Series comparison

-[PATCH 0/4] vhost-user-blk: live resize additional APIs
+[PATCH v4 0/3] vhost-user-blk: live resize additional APIs
-In vhost-user protocol we have VHOST_USER_BACKEND_CONFIG_CHANGE_MSG,
+v4:
-which backend may send to notify Qemu, that we should re-read the
+Fixes 01-02 from v3 are already merged.
-config, and notify the guest.
+: new, split out from 03
 : refacting vhost_user_blk_handle_config_change() split out to 02
     drop current_run_state_str() helper
     some rewordings (Markus)
-Still that's not always convenient: backend may not support this
+Vladimir Sementsov-Ogievskiy (3):
-message. Also, having QMP command to force config sync is more reliable
+  qdev-monitor: add option to report GenericError from find_device_state
-than waiting for notification from external program. It also may be
+  vhost-user-blk: split vhost_user_blk_sync_config()
-helpful for debug/restore: if we have changed disk size, but guest
+  qapi: introduce device-sync-config
 doesn't see that, it's good to have a separate QMP command to trigger
 resync of the config.
-So, the series proposes two experimental APIs:
+ hw/block/vhost-user-blk.c | 27 ++++++++++++-----
+ hw/virtio/virtio-pci.c    |  9 ++++++
-. x-device-sync-config command, to trigger config synchronization
+ include/hw/qdev-core.h    |  3 ++
+ qapi/qdev.json            | 23 ++++++++++++++
-. X_CONFIG_READ event, which notify management tool that guest read the
+ system/qdev-monitor.c     | 63 ++++++++++++++++++++++++++++++++++++---
-updated config. Of course, that can't guarantee that the guest correctly
+files changed, 114 insertions(+), 11 deletions(-)
 handled the updated config, but it's still better than nothing: for sure
 guest will not show new disk size until it read the updated config. So,
 management tool may wait for this event to report success to the user.
 The series is based on "[PATCH v8 0/4] pci hotplug tracking": it doesn't
 depend on it, but just modify same files, so I just to avoid extra
 conflicts.
 Based-on: <20231005092926.56231-1-vsementsov@yandex-team.ru>
 Vladimir Sementsov-Ogievskiy (4):
   vhost-user-blk: simplify and fix vhost_user_blk_handle_config_change
   qapi: introduce device-sync-config
   qapi: device-sync-config: check runstate
   qapi: introduce CONFIG_READ event
  hw/block/vhost-user-blk.c | 32 ++++++++++++++---------
  hw/virtio/virtio-pci.c    | 18 +++++++++++++
  include/hw/qdev-core.h    |  3 +++
  include/monitor/qdev.h    |  1 +
  include/sysemu/runstate.h |  1 +
  monitor/monitor.c         |  1 +
  qapi/qdev.json            | 36 ++++++++++++++++++++++++++
  softmmu/qdev-monitor.c    | 53 +++++++++++++++++++++++++++++++++++++++
  softmmu/runstate.c        |  5 ++++
 files changed, 138 insertions(+), 12 deletions(-)
 --
 .34.1

-[PATCH 3/4] qapi: device-sync-config: check runstate
+[PATCH v4 1/3] qdev-monitor: add option to report GenericError from find_device_state
-Command result is racy if allow it during migration. Let's allow the
+Here we just prepare for the following patch, making possible to report
-sync only in RUNNING state.
+GenericError as recommended.
 This patch doesn't aim to prevent further use of DeviceNotFound by
 future interfaces:
  - find_device_state() is used in blk_by_qdev_id() and qmp_get_blk()
    functions, which may lead to spread of DeviceNotFound anyway
  - also, nothing prevent simply copy-pasting find_device_state() calls
    with false argument
 Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
 ---
- include/sysemu/runstate.h |  1 +
+ system/qdev-monitor.c | 15 +++++++++++----
- softmmu/qdev-monitor.c    | 27 ++++++++++++++++++++++++++-
+file changed, 11 insertions(+), 4 deletions(-)
  softmmu/runstate.c        |  5 +++++
 files changed, 32 insertions(+), 1 deletion(-)
-diff --git a/include/sysemu/runstate.h b/include/sysemu/runstate.h
+diff --git a/system/qdev-monitor.c b/system/qdev-monitor.c
 index XXXXXXX..XXXXXXX 100644
---- a/include/sysemu/runstate.h
+--- a/system/qdev-monitor.c
-+++ b/include/sysemu/runstate.h
++++ b/system/qdev-monitor.c
-@@ -XXX,XX +XXX,XX @@
+@@ -XXX,XX +XXX,XX @@ void qmp_device_add(QDict *qdict, QObject **ret_data, Error **errp)
- #include "qemu/notify.h"
+     object_unref(OBJECT(dev));
+ }
- bool runstate_check(RunState state);
-+const char *current_run_state_str(void);
+-static DeviceState *find_device_state(const char *id, Error **errp)
- void runstate_set(RunState new_state);
++/*
- RunState runstate_get(void);
++ * Note that creating new APIs using error classes other than GenericError is
- bool runstate_is_running(void);
++ * not recommended. Set use_generic_error=true for new interfaces.
-diff --git a/softmmu/qdev-monitor.c b/softmmu/qdev-monitor.c
++ */
-index XXXXXXX..XXXXXXX 100644
++static DeviceState *find_device_state(const char *id, bool use_generic_error,
---- a/softmmu/qdev-monitor.c
++                                      Error **errp)
-+++ b/softmmu/qdev-monitor.c
+ {
-@@ -XXX,XX +XXX,XX @@
+     Object *obj = object_resolve_path_at(qdev_get_peripheral(), id);
- #include "monitor/monitor.h"
+     DeviceState *dev;
- #include "monitor/qdev.h"
- #include "sysemu/arch_init.h"
+     if (!obj) {
-+#include "sysemu/runstate.h"
+-        error_set(errp, ERROR_CLASS_DEVICE_NOT_FOUND,
- #include "qapi/error.h"
++        error_set(errp,
- #include "qapi/qapi-commands-qdev.h"
++                  (use_generic_error ?
- #include "qapi/qapi-events-qdev.h"
++                   ERROR_CLASS_GENERIC_ERROR : ERROR_CLASS_DEVICE_NOT_FOUND),
-@@ -XXX,XX +XXX,XX @@ int qdev_sync_config(DeviceState *dev, Error **errp)
+                   "Device '%s' not found", id);
+         return NULL;
- void qmp_x_device_sync_config(const char *id, Error **errp)
+     }
@@ -XXX,XX +XXX,XX @@ void qdev_unplug(DeviceState *dev, Error **errp)
  void qmp_device_del(const char *id, Error **errp)
  {
 -    DeviceState *dev = find_device_state(id, errp);
-+    MigrationState *s = migrate_get_current();
++    DeviceState *dev = find_device_state(id, false, errp);
-+    DeviceState *dev;
+     if (dev != NULL) {
-+
+         if (dev->pending_deleted_event &&
-+    /*
+             (dev->pending_deleted_expires_ms == 0 ||
-+     * During migration there is a race between syncing`config and migrating it,
+@@ -XXX,XX +XXX,XX @@ BlockBackend *blk_by_qdev_id(const char *id, Error **errp)
-+     * so let's just not allow it.
-+     *
+     GLOBAL_STATE_CODE();
-+     * Moreover, let's not rely on setting up interrupts in paused state, which
-+     * may be a part of migration process.
+-    dev = find_device_state(id, errp);
-+     */
++    dev = find_device_state(id, false, errp);
-+
+     if (dev == NULL) {
-+    if (migration_is_running(s->state)) {
+         return NULL;
 +        error_setg(errp, "Config synchronization is not allowed "
 +                   "during migration.");
 +        return;
 +    }
 +
 +    if (!runstate_is_running()) {
 +        error_setg(errp, "Config synchronization allowed only in '%s' state, "
 +                   "current state is '%s'", RunState_str(RUN_STATE_RUNNING),
 +                   current_run_state_str());
 +        return;
 +    }
 +
 +    dev = find_device_state(id, errp);
      if (!dev) {
          return;
      }
-diff --git a/softmmu/runstate.c b/softmmu/runstate.c
-index XXXXXXX..XXXXXXX 100644
---- a/softmmu/runstate.c
-+++ b/softmmu/runstate.c
-@@ -XXX,XX +XXX,XX @@ bool runstate_check(RunState state)
-     return current_run_state == state;
- }
-+const char *current_run_state_str(void)
-+{
-+    return RunState_str(current_run_state);
-+}
-+
- static void runstate_init(void)
- {
-     const RunStateTransition *p;
 --
 .34.1

