virtio-mem: unplug memory only during system resets, not device resets

We recently converted from the LegacyReset to the new reset framework
in commit c009a311e9 ("virtio-mem: Use new Resettable framework instead
of LegacyReset") to be able to use the ResetType to filter out wakeup
resets.

However, this change had an undesired implications: as we override the
Resettable interface methods in VirtIOMEMClass, the reset handler will
not only get called during system resets (i.e., qemu_devices_reset())
but also during any direct or indirect device rests (e.g.,
device_cold_reset()).

Further, we might now receive two reset callbacks during
qemu_devices_reset(), first when reset by a parent and later when reset
directly.

The memory state of virtio-mem devices is rather special: it's supposed to
be persistent/unchanged during most resets (similar to resetting a hard
disk will not destroy the data), unless actually cold-resetting the whole
system (different to a hard disk where a reboot will not destroy the data):
ripping out system RAM is something guest OSes don't particularly enjoy,
but we want to detect when rebooting to an OS that does not support
virtio-mem and wouldn't be able to detect+use the memory -- and we want
to force-defragment hotplugged memory to also shrink the usable device
memory region. So we rally want to catch system resets to do that.

On supported targets (e.g., x86), getting a cold reset on the
device/parent triggers is not that easy (but looks like PCI code
might trigger it), so this implication went unnoticed.

However, with upcoming s390x support it is problematic: during
kdump, s390x triggers a subsystem reset, ending up in
s390_machine_reset() and calling only subsystem_reset() instead of
qemu_devices_reset() -- because it's not a full system reset.

In subsystem_reset(), s390x performs a device_cold_reset() of any
TYPE_VIRTUAL_CSS_BRIDGE device, which ends up resetting all children,
including the virtio-mem device. Consequently, we wrongly detect a system
reset and unplug all device memory, resulting in hotplugged memory not
getting included in the crash dump -- undesired.

We really must not mess with hotplugged memory state during simple
device resets. To fix, create+register a new reset object that will only
get triggered during qemu_devices_reset() calls, but not during any other
resets as it is logically not the child of any other object.

Message-ID: <20241025104103.342188-1-david@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Juraj Marcin <jmarcin@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>

hw/virtio/virtio-mem.c

@@ -956,6 +956,7 @@ static void virtio_mem_device_realize(DeviceState *dev, Error **errp)
     VirtIOMEM *vmem = VIRTIO_MEM(dev);
     uint64_t page_size;
     RAMBlock *rb;
+    Object *obj;
     int ret;
 
     if (!vmem->memdev) {
@@ -1121,7 +1122,28 @@ static void virtio_mem_device_realize(DeviceState *dev, Error **errp)
         vmstate_register_any(VMSTATE_IF(vmem),
                              &vmstate_virtio_mem_device_early, vmem);
     }
-    qemu_register_resettable(OBJECT(vmem));
+
+    /*
+     * We only want to unplug all memory to start with a clean slate when
+     * it is safe for the guest -- during system resets that call
+     * qemu_devices_reset().
+     *
+     * We'll filter out selected qemu_devices_reset() calls used for other
+     * purposes, like resetting all devices during wakeup from suspend on
+     * x86 based on the reset type passed to qemu_devices_reset().
+     *
+     * Unplugging all memory during simple device resets can result in the VM
+     * unexpectedly losing RAM, corrupting VM state.
+     *
+     * Simple device resets (or resets triggered by getting a parent device
+     * reset) must not change the state of plugged memory blocks. Therefore,
+     * we need a dedicated reset object that only gets called during
+     * qemu_devices_reset().
+     */
+    obj = object_new(TYPE_VIRTIO_MEM_SYSTEM_RESET);
+    vmem->system_reset = VIRTIO_MEM_SYSTEM_RESET(obj);
+    vmem->system_reset->vmem = vmem;
+    qemu_register_resettable(obj);
 
     /*
      * Set ourselves as RamDiscardManager before the plug handler maps the
@@ -1141,7 +1163,10 @@ static void virtio_mem_device_unrealize(DeviceState *dev)
      * found via an address space anymore. Unset ourselves.
      */
     memory_region_set_ram_discard_manager(&vmem->memdev->mr, NULL);
-    qemu_unregister_resettable(OBJECT(vmem));
+
+    qemu_unregister_resettable(OBJECT(vmem->system_reset));
+    object_unref(OBJECT(vmem->system_reset));
+
     if (vmem->early_migration) {
         vmstate_unregister(VMSTATE_IF(vmem), &vmstate_virtio_mem_device_early,
                            vmem);
@@ -1841,38 +1866,12 @@ static void virtio_mem_unplug_request_check(VirtIOMEM *vmem, Error **errp)
     }
 }
 
