virtio-md-pci: Handle unplug of virtio based memory devices

While we fence unplug requests from the outside, the VM can still
trigger unplug of virtio based memory devices, for example, in Linux
doing on a virtio-mem-pci device:
    # echo 0 > /sys/bus/pci/slots/3/power

While doing that is not really expected to work without harming the
guest OS (e.g., removing a virtio-mem device while it still provides
memory), let's make sure that we properly handle it on the QEMU side.

We'll add support for unplugging of virtio-mem devices in some
configurations next.

Message-ID: <20230711153445.514112-5-david@redhat.com>
Tested-by: Mario Casquero <mcasquer@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>

hw/virtio/virtio-md-pci.c

@@ -14,6 +14,7 @@
 #include "hw/virtio/virtio-md-pci.h"
 #include "hw/mem/memory-device.h"
 #include "qapi/error.h"
+#include "qemu/error-report.h"
 
 void virtio_md_pci_pre_plug(VirtIOMDPCI *vmd, MachineState *ms, Error **errp)
 {
@@ -74,7 +75,27 @@ void virtio_md_pci_unplug_request(VirtIOMDPCI *vmd, MachineState *ms,
 
 void virtio_md_pci_unplug(VirtIOMDPCI *vmd, MachineState *ms, Error **errp)
 {
-    /* We don't support hot unplug of virtio based memory devices */
+    DeviceState *dev = DEVICE(vmd);
+    HotplugHandler *bus_handler = qdev_get_bus_hotplug_handler(dev);
+    MemoryDeviceState *md = MEMORY_DEVICE(vmd);
+    Error *local_err = NULL;
+
+    /* Unplug the memory device while it is still realized. */
+    memory_device_unplug(md, ms);
+
+    if (bus_handler) {
+        hotplug_handler_unplug(bus_handler, dev, &local_err);
+        if (local_err) {
+            /* Not expected to fail ... but still try to recover. */
+            memory_device_plug(md, ms);
+            error_propagate(errp, local_err);
+            return;
+        }
+    } else {
+        /* Very unexpected, but let's just try to do the right thing. */
+        warn_report("Unexpected unplug of virtio based memory device");
+        qdev_unrealize(dev);
+    }
 }
 
 static const TypeInfo virtio_md_pci_info = {