accel/kvm: notify when KVM VM file fd is about to be changed

Various subsystems might need to take some steps before the KVM file descriptor
for a virtual machine is changed. So a new boolean attribute is added to the
vmfd_notifier structure which is passed to the notifier callbacks.
vmfd_notifer.pre is true for pre-notification of vmfd change and false for
post notification. Notifier callback implementations can simply check
the boolean value for (vmfd_notifer*)->pre and can take actions for pre or
post vmfd change based on the value.

Subsequent patches will add callback implementations for specific components
that need this pre-notification.

Signed-off-by: Ani Sinha <anisinha@redhat.com>
Link: https://lore.kernel.org/r/20260225035000.385950-9-anisinha@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

accel/kvm/kvm-all.c

@@ -2654,6 +2654,13 @@ static int kvm_reset_vmfd(MachineState *ms)
     memory_listener_unregister(&kml->listener);
     memory_listener_unregister(&kvm_io_listener);
 
+    vmfd_notifier.pre = true;
+    ret = kvm_vmfd_change_notify(&err);
+    if (ret < 0) {
+        return ret;
+    }
+    assert(!err);
+
     if (s->vmfd >= 0) {
         close(s->vmfd);
     }
@@ -2695,6 +2702,8 @@ static int kvm_reset_vmfd(MachineState *ms)
      * notify everyone that vmfd has changed.
      */
     vmfd_notifier.vmfd = s->vmfd;
+    vmfd_notifier.pre = false;
+
     ret = kvm_vmfd_change_notify(&err);
     if (ret < 0) {
         return ret;

include/system/kvm.h

@@ -571,12 +571,14 @@ int kvm_convert_memory(hwaddr start, hwaddr size, bool to_private);
 /* argument to vmfd change notifier */
 typedef struct VmfdChangeNotifier {
     int vmfd;
+    bool pre;
 } VmfdChangeNotifier;
 
 /**
  * kvm_vmfd_add_change_notifier - register a notifier to get notified when
- * a KVM vm file descriptor changes as a part of the confidential guest "reset"
- * process. Various subsystems should use this mechanism to take actions such
+ * a KVM vm file descriptor changes or about to be changed as a part of the
+ * confidential guest "reset" process.
+ * Various subsystems should use this mechanism to take actions such
  * as creating new fds against this new vm file descriptor.
  * @n: notifier with return value.
  */