On confidential guests when the KVM virtual machine file descriptor changes as
a part of the reset process, event file descriptors needs to be reassociated
with the new KVM VM file descriptor. This is achieved with the help of a
callback handler that gets called when KVM VM file descriptor changes during
the confidential guest reset process.

This patch is tested on non-confidential platform only.

Signed-off-by: Ani Sinha <anisinha@redhat.com>
---
 hw/hyperv/trace-events |  1 +
 hw/hyperv/vmbus.c      | 37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 38 insertions(+)

diff --git a/hw/hyperv/trace-events b/hw/hyperv/trace-events
index 7963c215b1..d8c96f18e9 100644
--- a/hw/hyperv/trace-events
+++ b/hw/hyperv/trace-events
@@ -16,6 +16,7 @@ vmbus_gpadl_torndown(uint32_t gpadl_id) "gpadl #%d"
 vmbus_open_channel(uint32_t chan_id, uint32_t gpadl_id, uint32_t target_vp) "channel #%d gpadl #%d target vp %d"
 vmbus_channel_open(uint32_t chan_id, uint32_t status) "channel #%d status %d"
 vmbus_close_channel(uint32_t chan_id) "channel #%d"
+vmbus_handle_vmfd_change(void) ""
 
 # hv-balloon
 hv_balloon_state_change(const char *tostr) "-> %s"
diff --git a/hw/hyperv/vmbus.c b/hw/hyperv/vmbus.c
index c5bab5d245..64abe4c4c1 100644
--- a/hw/hyperv/vmbus.c
+++ b/hw/hyperv/vmbus.c
@@ -20,6 +20,7 @@
 #include "hw/hyperv/vmbus-bridge.h"
 #include "hw/core/sysbus.h"
 #include "exec/cpu-common.h"
+#include "system/kvm.h"
 #include "exec/target_page.h"
 #include "trace.h"
 
@@ -248,6 +249,12 @@ struct VMBus {
      * interrupt page
      */
     EventNotifier notifier;
+
+    /*
+     * Notifier to inform when vmfd is changed as a part of confidential guest
+     * reset mechanism.
+     */
+    NotifierWithReturn vmbus_vmfd_change_notifier;
 };
 
 static bool gpadl_full(VMBusGpadl *gpadl)
@@ -2347,6 +2354,33 @@ static void vmbus_dev_unrealize(DeviceState *dev)
     free_channels(vdev);
 }
 
+/*
+ * If the KVM fd changes because of VM reset in confidential guests,
+ * reassociate event fd with the new KVM fd.
+ */
+static int vmbus_handle_vmfd_change(NotifierWithReturn *notifier,
+                                    void *data, Error** errp)
+{
+    VMBus *vmbus = container_of(notifier, VMBus,
+                                vmbus_vmfd_change_notifier);
+    int ret = 0;
+
+    /* we are not interested in pre vmfd change notification */
+    if (((VmfdChangeNotifier *)data)->pre) {
+        return 0;
+    }
+
+    ret = hyperv_set_event_flag_handler(VMBUS_EVENT_CONNECTION_ID,
+                                            &vmbus->notifier);
+    /* if we are only using userland event handler, it may already exist */
+    if (ret != 0 && ret != -EEXIST) {
+        error_setg(errp, "hyperv set event handler failed with %d", ret);
+    }
+
+    trace_vmbus_handle_vmfd_change();
+    return ret;
+}
+
 static const Property vmbus_dev_props[] = {
     DEFINE_PROP_UUID("instanceid", VMBusDevice, instanceid),
 };
@@ -2429,6 +2463,9 @@ static void vmbus_realize(BusState *bus, Error **errp)
         goto clear_event_notifier;
     }
 
+    vmbus->vmbus_vmfd_change_notifier.notify = vmbus_handle_vmfd_change;
+    kvm_vmfd_add_change_notifier(&vmbus->vmbus_vmfd_change_notifier);
+
     return;
 
 clear_event_notifier:
-- 
2.42.0

On 18.02.2026 12:42, Ani Sinha wrote:
> On confidential guests when the KVM virtual machine file descriptor changes as
> a part of the reset process, event file descriptors needs to be reassociated
> with the new KVM VM file descriptor. This is achieved with the help of a
> callback handler that gets called when KVM VM file descriptor changes during
> the confidential guest reset process.
> 
> This patch is tested on non-confidential platform only.
> 
> Signed-off-by: Ani Sinha <anisinha@redhat.com>
> ---

I have also tested this patch with a non-confidential Windows guest
with VMBus, including live migrating it a few times.

No regressions observed, so:
Acked-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>

Thanks,
Maciej