clear qcow on rebuild, update fs mounting

internal/qmp/client.go

@@ -94,12 +94,12 @@ func (c *Client) AddChardev(id, socketPath string) mo.Result[struct{}] {
 	return mo.Ok(struct{}{})
 }
 
-// AddVhostUserFsDevice adds a vhost-user-fs-pci device connected to a chardev.
-// This is the second step of hot-mounting a filesystem.
-func (c *Client) AddVhostUserFsDevice(chardevID, tag string) mo.Result[struct{}] {
+// AddVhostUserFsDevice adds a vhost-user-fs-pci device connected to a chardev,
+// targeting a hotplug-capable PCIe root port bus.
+func (c *Client) AddVhostUserFsDevice(chardevID, tag, bus string) mo.Result[struct{}] {
 	// Device ID must be unique, derive from chardev ID
 	deviceID := "dev_" + chardevID
-	cmd := fmt.Sprintf(`{"execute":"device_add","arguments":{"driver":"vhost-user-fs-pci","chardev":"%s","tag":"%s","id":"%s","queue-size":1024}}`, chardevID, tag, deviceID)
+	cmd := fmt.Sprintf(`{"execute":"device_add","arguments":{"driver":"vhost-user-fs-pci","chardev":"%s","tag":"%s","id":"%s","queue-size":1024,"bus":"%s"}}`, chardevID, tag, deviceID, bus)
 	raw, err := c.monitor.Run([]byte(cmd))
 	if err != nil {
 		return mo.Err[struct{}](fmt.Errorf("failed to add vhost-user-fs device for %s: %w", tag, err))
@@ -116,9 +116,67 @@ func (c *Client) AddVhostUserFsDevice(chardevID, tag string) mo.Result[struct{}]
 	return mo.Ok(struct{}{})
 }
 
+// FindFreeHotplugBus queries PCI devices and returns the first hotplug root port
+// that has no child devices attached. Returns an error if all slots are occupied.
+func (c *Client) FindFreeHotplugBus(busPrefix string, slotCount int) mo.Result[string] {
+	// Query all PCI devices to find which buses are occupied
+	cmd := []byte(`{"execute":"query-pci"}`)
+	raw, err := c.monitor.Run(cmd)
+	if err != nil {
+		return mo.Err[string](fmt.Errorf("failed to query PCI devices: %w", err))
+	}
+
+	// Parse to find which hotplug buses have devices
+	var resp struct {
+		Return []struct {
+			Devices []struct {
+				Bus       int    `json:"bus"`
+				QdevID    string `json:"qdev_id"`
+				PCIBridge *struct {
+					Bus struct {
+						Number int `json:"number"`
+					} `json:"bus"`
+					Devices []json.RawMessage `json:"devices"`
+				} `json:"pci_bridge"`
+			} `json:"devices"`
+		} `json:"return"`
+	}
+	if err := json.Unmarshal(raw, &resp); err != nil {
+		return mo.Err[string](fmt.Errorf("failed to parse PCI response: %w", err))
+	}
+
+	// Build set of occupied hotplug buses
+	occupied := make(map[string]bool)
+	for _, bus := range resp.Return {
+		for _, dev := range bus.Devices {
+			if dev.PCIBridge != nil && len(dev.PCIBridge.Devices) > 0 {
+				occupied[dev.QdevID] = true
+			}
+		}
+	}
+
+	// Find first free hotplug slot
+	for i := 0; i < slotCount; i++ {
+		busID := fmt.Sprintf("%s%d", busPrefix, i)
+		if !occupied[busID] {
+			return mo.Ok(busID)
+		}
+	}
+
+	return mo.Err[string](fmt.Errorf("all %d hotplug slots are occupied", slotCount))
+}
+
 // HotMountFilesystem performs a complete hot-mount of a virtiofsd filesystem.
 // Requires the virtiofsd daemon to already be running and listening on socketPath.
-func (c *Client) HotMountFilesystem(tag, socketPath string) mo.Result[struct{}] {
+// Finds a free PCIe hotplug root port and attaches the device there.
+func (c *Client) HotMountFilesystem(tag, socketPath, busPrefix string, slotCount int) mo.Result[struct{}] {
+	// Find a free hotplug bus
+	busResult := c.FindFreeHotplugBus(busPrefix, slotCount)
+	if busResult.IsError() {
+		return mo.Err[struct{}](busResult.Error())
+	}
+	bus := busResult.MustGet()
+
 	// Use tag as chardev ID for simplicity
 	chardevID := tag
 
@@ -127,8 +185,8 @@ func (c *Client) HotMountFilesystem(tag, socketPath string) mo.Result[struct{}]
 		return result
 	}
 
-	// Step 2: Add device
-	if result := c.AddVhostUserFsDevice(chardevID, tag); result.IsError() {
+	// Step 2: Add device on the hotplug bus
+	if result := c.AddVhostUserFsDevice(chardevID, tag, bus); result.IsError() {
 		return result
 	}