runtime: qemu: Add reclaim_guest_freed_memory [BACKPORT]

Similar to what we've done for Cloud Hypervisor in the commit
9f76467cb7, we're backporting a runtime-rs
feature that would be benificial to have as part of the go runtime.

This allows users to use virito-balloon for the hypervisor to reclaim
memory freed by the guest.

Signed-off-by: Fabiano Fidêncio <fidencio@northflank.com>

src/runtime/virtcontainers/qemu.go

@@ -139,6 +139,7 @@ const (
 	scsiControllerID         = "scsi0"
 	rngID                    = "rng0"
 	fallbackFileBackedMemDir = "/dev/shm"
+	balloonID                = "balloon0"
 
 	qemuStopSandboxTimeoutSecs = 15
 
@@ -632,6 +633,9 @@ func (q *qemu) prepareInitdataMount(config *HypervisorConfig) error {
 }
 
 // CreateVM is the Hypervisor VM creation implementation for govmmQemu.
+// This function is complex and there's not much to be done about it, unfortunately.
+//
+//nolint:gocyclo
 func (q *qemu) CreateVM(ctx context.Context, id string, network Network, hypervisorConfig *HypervisorConfig) error {
 	// Save the tracing context
 	q.ctx = ctx
@@ -801,6 +805,20 @@ func (q *qemu) CreateVM(ctx context.Context, id string, network Network, hypervi
 		}
 	}
 
+	if q.config.ReclaimGuestFreedMemory && !q.config.ConfidentialGuest {
+		balloonDev := config.BalloonDev{
+			ID:                balloonID,
+			DeflateOnOOM:      true,
+			DisableModern:     false,
+			FreePageReporting: true,
+		}
+
+		qemuConfig.Devices, err = q.arch.appendBalloonDevice(ctx, qemuConfig.Devices, balloonDev)
+		if err != nil {
+			return err
+		}
+	}
+
 	if machine.Type == QemuQ35 || machine.Type == QemuVirt {
 		if err := q.createPCIeTopology(&qemuConfig, hypervisorConfig, machine.Type, network); err != nil {
 			q.Logger().WithError(err).Errorf("Cannot create PCIe topology")

src/runtime/virtcontainers/qemu_arch_base.go

@@ -116,6 +116,9 @@ type qemuArch interface {
 	// appendRNGDevice appends a RNG device to devices
 	appendRNGDevice(ctx context.Context, devices []govmmQemu.Device, rngDevice config.RNGDev) ([]govmmQemu.Device, error)
 
+	// appendBalloonDevice appends a Balloon device to devices
+	appendBalloonDevice(ctx context.Context, devices []govmmQemu.Device, BalloonDevice config.BalloonDev) ([]govmmQemu.Device, error)
+
 	// setEndpointDevicePath sets the appropriate PCI or CCW device path for an endpoint
 	setEndpointDevicePath(endpoint Endpoint, bridgeAddr int, devAddr string) error
 
@@ -738,6 +741,19 @@ func (q *qemuArchBase) appendRNGDevice(_ context.Context, devices []govmmQemu.De
 	return devices, nil
 }
 
+func (q *qemuArchBase) appendBalloonDevice(_ context.Context, devices []govmmQemu.Device, balloonDev config.BalloonDev) ([]govmmQemu.Device, error) {
+	devices = append(devices,
+		govmmQemu.BalloonDevice{
+			ID:                balloonDev.ID,
+			DeflateOnOOM:      balloonDev.DeflateOnOOM,
+			DisableModern:     balloonDev.DisableModern,
+			FreePageReporting: balloonDev.FreePageReporting,
+		},
+	)
+
+	return devices, nil
+}
+
 func (q *qemuArchBase) setEndpointDevicePath(endpoint Endpoint, bridgeAddr int, devAddr string) error {
 	bridgeSlot, err := types.PciSlotFromInt(bridgeAddr)
 	if err != nil {