-[PATCH 1/4] vhost-user-blk: simplify and fix vhost_user_blk_handle_config_change
+[PATCH v4 2/3] vhost-user-blk: split vhost_user_blk_sync_config()
-Let's not care about what was changed and update the whole config,
+Split vhost_user_blk_sync_config() out from
-reasons:
+vhost_user_blk_handle_config_change(), to be reused in the following
+commit.
 . config->geometry should be updated together with capacity, so we fix
    a bug.
 . Vhost-user protocol doesn't say anything about config change
    limitation. Silent ignore of changes doesn't seem to be correct.
 . vhost-user-vsock reads the whole config
 . on realize we don't do any checks on retrieved config, so no reason
    to care here
 Also, let's notify guest unconditionally:
 . So does vhost-user-vsock
 . We are going to reuse the functionality in new cases when we do want
    to notify the guest unconditionally. So, no reason to create extra
    branches in the logic.
 Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
 ---
- hw/block/vhost-user-blk.c | 11 +++--------
+ hw/block/vhost-user-blk.c | 26 +++++++++++++++++++-------
-file changed, 3 insertions(+), 8 deletions(-)
+file changed, 19 insertions(+), 7 deletions(-)
 diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
 index XXXXXXX..XXXXXXX 100644
 --- a/hw/block/vhost-user-blk.c
 +++ b/hw/block/vhost-user-blk.c
 @@ -XXX,XX +XXX,XX @@ static void vhost_user_blk_set_config(VirtIODevice *vdev, const uint8_t *config)
+     s->blkcfg.wce = blkcfg->wce;
+ }
++static int vhost_user_blk_sync_config(DeviceState *dev, Error **errp)
++{
++    int ret;
++    VirtIODevice *vdev = VIRTIO_DEVICE(dev);
++    VHostUserBlk *s = VHOST_USER_BLK(vdev);
++
++    ret = vhost_dev_get_config(&s->dev, (uint8_t *)&s->blkcfg,
++                               vdev->config_len, errp);
++    if (ret < 0) {
++        return ret;
++    }
++
++    memcpy(vdev->config, &s->blkcfg, vdev->config_len);
++    virtio_notify_config(vdev);
++
++    return 0;
++}
++
  static int vhost_user_blk_handle_config_change(struct vhost_dev *dev)
  {
      int ret;
--    struct virtio_blk_config blkcfg;
+-    VirtIODevice *vdev = dev->vdev;
-     VirtIODevice *vdev = dev->vdev;
+-    VHostUserBlk *s = VHOST_USER_BLK(dev->vdev);
      VHostUserBlk *s = VHOST_USER_BLK(dev->vdev);
      Error *local_err = NULL;
-@@ -XXX,XX +XXX,XX @@ static int vhost_user_blk_handle_config_change(struct vhost_dev *dev)
      if (!dev->started) {
          return 0;
      }
--    ret = vhost_dev_get_config(dev, (uint8_t *)&blkcfg,
+-    ret = vhost_dev_get_config(dev, (uint8_t *)&s->blkcfg,
-+    ret = vhost_dev_get_config(dev, (uint8_t *)&s->blkcfg,
+-                               vdev->config_len, &local_err);
-                                vdev->config_len, &local_err);
++    ret = vhost_user_blk_sync_config(DEVICE(dev->vdev), &local_err);
      if (ret < 0) {
          error_report_err(local_err);
          return ret;
      }
--    /* valid for resize only */
+-    memcpy(dev->vdev->config, &s->blkcfg, vdev->config_len);
--    if (blkcfg.capacity != s->blkcfg.capacity) {
+-    virtio_notify_config(dev->vdev);
--        s->blkcfg.capacity = blkcfg.capacity;
+-
 -        memcpy(dev->vdev->config, &s->blkcfg, vdev->config_len);
 -        virtio_notify_config(dev->vdev);
 -    }
 +    memcpy(dev->vdev->config, &s->blkcfg, vdev->config_len);
 +    virtio_notify_config(dev->vdev);
      return 0;
  }
 --
 .34.1

