diff --git a/hw/i386/microvm.c b/hw/i386/microvm.c
index 273abe28c9..17e3f2f152 100644
--- a/hw/i386/microvm.c
+++ b/hw/i386/microvm.c
@@ -198,6 +198,12 @@ static void microvm_devices_init(MicrovmMachineState *mms)
     }
 
     if (x86_machine_is_acpi_enabled(x86ms) && mms->pcie == ON_OFF_AUTO_ON) {
+        /* use topmost 25% of the address space available */
+        hwaddr phys_size = (hwaddr)1 << X86_CPU(first_cpu)->phys_bits;
+        if (phys_size > 0x1000000ll) {
+            mms->gpex.mmio64.size = phys_size / 4;
+            mms->gpex.mmio64.base = phys_size - mms->gpex.mmio64.size;
+        }
         mms->gpex.mmio32.base = PCIE_MMIO_BASE;
         mms->gpex.mmio32.size = PCIE_MMIO_SIZE;
         mms->gpex.ecam.base   = PCIE_ECAM_BASE;
@@ -383,6 +389,9 @@ static void microvm_fix_kernel_cmdline(MachineState *machine)
 static void microvm_device_pre_plug_cb(HotplugHandler *hotplug_dev,
                                        DeviceState *dev, Error **errp)
 {
+    X86CPU *cpu = X86_CPU(dev);
+
+    cpu->host_phys_bits = true; /* need reliable phys-bits */
     x86_cpu_pre_plug(hotplug_dev, dev, errp);
 }