kola/qemuexec: allow changing guest network

This is useful when you're nesting VMs and you want the first VM to
be able to access the host. The default host address that QEMU assigns
(e.g. 10.0.2.2, modifiable via the `host=...` netdev knob) doesn't
always work because it's not actually an IP address owned by the host,
but proxied by QEMU itself.

So the source appears to come from localhost, but in some contexts (e.g.
iSCSI), we need the host and the guest to agree that the same IP refers
to the host.

With this, one can start the first VM as usual (e.g. `cosa run`) and the
second VM within with e.g. `cosa run --usernet-addr 10.0.3.0/24` and be
able to talk back to the outer VM via the valid address 10.0.2.15.

To be clear, this can all be done with passthrough QEMU args, so this is
just about making it more convenient.

mantle/cmd/kola/qemuexec.go

@@ -73,6 +73,8 @@ var (
 
 	netboot    string
 	netbootDir string
+
+	usernetAddr string
 )
 
 const maxAdditionalNics = 16
@@ -102,6 +104,7 @@ func init() {
 	cmdQemuExec.Flags().StringVarP(&sshCommand, "ssh-command", "x", "", "Command to execute instead of spawning a shell")
 	cmdQemuExec.Flags().StringVarP(&netboot, "netboot", "", "", "Filepath to BOOTP program (e.g. PXELINUX/GRUB binary or iPXE script")
 	cmdQemuExec.Flags().StringVarP(&netbootDir, "netboot-dir", "", "", "Directory to serve over TFTP (default: BOOTP parent dir). If specified, --netboot is relative to this dir.")
+	cmdQemuExec.Flags().StringVarP(&usernetAddr, "usernet-addr", "", "", "Guest IP network (QEMU default is '10.0.2.0/24')")
 }
 
 func renderFragments(fragments []string, c *conf.Conf) error {
@@ -356,11 +359,11 @@ func runQemuExec(cmd *cobra.Command, args []string) error {
 	if cpuCountHost {
 		builder.Processors = -1
 	}
-	if usernet {
+	if usernet || usernetAddr != "" {
 		h := []platform.HostForwardPort{
 			{Service: "ssh", HostPort: 0, GuestPort: 22},
 		}
-		builder.EnableUsermodeNetworking(h)
+		builder.EnableUsermodeNetworking(h, usernetAddr)
 	}
 	if netboot != "" {
 		builder.SetNetbootP(netboot, netbootDir)

mantle/platform/machine/qemu/cluster.go

@@ -175,12 +175,12 @@ func (qc *Cluster) NewMachineWithQemuOptions(userdata *conf.UserData, options pl
 	}
 
 	if len(options.HostForwardPorts) > 0 {
-		builder.EnableUsermodeNetworking(options.HostForwardPorts)
+		builder.EnableUsermodeNetworking(options.HostForwardPorts, "")
 	} else {
 		h := []platform.HostForwardPort{
 			{Service: "ssh", HostPort: 0, GuestPort: 22},
 		}
-		builder.EnableUsermodeNetworking(h)
+		builder.EnableUsermodeNetworking(h, "")
 	}
 	if options.AdditionalNics > 0 {
 		builder.AddAdditionalNics(options.AdditionalNics)

mantle/platform/machine/qemuiso/cluster.go

@@ -124,12 +124,12 @@ func (qc *Cluster) NewMachineWithQemuOptions(userdata *conf.UserData, options pl
 	}
 
 	if len(options.HostForwardPorts) > 0 {
-		builder.EnableUsermodeNetworking(options.HostForwardPorts)
+		builder.EnableUsermodeNetworking(options.HostForwardPorts, "")
 	} else {
 		h := []platform.HostForwardPort{
 			{Service: "ssh", HostPort: 0, GuestPort: 22},
 		}
-		builder.EnableUsermodeNetworking(h)
+		builder.EnableUsermodeNetworking(h, "")
 	}
 
 	if options.AdditionalNics > 0 {

mantle/platform/qemu.go

@@ -478,6 +478,7 @@ type QemuBuilder struct {
 	ignitionRendered bool
 
 	UsermodeNetworking        bool
+	usermodeNetworkingAddr    string
 	RestrictNetworking        bool
 	requestedHostForwardPorts []HostForwardPort
 	additionalNics            int
@@ -599,9 +600,10 @@ func virtio(arch, device, args string) string {
 
 // EnableUsermodeNetworking configure forwarding for all requested ports,
 // via usermode network helpers.
-func (builder *QemuBuilder) EnableUsermodeNetworking(h []HostForwardPort) {
+func (builder *QemuBuilder) EnableUsermodeNetworking(h []HostForwardPort, usernetAddr string) {
 	builder.UsermodeNetworking = true
 	builder.requestedHostForwardPorts = h
+	builder.usermodeNetworkingAddr = usernetAddr
 }
 
 func (builder *QemuBuilder) SetNetbootP(filename, dir string) {
@@ -637,6 +639,9 @@ func (builder *QemuBuilder) setupNetworking() error {
 	if builder.RestrictNetworking {
 		netdev += ",restrict=on"
 	}
+	if builder.usermodeNetworkingAddr != "" {
+		netdev += ",net=" + builder.usermodeNetworkingAddr
+	}
 	if builder.netbootP != "" {
 		// do an early stat so we fail with a nicer error now instead of in the VM
 		if _, err := os.Stat(filepath.Join(builder.netbootDir, builder.netbootP)); err != nil {