-[PATCH 2/4] qapi: introduce device-sync-config
+[PATCH v4 3/3] qapi: introduce device-sync-config
 Add command to sync config from vhost-user backend to the device. It
 may be helpful when VHOST_USER_SLAVE_CONFIG_CHANGE_MSG failed or not
 triggered interrupt to the guest or just not available (not supported
 by vhost-user server).
+Command result is racy if allow it during migration. Let's allow the
+sync only in RUNNING state.
 Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
 ---
- hw/block/vhost-user-blk.c | 27 ++++++++++++++++++++-------
+ hw/block/vhost-user-blk.c |  1 +
- hw/virtio/virtio-pci.c    |  9 +++++++++
+ hw/virtio/virtio-pci.c    |  9 ++++++++
  include/hw/qdev-core.h    |  3 +++
- qapi/qdev.json            | 14 ++++++++++++++
+ qapi/qdev.json            | 23 +++++++++++++++++++
- softmmu/qdev-monitor.c    | 23 +++++++++++++++++++++++
+ system/qdev-monitor.c     | 48 +++++++++++++++++++++++++++++++++++++++
-files changed, 69 insertions(+), 7 deletions(-)
+files changed, 84 insertions(+)
 diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
 index XXXXXXX..XXXXXXX 100644
 --- a/hw/block/vhost-user-blk.c
 +++ b/hw/block/vhost-user-blk.c
-@@ -XXX,XX +XXX,XX @@ static void vhost_user_blk_set_config(VirtIODevice *vdev, const uint8_t *config)
-     s->blkcfg.wce = blkcfg->wce;
- }
-+static int vhost_user_blk_sync_config(DeviceState *dev, Error **errp)
-+{
-+    int ret;
-+    VirtIODevice *vdev = VIRTIO_DEVICE(dev);
-+    VHostUserBlk *s = VHOST_USER_BLK(vdev);
-+
-+    ret = vhost_dev_get_config(&s->dev, (uint8_t *)&s->blkcfg,
-+                               vdev->config_len, errp);
-+    if (ret < 0) {
-+        return ret;
-+    }
-+
-+    memcpy(vdev->config, &s->blkcfg, vdev->config_len);
-+    virtio_notify_config(vdev);
-+
-+    return 0;
-+}
-+
- static int vhost_user_blk_handle_config_change(struct vhost_dev *dev)
- {
-     int ret;
--    VirtIODevice *vdev = dev->vdev;
--    VHostUserBlk *s = VHOST_USER_BLK(dev->vdev);
-     Error *local_err = NULL;
-     if (!dev->started) {
-         return 0;
-     }
--    ret = vhost_dev_get_config(dev, (uint8_t *)&s->blkcfg,
--                               vdev->config_len, &local_err);
-+    ret = vhost_user_blk_sync_config(DEVICE(dev->vdev), &local_err);
-     if (ret < 0) {
-         error_report_err(local_err);
-         return ret;
-     }
--    memcpy(dev->vdev->config, &s->blkcfg, vdev->config_len);
--    virtio_notify_config(dev->vdev);
--
-     return 0;
- }
 @@ -XXX,XX +XXX,XX @@ static void vhost_user_blk_class_init(ObjectClass *klass, void *data)
      device_class_set_props(dc, vhost_user_blk_properties);
      dc->vmsd = &vmstate_vhost_user_blk;
 +    dc->sync_config = vhost_user_blk_sync_config;
 ...
 diff --git a/qapi/qdev.json b/qapi/qdev.json
 index XXXXXXX..XXXXXXX 100644
 --- a/qapi/qdev.json
 +++ b/qapi/qdev.json
 @@ -XXX,XX +XXX,XX @@
- # Since: 8.2
  ##
- { 'event': 'X_DEVICE_ON', 'data': 'DeviceAndPath' }
+ { 'event': 'DEVICE_UNPLUG_GUEST_ERROR',
    'data': { '*device': 'str', 'path': 'str' } }
 +
 +##
-+# @x-device-sync-config:
++# @device-sync-config:
 +#
-+# Sync config from backend to the guest.
++# Synchronize device configuration from host to guest part.  First,
 +# copy the configuration from the host part (backend) to the guest
 +# part (frontend).  Then notify guest software that device
 +# configuration changed.
 +# The command may be used to notify the guest about block device
 +# capcity change.  Currently only vhost-user-blk device supports
 +# this.
 +#
 +# @id: the device's ID or QOM path
 +#
-+# Returns: Nothing on success
++# Features:
 +#          If @id is not a valid device, DeviceNotFound
 +#
-+# Since: 8.2
++# @unstable: The command is experimental.
 +#
 +# Since: 9.1
 +##
-+{ 'command': 'x-device-sync-config', 'data': {'id': 'str'} }
++{ 'command': 'device-sync-config',
-diff --git a/softmmu/qdev-monitor.c b/softmmu/qdev-monitor.c
++  'features': [ 'unstable' ],
 +  'data': {'id': 'str'} }
 diff --git a/system/qdev-monitor.c b/system/qdev-monitor.c
 index XXXXXXX..XXXXXXX 100644
---- a/softmmu/qdev-monitor.c
+--- a/system/qdev-monitor.c
-+++ b/softmmu/qdev-monitor.c
++++ b/system/qdev-monitor.c
-@@ -XXX,XX +XXX,XX @@ HotplugInfo *qmp_x_query_hotplug(const char *id, Error **errp)
+@@ -XXX,XX +XXX,XX @@
-     return hotplug_handler_get_state(hotplug_ctrl, dev, errp);
+ #include "monitor/monitor.h"
  #include "monitor/qdev.h"
  #include "sysemu/arch_init.h"
 +#include "sysemu/runstate.h"
  #include "qapi/error.h"
  #include "qapi/qapi-commands-qdev.h"
  #include "qapi/qmp/dispatch.h"
@@ -XXX,XX +XXX,XX @@ void qmp_device_del(const char *id, Error **errp)
      }
  }
 +int qdev_sync_config(DeviceState *dev, Error **errp)
 +{
 +    DeviceClass *dc = DEVICE_GET_CLASS(dev);
 ...
 +    }
 +
 +    return dc->sync_config(dev, errp);
 +}
 +
-+void qmp_x_device_sync_config(const char *id, Error **errp)
++void qmp_device_sync_config(const char *id, Error **errp)
 +{
-+    DeviceState *dev = find_device_state(id, errp);
++    DeviceState *dev;
 +
 +    /*
 +     * During migration there is a race between syncing`configuration and
 +     * migrating it (if migrate first, that target would get outdated version),
 +     * so let's just not allow it.
 +     *
 +     * Moreover, let's not rely on setting up interrupts in paused
 +     * state, which may be a part of migration process.
 +     */
 +
 +    if (migration_is_running()) {
 +        error_setg(errp, "Config synchronization is not allowed "
 +                   "during migration");
 +        return;
 +    }
 +
 +    if (!runstate_is_running()) {
 +        error_setg(errp, "Config synchronization allowed only in '%s' state, "
 +                   "current state is '%s'", RunState_str(RUN_STATE_RUNNING),
 +                   RunState_str(runstate_get()));
 +        return;
 +    }
 +
 +    dev = find_device_state(id, true, errp);
 +    if (!dev) {
 +        return;
 +    }
 +
 +    qdev_sync_config(dev, errp);
 ...

