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Re: [PATCH v4 3/3] qapi: introduce device-sync-config

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [PATCH v4 3/3] qapi: introduce device-sync-config
Date: Tue, 30 Apr 2024 11:31:19 +0300
User-agent: Mozilla Thunderbird 
On 30.04.24 11:19, Markus Armbruster wrote:

Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru> writes:


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 |  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 091d2c6acf..2f301f380c 100644
--- a/hw/block/vhost-user-blk.c
+++ b/hw/block/vhost-user-blk.c
@@ -588,6 +588,7 @@ 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 b1d02f4b3d..0d91e8b5dc 100644
--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -2351,6 +2351,14 @@ 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);
@@ -2367,6 +2375,7 @@ 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 9228e96c87..87135bdcdf 100644
--- a/include/hw/qdev-core.h
+++ b/include/hw/qdev-core.h
@@ -95,6 +95,7 @@ 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.
@@ -162,6 +163,7 @@ struct DeviceClass {
      DeviceReset reset;
      DeviceRealize realize;
      DeviceUnrealize unrealize;
+    DeviceSyncConfig sync_config;
/** 
       * @vmsd: device state serialisation description for
@@ -546,6 +548,7 @@ 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 facaa0bc6a..fc5e125a45 100644
--- a/qapi/qdev.json
+++ b/qapi/qdev.json
@@ -161,3 +161,26 @@
  ##
  { '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.


Blank line here, please.


+# 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 264978aa40..47bfc0506e 100644
--- a/system/qdev-monitor.c
+++ b/system/qdev-monitor.c
@@ -23,6 +23,7 @@
  #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"
@@ -971,6 +972,53 @@ 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.


Wrap comment lines around column 70 for legibility, please.


+     *
+     * Moreover, let's not rely on setting up interrupts in paused
+     * state, which may be a part of migration process.


We discussed this in review of v3.  You wanted to check whether the
problem is real.  Is it?


We discussed it later than I've sent v4 :) No, I didn't check yet.




+     */
+
+    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, "


Suggest a line break after errp,


+                   "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;




--
Best regards,
Vladimir
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