[PATCH v1 3/5] virtio-mem: Prepare for unplug support of virtio-mem-pci devices

[David Hildenbrand]

In many cases, blindly unplugging a virtio-mem device is problematic. We
can only safely remove a device once:
* The guest is not expecting to be able to read unplugged memory
  (unplugged-inaccessible == on)
* The virtio-mem device does not have memory plugged (size == 0)
* The virtio-mem device does not have outstanding requests to the VM to
  plug memory (requested-size == 0)

So let's add a helper to check for that from the unplug-request code
from relevant machine hotplug handlers and disallow changing the
requested-size once an unplug request is pending.

Disallowing requested-size changes handles corner cases such as
(1) pausing the VM (2) requesting device unplug and (3) adjusting the
requested size. If the VM would plug memory (due to the requested size
change) before processing the unplug request, we would be in trouble.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 hw/virtio/virtio-mem-pci.c     | 42 +++++++++++++++++++++++++++++++---
 hw/virtio/virtio-mem-pci.h     |  2 ++
 hw/virtio/virtio-mem.c         | 24 +++++++++++++++++++
 include/hw/virtio/virtio-mem.h |  2 ++
 4 files changed, 67 insertions(+), 3 deletions(-)

diff --git a/hw/virtio/virtio-mem-pci.c b/hw/virtio/virtio-mem-pci.c
index b85c12668d..dcbab9713c 100644
--- a/hw/virtio/virtio-mem-pci.c
+++ b/hw/virtio/virtio-mem-pci.c
@@ -93,6 +93,42 @@ static void virtio_mem_pci_size_change_notify(Notifier 
*notifier, void *data)
     g_free(qom_path);
 }
 
+void virtio_mem_pci_unplug_request_check(VirtIOMEMPCI *pci_mem, Error **errp)
+{
+    virtio_mem_unplug_request_check(&pci_mem->vdev, errp);
+}
+
+static void virtio_mem_pci_get_requested_size(Object *obj, Visitor *v,
+                                              const char *name, void *opaque,
+                                              Error **errp)
+{
+    VirtIOMEMPCI *pci_mem = VIRTIO_MEM_PCI(obj);
+
+    object_property_get(OBJECT(&pci_mem->vdev), name, v, errp);
+}
+
+static void virtio_mem_pci_set_requested_size(Object *obj, Visitor *v,
+                                              const char *name, void *opaque,
+                                              Error **errp)
+{
+    VirtIOMEMPCI *pci_mem = VIRTIO_MEM_PCI(obj);
+    DeviceState *dev = DEVICE(obj);
+
+    /*
+     * If we passed virtio_mem_pci_unplug_request_check(), making sure that
+     * the requested size is 0, don't allow modifying the requested size
+     * anymore, otherwise the VM might end up hotplugging memory before
+     * handling the unplug request.
+     */
+    if (dev->pending_deleted_event) {
+        error_setg(errp, "'%s' cannot be changed if the device is in the"
+                   " process of unplug", name);
+        return;
+    }
+
+    object_property_set(OBJECT(&pci_mem->vdev), name, v, errp);
+}
+
 static void virtio_mem_pci_class_init(ObjectClass *klass, void *data)
 {
     DeviceClass *dc = DEVICE_CLASS(klass);
@@ -135,9 +171,9 @@ static void virtio_mem_pci_instance_init(Object *obj)
                               OBJECT(&dev->vdev), VIRTIO_MEM_BLOCK_SIZE_PROP);
     object_property_add_alias(obj, VIRTIO_MEM_SIZE_PROP, OBJECT(&dev->vdev),
                               VIRTIO_MEM_SIZE_PROP);
-    object_property_add_alias(obj, VIRTIO_MEM_REQUESTED_SIZE_PROP,
-                              OBJECT(&dev->vdev),
-                              VIRTIO_MEM_REQUESTED_SIZE_PROP);
+    object_property_add(obj, VIRTIO_MEM_REQUESTED_SIZE_PROP, "size",
+                        virtio_mem_pci_get_requested_size,
+                        virtio_mem_pci_set_requested_size, NULL, NULL);
 }
 
 static const VirtioPCIDeviceTypeInfo virtio_mem_pci_info = {
diff --git a/hw/virtio/virtio-mem-pci.h b/hw/virtio/virtio-mem-pci.h
index e636e1a48d..f8e8bc523a 100644
--- a/hw/virtio/virtio-mem-pci.h
+++ b/hw/virtio/virtio-mem-pci.h
@@ -32,4 +32,6 @@ struct VirtIOMEMPCI {
     Notifier size_change_notifier;
 };
 
+void virtio_mem_pci_unplug_request_check(VirtIOMEMPCI *pci_mem, Error **errp);
+
 #endif /* QEMU_VIRTIO_MEM_PCI_H */
diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c
index 538b695c29..e0dc7f04ea 100644
--- a/hw/virtio/virtio-mem.c
+++ b/hw/virtio/virtio-mem.c
@@ -1468,6 +1468,30 @@ static void 
virtio_mem_rdm_unregister_listener(RamDiscardManager *rdm,
     QLIST_REMOVE(rdl, next);
 }
 
+void virtio_mem_unplug_request_check(VirtIOMEM *vmem, Error **errp)
+{
+    if (vmem->unplugged_inaccessible == ON_OFF_AUTO_OFF) {
+        /*
+         * We could allow it with a usable region size of 0, but let's just
+         * not care about that legacy setting.
+         */
+        error_setg(errp, "virtio-mem device cannot get unplugged while"
+                   " '" VIRTIO_MEM_UNPLUGGED_INACCESSIBLE_PROP "' != 'on'");
+        return;
+    }
+
+    if (vmem->size) {
+        error_setg(errp, "virtio-mem device cannot get unplugged while"
+                   " '" VIRTIO_MEM_SIZE_PROP "' != '0'");
+        return;
+    }
+    if (vmem->requested_size) {
+        error_setg(errp, "virtio-mem device cannot get unplugged while"
+                   " '" VIRTIO_MEM_REQUESTED_SIZE_PROP "' != '0'");
+        return;
+    }
+}
+
 static void virtio_mem_class_init(ObjectClass *klass, void *data)
 {
     DeviceClass *dc = DEVICE_CLASS(klass);
diff --git a/include/hw/virtio/virtio-mem.h b/include/hw/virtio/virtio-mem.h
index f15e561785..dd5aec98f5 100644
--- a/include/hw/virtio/virtio-mem.h
+++ b/include/hw/virtio/virtio-mem.h
@@ -100,4 +100,6 @@ struct VirtIOMEMClass {
     void (*remove_size_change_notifier)(VirtIOMEM *vmem, Notifier *notifier);
 };
 
+void virtio_mem_unplug_request_check(VirtIOMEM *vmem, Error **errp);
+
 #endif
-- 
2.40.1