-[PATCH 4/4] qapi: introduce CONFIG_READ event
+Deleted patch
-Send a new event when guest reads virtio-pci config after
-virtio_notify_config() call.
-That's useful to check that guest fetched modified config, for example
-after resizing disk backend.
-Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
----
- hw/virtio/virtio-pci.c |  9 +++++++++
- include/monitor/qdev.h |  1 +
- monitor/monitor.c      |  1 +
- qapi/qdev.json         | 22 ++++++++++++++++++++++
- softmmu/qdev-monitor.c |  5 +++++
-files changed, 38 insertions(+)
-diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
-index XXXXXXX..XXXXXXX 100644
---- a/hw/virtio/virtio-pci.c
-+++ b/hw/virtio/virtio-pci.c
-@@ -XXX,XX +XXX,XX @@
- #include "hw/boards.h"
- #include "hw/virtio/virtio.h"
- #include "migration/qemu-file-types.h"
-+#include "monitor/qdev.h"
- #include "hw/pci/pci.h"
- #include "hw/pci/pci_bus.h"
- #include "hw/qdev-properties.h"
-@@ -XXX,XX +XXX,XX @@ static uint64_t virtio_pci_config_read(void *opaque, hwaddr addr,
-     }
-     addr -= config;
-+    if (vdev->generation > 0) {
-+        qdev_config_read_event(DEVICE(proxy));
-+    }
-+
-     switch (size) {
-     case 1:
-         val = virtio_config_readb(vdev, addr);
-@@ -XXX,XX +XXX,XX @@ static uint64_t virtio_pci_device_read(void *opaque, hwaddr addr,
-         return UINT64_MAX;
-     }
-+    if (vdev->generation > 0) {
-+        qdev_config_read_event(DEVICE(proxy));
-+    }
-+
-     switch (size) {
-     case 1:
-         val = virtio_config_modern_readb(vdev, addr);
-diff --git a/include/monitor/qdev.h b/include/monitor/qdev.h
-index XXXXXXX..XXXXXXX 100644
---- a/include/monitor/qdev.h
-+++ b/include/monitor/qdev.h
-@@ -XXX,XX +XXX,XX @@ DeviceState *qdev_device_add_from_qdict(const QDict *opts,
- const char *qdev_set_id(DeviceState *dev, char *id, Error **errp);
- void qdev_hotplug_device_on_event(DeviceState *dev);
-+void qdev_config_read_event(DeviceState *dev);
- DeviceAndPath *qdev_new_device_and_path(DeviceState *dev);
-diff --git a/monitor/monitor.c b/monitor/monitor.c
-index XXXXXXX..XXXXXXX 100644
---- a/monitor/monitor.c
-+++ b/monitor/monitor.c
-@@ -XXX,XX +XXX,XX @@ static MonitorQAPIEventConf monitor_qapi_event_conf[QAPI_EVENT__MAX] = {
-     [QAPI_EVENT_QUORUM_FAILURE]    = { 1000 * SCALE_MS },
-     [QAPI_EVENT_VSERPORT_CHANGE]   = { 1000 * SCALE_MS },
-     [QAPI_EVENT_MEMORY_DEVICE_SIZE_CHANGE] = { 1000 * SCALE_MS },
-+    [QAPI_EVENT_X_CONFIG_READ]   = { 300 * SCALE_MS },
- };
- /*
-diff --git a/qapi/qdev.json b/qapi/qdev.json
-index XXXXXXX..XXXXXXX 100644
---- a/qapi/qdev.json
-+++ b/qapi/qdev.json
-@@ -XXX,XX +XXX,XX @@
- # Since: 8.2
- ##
- { 'command': 'x-device-sync-config', 'data': {'id': 'str'} }
-+
-+##
-+# @X_CONFIG_READ:
-+#
-+# Emitted whenever guest reads virtio device config after config change.
-+#
-+# @device: device name
-+#
-+# @path: device path
-+#
-+# Since: 5.0.1-24
-+#
-+# Example:
-+#
-+# <- { "event": "X_CONFIG_READ",
-+#      "data": { "device": "virtio-net-pci-0",
-+#                "path": "/machine/peripheral/virtio-net-pci-0" },
-+#      "timestamp": { "seconds": 1265044230, "microseconds": 450486 } }
-+#
-+##
-+{ 'event': 'X_CONFIG_READ',
-+  'data': { '*device': 'str', 'path': 'str' } }
-diff --git a/softmmu/qdev-monitor.c b/softmmu/qdev-monitor.c
-index XXXXXXX..XXXXXXX 100644
---- a/softmmu/qdev-monitor.c
-+++ b/softmmu/qdev-monitor.c
-@@ -XXX,XX +XXX,XX @@ void qdev_hotplug_device_on_event(DeviceState *dev)
-     dev->device_on_event_sent = true;
-     qapi_event_send_x_device_on(dev->id, dev->canonical_path);
- }
-+
-+void qdev_config_read_event(DeviceState *dev)
-+{
-+    qapi_event_send_x_config_read(dev->id, dev->canonical_path);
-+}
---
-.34.1

In vhost-user protocol we have VHOST_USER_BACKEND_CONFIG_CHANGE_MSG,
which backend may send to notify Qemu, that we should re-read the
config, and notify the guest.

Still that's not always convenient: backend may not support this
message. Also, having QMP command to force config sync is more reliable
than waiting for notification from external program. It also may be
helpful for debug/restore: if we have changed disk size, but guest
doesn't see that, it's good to have a separate QMP command to trigger
resync of the config.

So, the series proposes two experimental APIs:

1. x-device-sync-config command, to trigger config synchronization

2. X_CONFIG_READ event, which notify management tool that guest read the
updated config. Of course, that can't guarantee that the guest correctly
handled the updated config, but it's still better than nothing: for sure
guest will not show new disk size until it read the updated config. So,
management tool may wait for this event to report success to the user.

The series is based on "[PATCH v8 0/4] pci hotplug tracking": it doesn't
depend on it, but just modify same files, so I just to avoid extra
conflicts.
Based-on: <20231005092926.56231-1-vsementsov@yandex-team.ru>

Vladimir Sementsov-Ogievskiy (4):
  vhost-user-blk: simplify and fix vhost_user_blk_handle_config_change
  qapi: introduce device-sync-config
  qapi: device-sync-config: check runstate
  qapi: introduce CONFIG_READ event

-- 
2.34.1

Let's not care about what was changed and update the whole config,
reasons:

1. config->geometry should be updated together with capacity, so we fix
   a bug.

2. Vhost-user protocol doesn't say anything about config change
   limitation. Silent ignore of changes doesn't seem to be correct.

