tests/functional/x86_64: add functional test to exercise vm fd change on reset

A new functional test is added that exercises the code changes related to
closing of the old KVM VM file descriptor and opening a new one upon VM reset.
This normally happens when confidential guests are reset but for
non-confidential guests, we use a special machine specific debug/test parameter
'x-change-vmfd-on-reset' to enable this behavior.
Only specific code changes related to re-initialisation of SEV-ES, SEV-SNP and
TDX platforms are not exercised in this test as they require hardware that
supports running confidential guests.

Signed-off-by: Ani Sinha <anisinha@redhat.com>
Link: https://lore.kernel.org/r/20260225035000.385950-34-anisinha@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

MAINTAINERS

@@ -157,6 +157,7 @@ M: Ani Sinha <anisinha@redhat.com>
 M: Paolo Bonzini <pbonzini@redhat.com>
 S: Maintained
 F: stubs/kvm.c
+F: tests/functional/x86_64/test_rebuild_vmfd.py
 
 Guest CPU cores (TCG)
 ---------------------

tests/functional/x86_64/meson.build

@@ -37,4 +37,5 @@ tests_x86_64_system_thorough = [
   'vhost_user_bridge',
   'virtio_balloon',
   'virtio_gpu',
+  'rebuild_vmfd',
 ]

tests/functional/x86_64/test_rebuild_vmfd.py

@@ -0,0 +1,136 @@
+#!/usr/bin/env python3
+#
+# Functional tests exercising guest KVM file descriptor change on reset.
+#
+# Copyright © 2026 Red Hat, Inc.
+#
+# Author:
+#  Ani Sinha <anisinha@redhat.com>
+#
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+import os
+from qemu.machine import machine
+
+from qemu_test import QemuSystemTest, Asset, exec_command_and_wait_for_pattern
+from qemu_test import wait_for_console_pattern
+
+class KVMGuest(QemuSystemTest):
+
+    # ASSET UKI was generated using
+    # https://gitlab.com/kraxel/edk2-tests/-/blob/unittest/tools/make-supermin.sh
+    ASSET_UKI = Asset('https://gitlab.com/anisinha/misc-artifacts/'
+                      '-/raw/main/uki.x86-64.efi?ref_type=heads',
+                      'e0f806bd1fa24111312e1fe849d2ee69808d4343930a5'
+                      'dc8c1688da17c65f576')
+    # ASSET_OVMF comes from /usr/share/edk2/ovmf/OVMF.stateless.fd of a
+    # fedora core 43 distribution which in turn comes from the
+    # edk2-ovmf-20251119-3.fc43.noarch rpm of that distribution.
+    ASSET_OVMF = Asset('https://gitlab.com/anisinha/misc-artifacts/'
+                       '-/raw/main/OVMF.stateless.fd?ref_type=heads',
+                       '58a4275aafa8774bd6b1540adceae4ea434b8db75b476'
+                       '11839ff47be88cfcf22')
+
+    def common_vm_setup(self, kvm_args=None, cpu_args=None):
+        self.set_machine('q35')
+        self.require_accelerator("kvm")
+
+        self.vm.set_console()
+        if kvm_args:
+            self.vm.add_args("-accel", "kvm,%s" %kvm_args)
+        else:
+            self.vm.add_args("-accel", "kvm")
+        self.vm.add_args("-smp", "2")
+        if cpu_args:
+            self.vm.add_args("-cpu", "host,%s" %cpu_args)
+        else:
+            self.vm.add_args("-cpu", "host")
+        self.vm.add_args("-m", "2G")
+        self.vm.add_args("-nographic", "-nodefaults")
+
+
+        self.uki_path = self.ASSET_UKI.fetch()
+        self.ovmf_path = self.ASSET_OVMF.fetch()
+
+        self.vm.add_args('-kernel', self.uki_path)
+        self.vm.add_args("-bios", self.ovmf_path)
+        # enable KVM VMFD change on reset for a non-coco VM
+        self.vm.add_args("-machine", "q35,x-change-vmfd-on-reset=on")
+
+        # enable tracing of basic vmfd change function
+        self.vm.add_args("--trace", "kvm_reset_vmfd")
+
+    def launch_vm(self):
+        try:
+            self.vm.launch()
+        except machine.VMLaunchFailure as e:
+            if "Xen HVM guest support not present" in e.output:
+                self.skipTest("KVM Xen support is not present "
+                              "(need v5.12+ kernel with CONFIG_KVM_XEN)")
+            elif "Property 'kvm-accel.xen-version' not found" in e.output:
+                self.skipTest("QEMU not built with CONFIG_XEN_EMU support")
+            else:
+                raise e
+
+        self.log.info('VM launched')
+        console_pattern = 'bash-5.1#'
+        wait_for_console_pattern(self, console_pattern)
+        self.log.info('VM ready with a bash prompt')
+
+    def vm_console_reset(self):
+        exec_command_and_wait_for_pattern(self, '/usr/sbin/reboot -f',
+                                          'reboot: machine restart')
+        console_pattern = '# --- Hello world ---'
+        wait_for_console_pattern(self, console_pattern)
+        self.vm.shutdown()
+
+    def vm_qmp_reset(self):
+        self.vm.qmp('system_reset')
+        console_pattern = '# --- Hello world ---'
+        wait_for_console_pattern(self, console_pattern)
+        self.vm.shutdown()
+
+    def check_logs(self):
+        self.assertRegex(self.vm.get_log(),
+                         r'kvm_reset_vmfd')
+        self.assertRegex(self.vm.get_log(),
+                         r'virtual machine state has been rebuilt')
+
+    def test_reset_console(self):
+        self.common_vm_setup()
+        self.launch_vm()
+        self.vm_console_reset()
+        self.check_logs()
+
+    def test_reset_qmp(self):
+        self.common_vm_setup()
+        self.launch_vm()
+        self.vm_qmp_reset()
+        self.check_logs()
+
+    def test_reset_kvmpit(self):
+        self.common_vm_setup()
+        self.vm.add_args("--trace", "kvmpit_post_vmfd_change")
+        self.launch_vm()
+        self.vm_console_reset()
+        self.assertRegex(self.vm.get_log(),
+                         r'kvmpit_post_vmfd_change')
+
+    def test_reset_xen_emulation(self):
+        self.common_vm_setup("xen-version=0x4000a,kernel-irqchip=split")
+        self.launch_vm()
+        self.vm_console_reset()
+        self.check_logs()
+
+    def test_reset_hyperv_vmbus(self):
+        self.common_vm_setup(None, "hv-syndbg,hv-relaxed,hv_time,hv-synic,"
+                             "hv-vpindex,hv-runtime,hv-stimer")
+        self.vm.add_args("-device", "vmbus-bridge,irq=15")
+        self.vm.add_args("-trace", "vmbus_handle_vmfd_change")
+        self.launch_vm()
+        self.vm_console_reset()
+        self.assertRegex(self.vm.get_log(),
+                         r'vmbus_handle_vmfd_change')
+
+if __name__ == '__main__':
+    QemuSystemTest.main()