On Fri, Oct 25, 2024 at 12:41:03PM +0200, David Hildenbrand wrote:
> 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.
>
> Cc: "Michael S. Tsirkin" <m...@redhat.com>
> Cc: Juraj Marcin <jmar...@redhat.com>
> Cc: Peter Maydell <peter.mayd...@linaro.org>
> Signed-off-by: David Hildenbrand <da...@redhat.com>
Acked-by: Michael S. Tsirkin <m...@redhat.com>
> ---
> hw/virtio/virtio-mem.c | 103 +++++++++++++++++++++++----------
> include/hw/virtio/virtio-mem.h | 12 +++-
> 2 files changed, 83 insertions(+), 32 deletions(-)
>
> diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c
> index ae1e81d7ba..08e0e9da1c 100644
> --- a/hw/virtio/virtio-mem.c
> +++ b/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;
> +}
> diff --git a/include/hw/virtio/virtio-mem.h b/include/hw/virtio/virtio-mem.h
> index a1af144c28..abde1c4101 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 +121,14 @@ 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 {
> --
> 2.46.1
