypervVirtualSwitch, hypervVirtualSwitch)
 	updateBoolFromFlag(cmd, &cc.HypervUseExternalSwitch, hypervUseExternalSwitch)

pkg/drivers/krunkit/krunkit.go

@@ -71,6 +71,7 @@ type Driver struct {
 	CPU            int
 	Memory         int
 	ExtraDisks     int
+	VirtiofsShare  []string
 	MACAddress     string
 	VmnetHelper    vmnet.Helper
 }
@@ -231,6 +232,15 @@ func (d *Driver) startKrunkit(socketPath string) error {
 			"--device", fmt.Sprintf("virtio-blk,path=%s", pkgdrivers.ExtraDiskPath(d.BaseDriver, i)))
 	}
 
+	for _, s := range d.VirtiofsShare {
+		vs, err := pkgdrivers.ParseVirtiofsShare(s)
+		if err != nil {
+			return err
+		}
+		args = append(args,
+			"--device", fmt.Sprintf("virtio-fs,sharedDir=%s,mountTag=%s", vs.SharedDir, vs.MountTag))
+	}
+
 	log.Debugf("executing: krunkit %s", strings.Join(args, " "))
 	cmd := exec.Command(driverName, args...)
 

pkg/drivers/vfkit/vfkit.go

@@ -72,6 +72,7 @@ type Driver struct {
 	CPU            int
 	Memory         int
 	ExtraDisks     int
+	VirtiofsShare  []string
 	Network        string        // "", "nat", "vmnet-shared"
 	MACAddress     string        // For network=nat, network=""
 	VmnetHelper    *vmnet.Helper // For network=vmnet-shared
@@ -312,6 +313,15 @@ func (d *Driver) startVfkit(socketPath string) error {
 	startCmd = append(startCmd,
 		"--device", fmt.Sprintf("virtio-serial,logFilePath=%s", serialPath))
 
+	for _, s := range d.VirtiofsShare {
+		vs, err := pkgdrivers.ParseVirtiofsShare(s)
+		if err != nil {
+			return err
+		}
+		startCmd = append(startCmd,
+			"--device", fmt.Sprintf("virtio-fs,sharedDir=%s,mountTag=%s", vs.SharedDir, vs.MountTag))
+	}
+
 	log.Debugf("executing: vfkit %s", strings.Join(startCmd, " "))
 	os.Remove(d.sockfilePath())
 	cmd := exec.Command("vfkit", startCmd...)

pkg/drivers/virtiofs.go

@@ -0,0 +1,60 @@
+/*
+Copyright 2025 The Kubernetes Authors All rights reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package drivers
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// VirtiofsShare is a directory on the host shared with the guest using
+// virtiofs.
+type VirtiofsShare struct {
+	SharedDir string
+	MountTag  string
+}
+
+// ParseVirtiofsShare parses --virtiofs-share argument and returns a validated VirtiofsShare.
+// - SharedDir is an absolute path to existing directory to share with the guest.
+// - MountTag is a non-empty string without leading or trailing whitespace.
+func ParseVirtiofsShare(s string) (*VirtiofsShare, error) {
+	pair := strings.SplitN(s, ":", 2)
+	if len(pair) != 2 {
+		return nil, fmt.Errorf("invalid virtiofsShare string %q: (expected '/shared-dir:mount-tag')", s)
+	}
+
+	sharedDir := pair[0]
+	mountTag := pair[1]
+
+	if !filepath.IsAbs(sharedDir) {
+		return nil, fmt.Errorf("shared directory %q is not absolute path", sharedDir)
+	}
+
+	if fs, err := os.Stat(sharedDir); err != nil {
+		return nil, fmt.Errorf("failed to validate shared directory %q: %w", sharedDir, err)
+	} else if !fs.IsDir() {
+		return nil, fmt.Errorf("shared directory %q is not a directory", sharedDir)
+	}
+
+	if mountTag == "" || mountTag != strings.TrimSpace(mountTag) {
+		return nil, fmt.Errorf("invalid mount tag: %q", mountTag)
+	}
+
+	return &VirtiofsShare{SharedDir: sharedDir, MountTag: mountTag}, nil
+}

pkg/minikube/config/types.go

@@ -63,6 +63,7 @@ type ClusterConfig struct {
 	DisableDriverMounts     bool     // Only used by virtualbox
 	NFSShare                []string
 	NFSSharesRoot           string
+	VirtiofsShare           []string
 	UUID                    string // Only used by hyperkit to restore the mac address
 	NoVTXCheck              bool   // Only used by virtualbox
 	DNSProxy                bool   // Only used by virtualbox

pkg/minikube/registry/drvs/krunkit/krunkit.go

@@ -76,6 +76,7 @@ func configure(cfg config.ClusterConfig, n config.Node) (interface{}, error) {
 		Memory:         cfg.Memory,
 		CPU:            cfg.CPUs,
 		ExtraDisks:     cfg.ExtraDisks,
+		VirtiofsShare:  cfg.VirtiofsShare,
 		VmnetHelper: vmnet.Helper{
 			MachineDir:  filepath.Join(storePath, "machines", machineName),
 			InterfaceID: u,

pkg/minikube/registry/drvs/vfkit/vfkit.go

@@ -99,6 +99,7 @@ func configure(cfg config.ClusterConfig, n config.Node) (interface{}, error) {
 		Memory:         cfg.Memory,
 		CPU:            cfg.CPUs,
 		ExtraDisks:     cfg.ExtraDisks,
+		VirtiofsShare:  cfg.VirtiofsShare,
 		Network:        cfg.Network,
 		MACAddress:     mac,
 		VmnetHelper:    helper,