libxl: don't try to manipulate json config for stubdomain

Stubdomain do not have it's own config file - its configuration is
derived from target domains. Do not try to manipulate it when attaching
PCI device.

This bug prevented starting HVM with stubdomain and PCI passthrough
device attached.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>

tools/libxl/libxl_pci.c

@@ -126,8 +126,11 @@ static int libxl__device_pci_add_xenstore(libxl__gc *gc,
     int rc;
     libxl_domain_config d_config;
     libxl__domain_userdata_lock *lock = NULL;
+    bool is_stubdomain = libxl_is_stubdom(CTX, domid, NULL);
 
-    libxl_domain_config_init(&d_config);
+    /* Stubdomain doesn't have own config. */
+    if (!is_stubdomain)
+        libxl_domain_config_init(&d_config);
 
     be_path = libxl__domain_device_backend_path(gc, 0, domid, 0,
                                                 LIBXL__DEVICE_KIND_PCI);
@@ -153,27 +156,35 @@ static int libxl__device_pci_add_xenstore(libxl__gc *gc,
     if (!starting)
         flexarray_append_pair(back, "state", GCSPRINTF("%d", XenbusStateReconfiguring));
 
-    lock = libxl__lock_domain_userdata(gc, domid);
-    if (!lock) {
-        rc = ERROR_LOCK_FAIL;
-        goto out;
+    /*
+     * Stubdomin config is derived from its target domain, it doesn't have
+     * its own file.
+     */
+    if (!is_stubdomain) {
+        lock = libxl__lock_domain_userdata(gc, domid);
+        if (!lock) {
+            rc = ERROR_LOCK_FAIL;
+            goto out;
+        }
+
+        rc = libxl__get_domain_configuration(gc, domid, &d_config);
+        if (rc) goto out;
+
+        device_add_domain_config(gc, &d_config, &libxl__pcidev_devtype,
+                                 pcidev);
+
+        rc = libxl__dm_check_start(gc, &d_config, domid);
+        if (rc) goto out;
     }
 
-    rc = libxl__get_domain_configuration(gc, domid, &d_config);
-    if (rc) goto out;
-
-    device_add_domain_config(gc, &d_config, &libxl__pcidev_devtype,
-                             pcidev);
-
-    rc = libxl__dm_check_start(gc, &d_config, domid);
-    if (rc) goto out;
-
     for (;;) {
         rc = libxl__xs_transaction_start(gc, &t);
         if (rc) goto out;
 
-        rc = libxl__set_domain_configuration(gc, domid, &d_config);
-        if (rc) goto out;
+        if (lock) {
+            rc = libxl__set_domain_configuration(gc, domid, &d_config);
+            if (rc) goto out;
+        }
 
         libxl__xs_writev(gc, t, be_path, libxl__xs_kvs_of_flexarray(gc, back));
 
@@ -185,7 +196,8 @@ static int libxl__device_pci_add_xenstore(libxl__gc *gc,
 out:
     libxl__xs_transaction_abort(gc, &t);
     if (lock) libxl__unlock_domain_userdata(lock);
-    libxl_domain_config_dispose(&d_config);
+    if (!is_stubdomain)
+        libxl_domain_config_dispose(&d_config);
     return rc;
 }