3. vhost-user-vsock reads the whole config

4. on realize we don't do any checks on retrieved config, so no reason
   to care here

Also, let's notify guest unconditionally:

1. So does vhost-user-vsock

2. We are going to reuse the functionality in new cases when we do want
   to notify the guest unconditionally. So, no reason to create extra
   branches in the logic.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
---
 hw/block/vhost-user-blk.c | 11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
index XXXXXXX..XXXXXXX 100644
--- a/hw/block/vhost-user-blk.c
+++ b/hw/block/vhost-user-blk.c
@@ -XXX,XX +XXX,XX @@ static void vhost_user_blk_set_config(VirtIODevice *vdev, const uint8_t *config)
 static int vhost_user_blk_handle_config_change(struct vhost_dev *dev)
 {
     int ret;
-    struct virtio_blk_config blkcfg;
     VirtIODevice *vdev = dev->vdev;
     VHostUserBlk *s = VHOST_USER_BLK(dev->vdev);
     Error *local_err = NULL;
@@ -XXX,XX +XXX,XX @@ static int vhost_user_blk_handle_config_change(struct vhost_dev *dev)
         return 0;
     }
 
-    ret = vhost_dev_get_config(dev, (uint8_t *)&blkcfg,
+    ret = vhost_dev_get_config(dev, (uint8_t *)&s->blkcfg,
                                vdev->config_len, &local_err);
     if (ret < 0) {
         error_report_err(local_err);
         return ret;
     }
 
-    /* valid for resize only */
-    if (blkcfg.capacity != s->blkcfg.capacity) {
-        s->blkcfg.capacity = blkcfg.capacity;
-        memcpy(dev->vdev->config, &s->blkcfg, vdev->config_len);
-        virtio_notify_config(dev->vdev);
-    }
+    memcpy(dev->vdev->config, &s->blkcfg, vdev->config_len);
+    virtio_notify_config(dev->vdev);
 
     return 0;
 }
-- 
2.34.1

Add command to sync config from vhost-user backend to the device. It
may be helpful when VHOST_USER_SLAVE_CONFIG_CHANGE_MSG failed or not
triggered interrupt to the guest or just not available (not supported
by vhost-user server).

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
---
 hw/block/vhost-user-blk.c | 27 ++++++++++++++++++++-------
 hw/virtio/virtio-pci.c    |  9 +++++++++
 include/hw/qdev-core.h    |  3 +++
 qapi/qdev.json            | 14 ++++++++++++++
 softmmu/qdev-monitor.c    | 23 +++++++++++++++++++++++
 5 files changed, 69 insertions(+), 7 deletions(-)

diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
index XXXXXXX..XXXXXXX 100644
--- a/hw/block/vhost-user-blk.c
+++ b/hw/block/vhost-user-blk.c
@@ -XXX,XX +XXX,XX @@ static void vhost_user_blk_set_config(VirtIODevice *vdev, const uint8_t *config)
     s->blkcfg.wce = blkcfg->wce;
 }
 
+static int vhost_user_blk_sync_config(DeviceState *dev, Error **errp)
+{
+    int ret;
+    VirtIODevice *vdev = VIRTIO_DEVICE(dev);
+    VHostUserBlk *s = VHOST_USER_BLK(vdev);
+
+    ret = vhost_dev_get_config(&s->dev, (uint8_t *)&s->blkcfg,
+                               vdev->config_len, errp);
+    if (ret < 0) {
+        return ret;
+    }
+
+    memcpy(vdev->config, &s->blkcfg, vdev->config_len);
+    virtio_notify_config(vdev);
+
+    return 0;
+}
+
 static int vhost_user_blk_handle_config_change(struct vhost_dev *dev)
 {
     int ret;
-    VirtIODevice *vdev = dev->vdev;
-    VHostUserBlk *s = VHOST_USER_BLK(dev->vdev);
     Error *local_err = NULL;
 
     if (!dev->started) {
         return 0;
     }
 
-    ret = vhost_dev_get_config(dev, (uint8_t *)&s->blkcfg,
-                               vdev->config_len, &local_err);
+    ret = vhost_user_blk_sync_config(DEVICE(dev->vdev), &local_err);
     if (ret < 0) {
         error_report_err(local_err);
         return ret;
     }
 
-    memcpy(dev->vdev->config, &s->blkcfg, vdev->config_len);
-    virtio_notify_config(dev->vdev);
-
     return 0;
 }
 
@@ -XXX,XX +XXX,XX @@ static void vhost_user_blk_class_init(ObjectClass *klass, void *data)
 
     device_class_set_props(dc, vhost_user_blk_properties);
     dc->vmsd = &vmstate_vhost_user_blk;
+    dc->sync_config = vhost_user_blk_sync_config;
     set_bit(DEVICE_CATEGORY_STORAGE, dc->categories);
     vdc->realize = vhost_user_blk_device_realize;
     vdc->unrealize = vhost_user_blk_device_unrealize;
diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index XXXXXXX..XXXXXXX 100644
--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -XXX,XX +XXX,XX @@ static void virtio_pci_dc_realize(DeviceState *qdev, Error **errp)
     vpciklass->parent_dc_realize(qdev, errp);
 }
 
+static int virtio_pci_sync_config(DeviceState *dev, Error **errp)
+{
+    VirtIOPCIProxy *proxy = VIRTIO_PCI(dev);
+    VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
+
+    return qdev_sync_config(DEVICE(vdev), errp);
+}
+
 static void virtio_pci_class_init(ObjectClass *klass, void *data)
 {
     DeviceClass *dc = DEVICE_CLASS(klass);
@@ -XXX,XX +XXX,XX @@ static void virtio_pci_class_init(ObjectClass *klass, void *data)
     device_class_set_parent_realize(dc, virtio_pci_dc_realize,
                                     &vpciklass->parent_dc_realize);
     rc->phases.hold = virtio_pci_bus_reset_hold;
+    dc->sync_config = virtio_pci_sync_config;
 }
 
 static const TypeInfo virtio_pci_info = {
diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
index XXXXXXX..XXXXXXX 100644
--- a/include/hw/qdev-core.h
+++ b/include/hw/qdev-core.h
@@ -XXX,XX +XXX,XX @@ typedef void (*DeviceUnrealize)(DeviceState *dev);
 typedef void (*DeviceReset)(DeviceState *dev);
 typedef void (*BusRealize)(BusState *bus, Error **errp);
 typedef void (*BusUnrealize)(BusState *bus);
+typedef int (*DeviceSyncConfig)(DeviceState *dev, Error **errp);
 
 /**
  * struct DeviceClass - The base class for all devices.
@@ -XXX,XX +XXX,XX @@ struct DeviceClass {
     DeviceReset reset;
     DeviceRealize realize;
     DeviceUnrealize unrealize;
+    DeviceSyncConfig sync_config;
 
