Hi David,
On 25/10/24 11:41, 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>
---
hw/virtio/virtio-mem.c | 103 +++++++++++++++++++++++----------
include/hw/virtio/virtio-mem.h | 12 +++-
2 files changed, 83 insertions(+), 32 deletions(-)
+OBJECT_DEFINE_SIMPLE_TYPE_WITH_INTERFACES(VirtioMemSystemReset,
virtio_mem_system_reset, VIRTIO_MEM_SYSTEM_RESET, OBJECT, {
TYPE_RESETTABLE_INTERFACE }, { })
Please fix style when applying :)
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
+struct VirtioMemSystemReset {
+ Object parent;
And add a newline here, thanks!
ResettableState reset_state;
+ VirtIOMEM *vmem;
};
struct VirtIOMEMClass {