-static ResettableState *virtio_mem_get_reset_state(Object *obj)
-{
-    VirtIOMEM *vmem = VIRTIO_MEM(obj);
-    return &vmem->reset_state;
-}
-
-static void virtio_mem_system_reset_hold(Object *obj, ResetType type)
-{
-    VirtIOMEM *vmem = VIRTIO_MEM(obj);
-
-    /*
-     * When waking up from standby/suspend-to-ram, do not unplug any memory.
-     */
-    if (type == RESET_TYPE_WAKEUP) {
-        return;
-    }
-
-    /*
-     * During usual resets, we will unplug all memory and shrink the usable
-     * region size. This is, however, not possible in all scenarios. Then,
-     * the guest has to deal with this manually (VIRTIO_MEM_REQ_UNPLUG_ALL).
-     */
-    virtio_mem_unplug_all(vmem);
-}
-
 static void virtio_mem_class_init(ObjectClass *klass, void *data)
 {
     DeviceClass *dc = DEVICE_CLASS(klass);
     VirtioDeviceClass *vdc = VIRTIO_DEVICE_CLASS(klass);
     VirtIOMEMClass *vmc = VIRTIO_MEM_CLASS(klass);
     RamDiscardManagerClass *rdmc = RAM_DISCARD_MANAGER_CLASS(klass);
-    ResettableClass *rc = RESETTABLE_CLASS(klass);
 
     device_class_set_props(dc, virtio_mem_properties);
     dc->vmsd = &vmstate_virtio_mem;
@@ -1899,9 +1898,6 @@ static void virtio_mem_class_init(ObjectClass *klass, void *data)
     rdmc->replay_discarded = virtio_mem_rdm_replay_discarded;
     rdmc->register_listener = virtio_mem_rdm_register_listener;
     rdmc->unregister_listener = virtio_mem_rdm_unregister_listener;
-
-    rc->get_state = virtio_mem_get_reset_state;
-    rc->phases.hold = virtio_mem_system_reset_hold;
 }
 
 static const TypeInfo virtio_mem_info = {
@@ -1924,3 +1920,48 @@ static void virtio_register_types(void)
 }
 
 type_init(virtio_register_types)
+
+OBJECT_DEFINE_SIMPLE_TYPE_WITH_INTERFACES(VirtioMemSystemReset, virtio_mem_system_reset, VIRTIO_MEM_SYSTEM_RESET, OBJECT, { TYPE_RESETTABLE_INTERFACE }, { })
+
+static void virtio_mem_system_reset_init(Object *obj)
+{
+}
+
+static void virtio_mem_system_reset_finalize(Object *obj)
+{
+}
+
+static ResettableState *virtio_mem_system_reset_get_state(Object *obj)
+{
+    VirtioMemSystemReset *vmem_reset = VIRTIO_MEM_SYSTEM_RESET(obj);
+
+    return &vmem_reset->reset_state;
+}
+
+static void virtio_mem_system_reset_hold(Object *obj, ResetType type)
+{
+    VirtioMemSystemReset *vmem_reset = VIRTIO_MEM_SYSTEM_RESET(obj);
+    VirtIOMEM *vmem = vmem_reset->vmem;
+
+    /*
+     * When waking up from standby/suspend-to-ram, do not unplug any memory.
+     */
+    if (type == RESET_TYPE_WAKEUP) {
+        return;
+    }
+
+    /*
+     * During usual resets, we will unplug all memory and shrink the usable
+     * region size. This is, however, not possible in all scenarios. Then,
+     * the guest has to deal with this manually (VIRTIO_MEM_REQ_UNPLUG_ALL).
+     */
+    virtio_mem_unplug_all(vmem);
+}
+
+static void virtio_mem_system_reset_class_init(ObjectClass *klass, void *data)
+{
+    ResettableClass *rc = RESETTABLE_CLASS(klass);
+
+    rc->get_state = virtio_mem_system_reset_get_state;
+    rc->phases.hold = virtio_mem_system_reset_hold;
+}

include/hw/virtio/virtio-mem.h

@@ -25,6 +25,10 @@
 OBJECT_DECLARE_TYPE(VirtIOMEM, VirtIOMEMClass,
                     VIRTIO_MEM)
 
+#define TYPE_VIRTIO_MEM_SYSTEM_RESET "virtio-mem-system-reset"
+
+OBJECT_DECLARE_SIMPLE_TYPE(VirtioMemSystemReset, VIRTIO_MEM_SYSTEM_RESET)
+
 #define VIRTIO_MEM_MEMDEV_PROP "memdev"
 #define VIRTIO_MEM_NODE_PROP "node"
 #define VIRTIO_MEM_SIZE_PROP "size"
@@ -117,8 +121,15 @@ struct VirtIOMEM {
     /* listeners to notify on plug/unplug activity. */
     QLIST_HEAD(, RamDiscardListener) rdl_list;
 
-    /* State of the resettable container */
+    /* Catch system resets -> qemu_devices_reset() only. */
+    VirtioMemSystemReset *system_reset;
+};
+
+struct VirtioMemSystemReset {
+    Object parent;
+
     ResettableState reset_state;
+    VirtIOMEM *vmem;
 };
 
 struct VirtIOMEMClass {