     /**
      * @vmsd: device state serialisation description for
@@ -XXX,XX +XXX,XX @@ bool qdev_hotplug_allowed(DeviceState *dev, Error **errp);
  */
 HotplugHandler *qdev_get_hotplug_handler(DeviceState *dev);
 void qdev_unplug(DeviceState *dev, Error **errp);
+int qdev_sync_config(DeviceState *dev, Error **errp);
 void qdev_simple_device_unplug_cb(HotplugHandler *hotplug_dev,
                                   DeviceState *dev, Error **errp);
 void qdev_machine_creation_done(void);
diff --git a/qapi/qdev.json b/qapi/qdev.json
index XXXXXXX..XXXXXXX 100644
--- a/qapi/qdev.json
+++ b/qapi/qdev.json
@@ -XXX,XX +XXX,XX @@
 # Since: 8.2
 ##
 { 'event': 'X_DEVICE_ON', 'data': 'DeviceAndPath' }
+
+##
+# @x-device-sync-config:
+#
+# Sync config from backend to the guest.
+#
+# @id: the device's ID or QOM path
+#
+# Returns: Nothing on success
+#          If @id is not a valid device, DeviceNotFound
+#
+# Since: 8.2
+##
+{ 'command': 'x-device-sync-config', 'data': {'id': 'str'} }
diff --git a/softmmu/qdev-monitor.c b/softmmu/qdev-monitor.c
index XXXXXXX..XXXXXXX 100644
--- a/softmmu/qdev-monitor.c
+++ b/softmmu/qdev-monitor.c
@@ -XXX,XX +XXX,XX @@ HotplugInfo *qmp_x_query_hotplug(const char *id, Error **errp)
     return hotplug_handler_get_state(hotplug_ctrl, dev, errp);
 }
 
+int qdev_sync_config(DeviceState *dev, Error **errp)
+{
+    DeviceClass *dc = DEVICE_GET_CLASS(dev);
+
+    if (!dc->sync_config) {
+        error_setg(errp, "device-sync-config is not supported for '%s'",
+                   object_get_typename(OBJECT(dev)));
+        return -ENOTSUP;
+    }
+
+    return dc->sync_config(dev, errp);
+}
+
+void qmp_x_device_sync_config(const char *id, Error **errp)
+{
+    DeviceState *dev = find_device_state(id, errp);
+    if (!dev) {
+        return;
+    }
+
+    qdev_sync_config(dev, errp);
+}
+
 void hmp_device_add(Monitor *mon, const QDict *qdict)
 {
     Error *err = NULL;
-- 
2.34.1

Command result is racy if allow it during migration. Let's allow the
sync only in RUNNING state.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
---
 include/sysemu/runstate.h |  1 +
 softmmu/qdev-monitor.c    | 27 ++++++++++++++++++++++++++-
 softmmu/runstate.c        |  5 +++++
 3 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/include/sysemu/runstate.h b/include/sysemu/runstate.h
index XXXXXXX..XXXXXXX 100644
--- a/include/sysemu/runstate.h
+++ b/include/sysemu/runstate.h
@@ -XXX,XX +XXX,XX @@
 #include "qemu/notify.h"
 
 bool runstate_check(RunState state);
+const char *current_run_state_str(void);
 void runstate_set(RunState new_state);
 RunState runstate_get(void);
 bool runstate_is_running(void);
diff --git a/softmmu/qdev-monitor.c b/softmmu/qdev-monitor.c
index XXXXXXX..XXXXXXX 100644
--- a/softmmu/qdev-monitor.c
+++ b/softmmu/qdev-monitor.c
@@ -XXX,XX +XXX,XX @@
 #include "monitor/monitor.h"
 #include "monitor/qdev.h"
 #include "sysemu/arch_init.h"
+#include "sysemu/runstate.h"
 #include "qapi/error.h"
 #include "qapi/qapi-commands-qdev.h"
 #include "qapi/qapi-events-qdev.h"
@@ -XXX,XX +XXX,XX @@ int qdev_sync_config(DeviceState *dev, Error **errp)
 
 void qmp_x_device_sync_config(const char *id, Error **errp)
 {
-    DeviceState *dev = find_device_state(id, errp);
+    MigrationState *s = migrate_get_current();
+    DeviceState *dev;
+
+    /*
+     * During migration there is a race between syncing`config and migrating it,
+     * so let's just not allow it.
+     *
+     * Moreover, let's not rely on setting up interrupts in paused state, which
+     * may be a part of migration process.
+     */
+
+    if (migration_is_running(s->state)) {
+        error_setg(errp, "Config synchronization is not allowed "
+                   "during migration.");
+        return;
+    }
+
+    if (!runstate_is_running()) {
+        error_setg(errp, "Config synchronization allowed only in '%s' state, "
+                   "current state is '%s'", RunState_str(RUN_STATE_RUNNING),
+                   current_run_state_str());
+        return;
+    }
+
+    dev = find_device_state(id, errp);
     if (!dev) {
         return;
     }
diff --git a/softmmu/runstate.c b/softmmu/runstate.c
index XXXXXXX..XXXXXXX 100644
--- a/softmmu/runstate.c
+++ b/softmmu/runstate.c
@@ -XXX,XX +XXX,XX @@ bool runstate_check(RunState state)
     return current_run_state == state;
 }
 
+const char *current_run_state_str(void)
+{
+    return RunState_str(current_run_state);
+}
+
 static void runstate_init(void)
 {
     const RunStateTransition *p;
-- 
2.34.1

Send a new event when guest reads virtio-pci config after
virtio_notify_config() call.

That's useful to check that guest fetched modified config, for example
after resizing disk backend.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
---
 hw/virtio/virtio-pci.c |  9 +++++++++
 include/monitor/qdev.h |  1 +
 monitor/monitor.c      |  1 +
 qapi/qdev.json         | 22 ++++++++++++++++++++++
 softmmu/qdev-monitor.c |  5 +++++
 5 files changed, 38 insertions(+)

diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index XXXXXXX..XXXXXXX 100644
--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -XXX,XX +XXX,XX @@
 #include "hw/boards.h"
 #include "hw/virtio/virtio.h"
 #include "migration/qemu-file-types.h"
+#include "monitor/qdev.h"
 #include "hw/pci/pci.h"
 #include "hw/pci/pci_bus.h"
 #include "hw/qdev-properties.h"
@@ -XXX,XX +XXX,XX @@ static uint64_t virtio_pci_config_read(void *opaque, hwaddr addr,
     }
     addr -= config;
 
+    if (vdev->generation > 0) {
+        qdev_config_read_event(DEVICE(proxy));
+    }
+
     switch (size) {
     case 1:
         val = virtio_config_readb(vdev, addr);
@@ -XXX,XX +XXX,XX @@ static uint64_t virtio_pci_device_read(void *opaque, hwaddr addr,
         return UINT64_MAX;
     }
 
+    if (vdev->generation > 0) {
+        qdev_config_read_event(DEVICE(proxy));
+    }
+
     switch (size) {
     case 1:
         val = virtio_config_modern_readb(vdev, addr);
diff --git a/include/monitor/qdev.h b/include/monitor/qdev.h
index XXXXXXX..XXXXXXX 100644
--- a/include/monitor/qdev.h
+++ b/include/monitor/qdev.h
@@ -XXX,XX +XXX,XX @@ DeviceState *qdev_device_add_from_qdict(const QDict *opts,
 const char *qdev_set_id(DeviceState *dev, char *id, Error **errp);
 
 void qdev_hotplug_device_on_event(DeviceState *dev);
+void qdev_config_read_event(DeviceState *dev);
 
