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

We recently converted from the LegacyReset to the new reset framework
in commit c009a311e939 ("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>

1 files changed, 12 insertions, 1 deletions

diff --git a/include/hw/virtio/virtio-mem.h b/include/hw/virtio/virtio-mem.h
index a1af144..550ce58 100644
--- a/include/hw/virtio/virtio-mem.h
+++ b/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 {