[Xen-devel] [PATCH v5] libxl: fix migration of PV and PVH domUs with and without qemu

[Olaf Hering <olaf@xxxxxxxxx>]

If a domU has a qemu-xen instance attached, it is required to call qemus
"xen-save-devices-state" method. Without it, the receiving side of a PV or
PVH migration may be unable to lock the image:

xen be: qdisk-51712: xen be: qdisk-51712: error: Failed to get "write" lock
error: Failed to get "write" lock
xen be: qdisk-51712: xen be: qdisk-51712: initialise() failed
initialise() failed

To fix this bug, libxl__domain_suspend_device_model() and
libxl__domain_resume_device_model() have to be called not only for HVM,
but also if the active device_model is QEMU_XEN.

Unfortunately, libxl__domain_build_info_setdefault() used to hardcode
b_info->device_model_version to QEMU_XEN if it does not know it any
better. As a result libxl__device_model_version_running() will return
incorrect values. This breaks domUs without a device_model.
libxl__qmp_stop() would wait 10 seconds in qmp_open() for a qemu that
will never appear. During this long timeframe the domU remains in state
paused on the sending side. As a result network connections may be
dropped. Once this bug is fixed as well, by just removing the assumption
that every domU has a QEMU_XEN, there is no code to actually initialise
b_info->device_model_version.

There is a helper function libxl__need_xenpv_qemu(), which is used in
various places to decide if a device_model has to be spawned. This
function can not be used as is, just to fill device_model_version,
because store_libxl_entry() was already called earlier.

Introduce LIBXL_DEVICE_MODEL_VERSION_NONE for PV and PVH that have no
need for a device_model to make the state explicit. Indicate this new
state via LIBXL_HAVE macro in libxl.h.

v05:
- move initialization of device_model_version to extra commit
- return error from libxl__need_xenpv_qemu
- add LIBXL_HAVE_DEVICE_MODEL_VERSION_NONE
v04:
- make sure device_model_stubdomain is initialized
v03:
- rearrange code to make sure device_model_version is initialized before
  store_libxl_entry() is called
v02:
- update wording in a comment
- remove stale goto in domcreate_launch_dm
- initialize ret in libxl__need_xenpv_qemu

Signed-off-by: Olaf Hering <olaf@xxxxxxxxx>
Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Cc: Anthony PERARD <anthony.perard@xxxxxxxxxx>
---
 tools/libxl/libxl.h             |  7 +++++++
 tools/libxl/libxl_create.c      | 17 ++++++++++++++---
 tools/libxl/libxl_dom_suspend.c |  8 ++++++--
 tools/libxl/libxl_types.idl     |  1 +
 4 files changed, 28 insertions(+), 5 deletions(-)

diff --git a/tools/libxl/libxl.h b/tools/libxl/libxl.h
index 482499a6c0..e0f6916b66 100644
--- a/tools/libxl/libxl.h
+++ b/tools/libxl/libxl.h
@@ -1182,6 +1182,13 @@ void libxl_mac_copy(libxl_ctx *ctx, libxl_mac *dst, 
const libxl_mac *src);
  */
 #define LIBXL_HAVE_PVCALLS 1
 
+/*
+ * LIBXL_HAVE_DEVICE_MODEL_VERSION_NONE
+ *
+ * If this is defined, libxl will only run a device-model if required.
+ */
+#define LIBXL_HAVE_DEVICE_MODEL_VERSION_NONE 1
+
 typedef char **libxl_string_list;
 void libxl_string_list_dispose(libxl_string_list *sl);
 int libxl_string_list_length(const libxl_string_list *sl);
diff --git a/tools/libxl/libxl_create.c b/tools/libxl/libxl_create.c
index 3f0431cc84..64336b0d29 100644
--- a/tools/libxl/libxl_create.c
+++ b/tools/libxl/libxl_create.c
@@ -47,9 +47,20 @@ int libxl__domain_set_device_model(libxl__gc *gc, 
libxl_domain_config *d_config)
         }
         break;
     default:
-        b_info->device_model_version =
-            LIBXL_DEVICE_MODEL_VERSION_QEMU_XEN_TRADITIONAL;
-        break;
+        ret = libxl__need_xenpv_qemu(gc, d_config);
+        switch (ret) {
+        case 1:
+            d_config->b_info.device_model_version =
+                LIBXL_DEVICE_MODEL_VERSION_QEMU_XEN;
+            break;
+        case 0:
+            d_config->b_info.device_model_version =
+                LIBXL_DEVICE_MODEL_VERSION_NONE;
+            break;
+        default:
+            LOGE(ERROR, "Unable to determine QEMU requisite");
+            return ret;
+        }
     }
 
     if (b_info->device_model_version == LIBXL_DEVICE_MODEL_VERSION_QEMU_XEN) {
diff --git a/tools/libxl/libxl_dom_suspend.c b/tools/libxl/libxl_dom_suspend.c
index d1af3a6573..c492fe5dd1 100644
--- a/tools/libxl/libxl_dom_suspend.c
+++ b/tools/libxl/libxl_dom_suspend.c
@@ -379,7 +379,9 @@ static void 
domain_suspend_common_guest_suspended(libxl__egc *egc,
     libxl__ev_xswatch_deregister(gc, &dsps->guest_watch);
     libxl__ev_time_deregister(gc, &dsps->guest_timeout);
 
-    if (dsps->type == LIBXL_DOMAIN_TYPE_HVM) {
+    if (dsps->type == LIBXL_DOMAIN_TYPE_HVM ||
+        libxl__device_model_version_running(gc, dsps->domid) ==
+        LIBXL_DEVICE_MODEL_VERSION_QEMU_XEN) {
         dsps->callback_device_model_done = domain_suspend_common_done;
         libxl__domain_suspend_device_model(egc, dsps); /* must be last */
         return;
@@ -459,7 +461,9 @@ int libxl__domain_resume(libxl__gc *gc, uint32_t domid, int 
suspend_cancel)
         goto out;
     }
 
-    if (type == LIBXL_DOMAIN_TYPE_HVM) {
+    if (type == LIBXL_DOMAIN_TYPE_HVM ||
+        libxl__device_model_version_running(gc, domid) ==
+        LIBXL_DEVICE_MODEL_VERSION_QEMU_XEN) {
         rc = libxl__domain_resume_device_model(gc, domid);
         if (rc) {
             LOGD(ERROR, domid, "failed to resume device model:%d", rc);
diff --git a/tools/libxl/libxl_types.idl b/tools/libxl/libxl_types.idl
index cb4702fd7a..75bde095bc 100644
--- a/tools/libxl/libxl_types.idl
+++ b/tools/libxl/libxl_types.idl
@@ -106,6 +106,7 @@ libxl_device_model_version = 
Enumeration("device_model_version", [
     (0, "UNKNOWN"),
     (1, "QEMU_XEN_TRADITIONAL"), # Historical qemu-xen device model (qemu-dm)
     (2, "QEMU_XEN"),             # Upstream based qemu-xen device model
+    (3, "NONE"),
     ])
 
 libxl_console_type = Enumeration("console_type", [

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