 DeviceAndPath *qdev_new_device_and_path(DeviceState *dev);
 
diff --git a/monitor/monitor.c b/monitor/monitor.c
index XXXXXXX..XXXXXXX 100644
--- a/monitor/monitor.c
+++ b/monitor/monitor.c
@@ -XXX,XX +XXX,XX @@ static MonitorQAPIEventConf monitor_qapi_event_conf[QAPI_EVENT__MAX] = {
     [QAPI_EVENT_QUORUM_FAILURE]    = { 1000 * SCALE_MS },
     [QAPI_EVENT_VSERPORT_CHANGE]   = { 1000 * SCALE_MS },
     [QAPI_EVENT_MEMORY_DEVICE_SIZE_CHANGE] = { 1000 * SCALE_MS },
+    [QAPI_EVENT_X_CONFIG_READ]   = { 300 * SCALE_MS },
 };
 
 /*
diff --git a/qapi/qdev.json b/qapi/qdev.json
index XXXXXXX..XXXXXXX 100644
--- a/qapi/qdev.json
+++ b/qapi/qdev.json
@@ -XXX,XX +XXX,XX @@
 # Since: 8.2
 ##
 { 'command': 'x-device-sync-config', 'data': {'id': 'str'} }
+
+##
+# @X_CONFIG_READ:
+#
+# Emitted whenever guest reads virtio device config after config change.
+#
+# @device: device name
+#
+# @path: device path
+#
+# Since: 5.0.1-24
+#
+# Example:
+#
+# <- { "event": "X_CONFIG_READ",
+#      "data": { "device": "virtio-net-pci-0",
+#                "path": "/machine/peripheral/virtio-net-pci-0" },
+#      "timestamp": { "seconds": 1265044230, "microseconds": 450486 } }
+#
+##
+{ 'event': 'X_CONFIG_READ',
+  'data': { '*device': 'str', 'path': 'str' } }
diff --git a/softmmu/qdev-monitor.c b/softmmu/qdev-monitor.c
index XXXXXXX..XXXXXXX 100644
--- a/softmmu/qdev-monitor.c
+++ b/softmmu/qdev-monitor.c
@@ -XXX,XX +XXX,XX @@ void qdev_hotplug_device_on_event(DeviceState *dev)
     dev->device_on_event_sent = true;
     qapi_event_send_x_device_on(dev->id, dev->canonical_path);
 }
+
+void qdev_config_read_event(DeviceState *dev)
+{
+    qapi_event_send_x_config_read(dev->id, dev->canonical_path);
+}
-- 
2.34.1

Here we just prepare for the following patch, making possible to report
GenericError as recommended.

This patch doesn't aim to prevent further use of DeviceNotFound by
future interfaces:

- find_device_state() is used in blk_by_qdev_id() and qmp_get_blk()
   functions, which may lead to spread of DeviceNotFound anyway
 - also, nothing prevent simply copy-pasting find_device_state() calls
   with false argument

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
---
 system/qdev-monitor.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/system/qdev-monitor.c b/system/qdev-monitor.c
index XXXXXXX..XXXXXXX 100644
--- a/system/qdev-monitor.c
+++ b/system/qdev-monitor.c
@@ -XXX,XX +XXX,XX @@ void qmp_device_add(QDict *qdict, QObject **ret_data, Error **errp)
     object_unref(OBJECT(dev));
 }
 
-static DeviceState *find_device_state(const char *id, Error **errp)
+/*
+ * Note that creating new APIs using error classes other than GenericError is
+ * not recommended. Set use_generic_error=true for new interfaces.
+ */
+static DeviceState *find_device_state(const char *id, bool use_generic_error,
+                                      Error **errp)
 {
     Object *obj = object_resolve_path_at(qdev_get_peripheral(), id);
     DeviceState *dev;
 
     if (!obj) {
-        error_set(errp, ERROR_CLASS_DEVICE_NOT_FOUND,
+        error_set(errp,
+                  (use_generic_error ?
+                   ERROR_CLASS_GENERIC_ERROR : ERROR_CLASS_DEVICE_NOT_FOUND),
                   "Device '%s' not found", id);
         return NULL;
     }
@@ -XXX,XX +XXX,XX @@ void qdev_unplug(DeviceState *dev, Error **errp)
 
 void qmp_device_del(const char *id, Error **errp)
 {
-    DeviceState *dev = find_device_state(id, errp);
+    DeviceState *dev = find_device_state(id, false, errp);
     if (dev != NULL) {
         if (dev->pending_deleted_event &&
             (dev->pending_deleted_expires_ms == 0 ||
@@ -XXX,XX +XXX,XX @@ BlockBackend *blk_by_qdev_id(const char *id, Error **errp)
 
     GLOBAL_STATE_CODE();
 
-    dev = find_device_state(id, errp);
+    dev = find_device_state(id, false, errp);
     if (dev == NULL) {
         return NULL;
     }
-- 
2.34.1

Split vhost_user_blk_sync_config() out from
vhost_user_blk_handle_config_change(), to be reused in the following
commit.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
---
 hw/block/vhost-user-blk.c | 26 +++++++++++++++++++-------
 1 file changed, 19 insertions(+), 7 deletions(-)

diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
index XXXXXXX..XXXXXXX 100644
--- a/hw/block/vhost-user-blk.c
+++ b/hw/block/vhost-user-blk.c
@@ -XXX,XX +XXX,XX @@ static void vhost_user_blk_set_config(VirtIODevice *vdev, const uint8_t *config)
     s->blkcfg.wce = blkcfg->wce;
 }
 
+static int vhost_user_blk_sync_config(DeviceState *dev, Error **errp)
+{
+    int ret;
+    VirtIODevice *vdev = VIRTIO_DEVICE(dev);
+    VHostUserBlk *s = VHOST_USER_BLK(vdev);
+
+    ret = vhost_dev_get_config(&s->dev, (uint8_t *)&s->blkcfg,
+                               vdev->config_len, errp);
+    if (ret < 0) {
+        return ret;
+    }
+
+    memcpy(vdev->config, &s->blkcfg, vdev->config_len);
+    virtio_notify_config(vdev);
+
+    return 0;
+}
+
 static int vhost_user_blk_handle_config_change(struct vhost_dev *dev)
 {
     int ret;
-    VirtIODevice *vdev = dev->vdev;
-    VHostUserBlk *s = VHOST_USER_BLK(dev->vdev);
     Error *local_err = NULL;
 
     if (!dev->started) {
         return 0;
     }
 
-    ret = vhost_dev_get_config(dev, (uint8_t *)&s->blkcfg,
-                               vdev->config_len, &local_err);
+    ret = vhost_user_blk_sync_config(DEVICE(dev->vdev), &local_err);
     if (ret < 0) {
         error_report_err(local_err);
         return ret;
     }
 
-    memcpy(dev->vdev->config, &s->blkcfg, vdev->config_len);
-    virtio_notify_config(dev->vdev);
-
     return 0;
 }
 
