vhost-user-fs: Implement internal migration

A virtio-fs device's VM state consists of:
- the virtio device (vring) state (VMSTATE_VIRTIO_DEVICE)
- the back-end's (virtiofsd's) internal state

We get/set the latter via the new vhost operations to transfer migratory
state.  It is its own dedicated subsection, so that for external
migration, it can be disabled.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
Message-Id: <20231016134243.68248-8-hreitz@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio/vhost-user-fs.c

@@ -298,9 +298,108 @@ static struct vhost_dev *vuf_get_vhost(VirtIODevice *vdev)
     return &fs->vhost_dev;
 }
 
+/**
+ * Fetch the internal state from virtiofsd and save it to `f`.
+ */
+static int vuf_save_state(QEMUFile *f, void *pv, size_t size,
+                          const VMStateField *field, JSONWriter *vmdesc)
+{
+    VirtIODevice *vdev = pv;
+    VHostUserFS *fs = VHOST_USER_FS(vdev);
+    Error *local_error = NULL;
+    int ret;
+
+    ret = vhost_save_backend_state(&fs->vhost_dev, f, &local_error);
+    if (ret < 0) {
+        error_reportf_err(local_error,
+                          "Error saving back-end state of %s device %s "
+                          "(tag: \"%s\"): ",
+                          vdev->name, vdev->parent_obj.canonical_path,
+                          fs->conf.tag ?: "<none>");
+        return ret;
+    }
+
+    return 0;
+}
+
+/**
+ * Load virtiofsd's internal state from `f` and send it over to virtiofsd.
+ */
+static int vuf_load_state(QEMUFile *f, void *pv, size_t size,
+                          const VMStateField *field)
+{
+    VirtIODevice *vdev = pv;
+    VHostUserFS *fs = VHOST_USER_FS(vdev);
+    Error *local_error = NULL;
+    int ret;
+
+    ret = vhost_load_backend_state(&fs->vhost_dev, f, &local_error);
+    if (ret < 0) {
+        error_reportf_err(local_error,
+                          "Error loading back-end state of %s device %s "
+                          "(tag: \"%s\"): ",
+                          vdev->name, vdev->parent_obj.canonical_path,
+                          fs->conf.tag ?: "<none>");
+        return ret;
+    }
+
+    return 0;
+}
+
+static bool vuf_is_internal_migration(void *opaque)
+{
+    /* TODO: Return false when an external migration is requested */
+    return true;
+}
+
+static int vuf_check_migration_support(void *opaque)
+{
+    VirtIODevice *vdev = opaque;
+    VHostUserFS *fs = VHOST_USER_FS(vdev);
+
+    if (!vhost_supports_device_state(&fs->vhost_dev)) {
+        error_report("Back-end of %s device %s (tag: \"%s\") does not support "
+                     "migration through qemu",
+                     vdev->name, vdev->parent_obj.canonical_path,
+                     fs->conf.tag ?: "<none>");
+        return -ENOTSUP;
+    }
+
+    return 0;
+}
+
+static const VMStateDescription vuf_backend_vmstate;
+
 static const VMStateDescription vuf_vmstate = {
     .name = "vhost-user-fs",
-    .unmigratable = 1,
+    .version_id = 0,
+    .fields = (VMStateField[]) {
+        VMSTATE_VIRTIO_DEVICE,
+        VMSTATE_END_OF_LIST()
+    },
+    .subsections = (const VMStateDescription * []) {
+        &vuf_backend_vmstate,
+        NULL,
+    }
+};
+
+static const VMStateDescription vuf_backend_vmstate = {
+    .name = "vhost-user-fs-backend",
+    .version_id = 0,
+    .needed = vuf_is_internal_migration,
+    .pre_load = vuf_check_migration_support,
+    .pre_save = vuf_check_migration_support,
+    .fields = (VMStateField[]) {
+        {
+            .name = "back-end",
+            .info = &(const VMStateInfo) {
+                .name = "virtio-fs back-end state",
+                .get = vuf_load_state,
+                .put = vuf_save_state,
+            },
+        },
+        VMSTATE_END_OF_LIST()
+    },
 };
 
 static Property vuf_properties[] = {