[PATCH v2] monitor: Rate-limit MEMORY_DEVICE_SIZE_CHANGE qapi events per device

[David Hildenbrand]

We want to rate-limit MEMORY_DEVICE_SIZE_CHANGE events per device,
otherwise we can lose some events for devices. As we might not always have
a device id, add the qom-path to the event and use that to rate-limit
per device.

This was noticed by starting a VM with two virtio-mem devices that each
have a requested size > 0. The Linux guest will initialize both devices
in parallel, resulting in losing MEMORY_DEVICE_SIZE_CHANGE events for
one of the devices.

Fixes: 722a3c783ef4 ("virtio-pci: Send qapi events when the virtio-mem size 
changes")
Suggested-by: Markus Armbruster <arm...@redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilb...@redhat.com>
Cc: Markus Armbruster <arm...@redhat.com>
Cc: Michael S. Tsirkin <m...@redhat.com>
Cc: Eric Blake <ebl...@redhat.com>
Cc: Igor Mammedov <imamm...@redhat.com>
Cc: Michal Privoznik <mpriv...@redhat.com>
Signed-off-by: David Hildenbrand <da...@redhat.com>
---

Follow up of:
    https://lkml.kernel.org/r/20210921102434.24273-1-da...@redhat.com

v1 -> v2:
- Add the qom-path and use that identifier to rate-limit per device
- Rephrase subject/description

---
 hw/virtio/virtio-mem-pci.c | 3 ++-
 monitor/monitor.c          | 9 +++++++++
 qapi/machine.json          | 5 ++++-
 3 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/hw/virtio/virtio-mem-pci.c b/hw/virtio/virtio-mem-pci.c
index fa5395cd88..dd5085497f 100644
--- a/hw/virtio/virtio-mem-pci.c
+++ b/hw/virtio/virtio-mem-pci.c
@@ -87,6 +87,7 @@ static void virtio_mem_pci_size_change_notify(Notifier 
*notifier, void *data)
     VirtIOMEMPCI *pci_mem = container_of(notifier, VirtIOMEMPCI,
                                          size_change_notifier);
     DeviceState *dev = DEVICE(pci_mem);
+    const char * qom_path = object_get_canonical_path(OBJECT(dev));
     const uint64_t * const size_p = data;
     const char *id = NULL;
 
@@ -94,7 +95,7 @@ static void virtio_mem_pci_size_change_notify(Notifier 
*notifier, void *data)
         id = g_strdup(dev->id);
     }
 
-    qapi_event_send_memory_device_size_change(!!id, id, *size_p);
+    qapi_event_send_memory_device_size_change(!!id, id, *size_p, qom_path);
 }
 
 static void virtio_mem_pci_class_init(ObjectClass *klass, void *data)
diff --git a/monitor/monitor.c b/monitor/monitor.c
index 46a171bca6..21c7a68758 100644
--- a/monitor/monitor.c
+++ b/monitor/monitor.c
@@ -474,6 +474,10 @@ static unsigned int qapi_event_throttle_hash(const void 
*key)
         hash += g_str_hash(qdict_get_str(evstate->data, "node-name"));
     }
 
+    if (evstate->event == QAPI_EVENT_MEMORY_DEVICE_SIZE_CHANGE) {
+        hash += g_str_hash(qdict_get_str(evstate->data, "qom-path"));
+    }
+
     return hash;
 }
 
@@ -496,6 +500,11 @@ static gboolean qapi_event_throttle_equal(const void *a, 
const void *b)
                        qdict_get_str(evb->data, "node-name"));
     }
 
+    if (eva->event == QAPI_EVENT_MEMORY_DEVICE_SIZE_CHANGE) {
+        return !strcmp(qdict_get_str(eva->data, "qom-path"),
+                       qdict_get_str(evb->data, "qom-path"));
+    }
+
     return TRUE;
 }
 
diff --git a/qapi/machine.json b/qapi/machine.json
index 157712f006..2487c92f18 100644
--- a/qapi/machine.json
+++ b/qapi/machine.json
@@ -1245,8 +1245,11 @@
 # action).
 #
 # @id: device's ID
+#
 # @size: the new size of memory that the device provides
 #
+# @qom-path: path to the device object in the QOM tree (since 6.2)
+#
 # Note: this event is rate-limited.
 #
 # Since: 5.1
@@ -1259,7 +1262,7 @@
 #
 ##
 { 'event': 'MEMORY_DEVICE_SIZE_CHANGE',
-  'data': { '*id': 'str', 'size': 'size' } }
+  'data': { '*id': 'str', 'size': 'size', 'qom-path' : 'str'} }
 
 
 ##
-- 
2.31.1