-- 
2.34.1

Command result is racy if allow it during migration. Let's allow the
sync only in RUNNING state.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
---
 hw/block/vhost-user-blk.c |  1 +
 hw/virtio/virtio-pci.c    |  9 ++++++++
 include/hw/qdev-core.h    |  3 +++
 qapi/qdev.json            | 23 +++++++++++++++++++
 system/qdev-monitor.c     | 48 +++++++++++++++++++++++++++++++++++++++
 5 files changed, 84 insertions(+)

diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
index XXXXXXX..XXXXXXX 100644
--- a/hw/block/vhost-user-blk.c
+++ b/hw/block/vhost-user-blk.c
@@ -XXX,XX +XXX,XX @@ static void vhost_user_blk_class_init(ObjectClass *klass, void *data)
 
     device_class_set_props(dc, vhost_user_blk_properties);
     dc->vmsd = &vmstate_vhost_user_blk;
+    dc->sync_config = vhost_user_blk_sync_config;
     set_bit(DEVICE_CATEGORY_STORAGE, dc->categories);
     vdc->realize = vhost_user_blk_device_realize;
     vdc->unrealize = vhost_user_blk_device_unrealize;
diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index XXXXXXX..XXXXXXX 100644
--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -XXX,XX +XXX,XX @@ static void virtio_pci_dc_realize(DeviceState *qdev, Error **errp)
     vpciklass->parent_dc_realize(qdev, errp);
 }
 
+static int virtio_pci_sync_config(DeviceState *dev, Error **errp)
+{
+    VirtIOPCIProxy *proxy = VIRTIO_PCI(dev);
+    VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
+
+    return qdev_sync_config(DEVICE(vdev), errp);
+}
+
 static void virtio_pci_class_init(ObjectClass *klass, void *data)
 {
     DeviceClass *dc = DEVICE_CLASS(klass);
@@ -XXX,XX +XXX,XX @@ static void virtio_pci_class_init(ObjectClass *klass, void *data)
     device_class_set_parent_realize(dc, virtio_pci_dc_realize,
                                     &vpciklass->parent_dc_realize);
     rc->phases.hold = virtio_pci_bus_reset_hold;
+    dc->sync_config = virtio_pci_sync_config;
 }
 
 static const TypeInfo virtio_pci_info = {
diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
index XXXXXXX..XXXXXXX 100644
--- a/include/hw/qdev-core.h
+++ b/include/hw/qdev-core.h
@@ -XXX,XX +XXX,XX @@ typedef void (*DeviceUnrealize)(DeviceState *dev);
 typedef void (*DeviceReset)(DeviceState *dev);
 typedef void (*BusRealize)(BusState *bus, Error **errp);
 typedef void (*BusUnrealize)(BusState *bus);
+typedef int (*DeviceSyncConfig)(DeviceState *dev, Error **errp);
 
 /**
  * struct DeviceClass - The base class for all devices.
@@ -XXX,XX +XXX,XX @@ struct DeviceClass {
     DeviceReset reset;
     DeviceRealize realize;
     DeviceUnrealize unrealize;
+    DeviceSyncConfig sync_config;
 
     /**
      * @vmsd: device state serialisation description for
@@ -XXX,XX +XXX,XX @@ bool qdev_hotplug_allowed(DeviceState *dev, Error **errp);
  */
 HotplugHandler *qdev_get_hotplug_handler(DeviceState *dev);
 void qdev_unplug(DeviceState *dev, Error **errp);
+int qdev_sync_config(DeviceState *dev, Error **errp);
 void qdev_simple_device_unplug_cb(HotplugHandler *hotplug_dev,
                                   DeviceState *dev, Error **errp);
 void qdev_machine_creation_done(void);
diff --git a/qapi/qdev.json b/qapi/qdev.json
index XXXXXXX..XXXXXXX 100644
--- a/qapi/qdev.json
+++ b/qapi/qdev.json
@@ -XXX,XX +XXX,XX @@
 ##
 { 'event': 'DEVICE_UNPLUG_GUEST_ERROR',
   'data': { '*device': 'str', 'path': 'str' } }
+
+##
+# @device-sync-config:
+#
+# Synchronize device configuration from host to guest part.  First,
+# copy the configuration from the host part (backend) to the guest
+# part (frontend).  Then notify guest software that device
+# configuration changed.
+# The command may be used to notify the guest about block device
+# capcity change.  Currently only vhost-user-blk device supports
+# this.
+#
+# @id: the device's ID or QOM path
+#
+# Features:
+#
+# @unstable: The command is experimental.
+#
+# Since: 9.1
+##
+{ 'command': 'device-sync-config',
+  'features': [ 'unstable' ],
+  'data': {'id': 'str'} }
diff --git a/system/qdev-monitor.c b/system/qdev-monitor.c
index XXXXXXX..XXXXXXX 100644
--- a/system/qdev-monitor.c
+++ b/system/qdev-monitor.c
@@ -XXX,XX +XXX,XX @@
 #include "monitor/monitor.h"
 #include "monitor/qdev.h"
 #include "sysemu/arch_init.h"
+#include "sysemu/runstate.h"
 #include "qapi/error.h"
 #include "qapi/qapi-commands-qdev.h"
 #include "qapi/qmp/dispatch.h"
@@ -XXX,XX +XXX,XX @@ void qmp_device_del(const char *id, Error **errp)
     }
 }
 
+int qdev_sync_config(DeviceState *dev, Error **errp)
+{
+    DeviceClass *dc = DEVICE_GET_CLASS(dev);
+
+    if (!dc->sync_config) {
+        error_setg(errp, "device-sync-config is not supported for '%s'",
+                   object_get_typename(OBJECT(dev)));
+        return -ENOTSUP;
+    }
+
+    return dc->sync_config(dev, errp);
+}
+
+void qmp_device_sync_config(const char *id, Error **errp)
+{
+    DeviceState *dev;
+
+    /*
+     * During migration there is a race between syncing`configuration and
+     * migrating it (if migrate first, that target would get outdated version),
+     * so let's just not allow it.
+     *
+     * Moreover, let's not rely on setting up interrupts in paused
+     * state, which may be a part of migration process.
+     */
+
+    if (migration_is_running()) {
+        error_setg(errp, "Config synchronization is not allowed "
+                   "during migration");
+        return;
+    }
+
+    if (!runstate_is_running()) {
+        error_setg(errp, "Config synchronization allowed only in '%s' state, "
+                   "current state is '%s'", RunState_str(RUN_STATE_RUNNING),
+                   RunState_str(runstate_get()));
+        return;
+    }
+
+    dev = find_device_state(id, true, errp);
+    if (!dev) {
+        return;
+    }
+
+    qdev_sync_config(dev, errp);
+}
+
 void hmp_device_add(Monitor *mon, const QDict *qdict)
 {
     Error *err = NULL;
-- 
2.34.1