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

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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>
This commit is contained in:
David Hildenbrand 2024-10-25 12:41:03 +02:00
parent 9863d46a5a
commit 713484d038
2 changed files with 84 additions and 32 deletions
Download patch file Download diff file Expand all files Collapse all files

103
hw/virtio/virtio-mem.c
View file

@ -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;
}