Reset in QEMU: the Resettable interface

The reset of qemu objects is handled using the resettable interface declared in include/hw/resettable.h.

This interface allows objects to be grouped (on a tree basis); so that the whole group can be reset consistently. Each individual member object does not have to care about others; in particular, problems of order (which object is reset first) are addressed.

The main object types which implement this interface are DeviceClass and BusClass.

Triggering reset

This section documents the APIs which “users” of a resettable object should use to control it. All resettable control functions must be called while holding the BQL.

You can apply a reset to an object using resettable_assert_reset(). You need to call resettable_release_reset() to release the object from reset. To instantly reset an object, without keeping it in reset state, just call resettable_reset(). These functions take two parameters: a pointer to the object to reset and a reset type.

The Resettable interface handles reset types with an enum ResetType:

RESET_TYPE_COLD: Cold reset is supported by every resettable object. In QEMU, it means we reset to the initial state corresponding to the start of QEMU; this might differ from what is a real hardware cold reset. It differs from other resets (like warm or bus resets) which may keep certain parts untouched.
RESET_TYPE_SNAPSHOT_LOAD: This is called for a reset which is being done to put the system into a clean state prior to loading a snapshot. (This corresponds to a reset with SHUTDOWN_CAUSE_SNAPSHOT_LOAD.) Almost all devices should treat this the same as RESET_TYPE_COLD. The main exception is devices which have some non-deterministic state they want to reinitialize to a different value on each cold reset, such as RNG seed information, and which they must not reinitialize on a snapshot-load reset.

Devices which implement reset methods must treat any unknown ResetType as equivalent to RESET_TYPE_COLD; this will reduce the amount of existing code we need to change if we add more types in future.

Calling resettable_reset() is equivalent to calling resettable_assert_reset() then resettable_release_reset(). It is possible to interleave multiple calls to these three functions. There may be several reset sources/controllers of a given object. The interface handles everything and the different reset controllers do not need to know anything about each others. The object will leave reset state only when each other controllers end their reset operation. This point is handled internally by maintaining a count of in-progress resets; it is crucial to call resettable_release_reset() one time and only one time per resettable_assert_reset() call.

For now migration of a device or bus in reset is not supported. Care must be taken not to delay resettable_release_reset() after its resettable_assert_reset() counterpart.

Note that, since resettable is an interface, the API takes a simple Object as parameter. Still, it is a programming error to call a resettable function on a non-resettable object and it will trigger a run time assert error. Since most calls to resettable interface are done through base class functions, such an error is not likely to happen.

For Devices and Buses, the following helper functions exist:

device_cold_reset()
bus_cold_reset()

These are simple wrappers around resettable_reset() function; they only cast the Device or Bus into an Object and pass the cold reset type. When possible prefer to use these functions instead of resettable_reset().

Device and bus functions co-exist because there can be semantic differences between resetting a bus and resetting the controller bridge which owns it. For example, consider a SCSI controller. Resetting the controller puts all its registers back to what reset state was as well as reset everything on the SCSI bus, whereas resetting just the SCSI bus only resets everything that’s on it but not the controller.

Multi-phase mechanism

This section documents the internals of the resettable interface.

The resettable interface uses a multi-phase system to relieve objects and machines from reset ordering problems. To address this, the reset operation of an object is split into three well defined phases.

When resetting several objects (for example the whole machine at simulation startup), all first phases of all objects are executed, then all second phases and then all third phases.

The three phases are:

The enter phase is executed when the object enters reset. It resets only local state of the object; it must not do anything that has a side-effect on other objects, such as raising or lowering a qemu_irq line or reading or writing guest memory.
The hold phase is executed for entry into reset, once every object in the group which is being reset has had its enter phase executed. At this point devices can do actions that affect other objects.
The exit phase is executed when the object leaves the reset state. Actions affecting other objects are permitted.

As said in previous section, the interface maintains a count of reset. This count is used to ensure phases are executed only when required. enter and hold phases are executed only when asserting reset for the first time (if an object is already in reset state when calling resettable_assert_reset() or resettable_reset(), they are not executed). The exit phase is executed only when the last reset operation ends. Therefore the object does not need to care how many of reset controllers it has and how many of them have started a reset.

Handling reset in a resettable object

This section documents the APIs that an implementation of a resettable object must provide and what functions it has access to. It is intended for people who want to implement or convert a class which has the resettable interface; for example when specializing an existing device or bus.

Methods to implement

Three methods should be defined or left empty. Each method corresponds to a phase of the reset; they are name phases.enter(), phases.hold() and phases.exit(). They all take the object as parameter. The enter method also take the reset type as second parameter.

When extending an existing class, these methods may need to be extended too. The resettable_class_set_parent_phases() class function may be used to backup parent class methods.

Here follows an example to implement reset for a Device which sets an IO while in reset.

static void mydev_reset_enter(Object *obj, ResetType type)
{
    MyDevClass *myclass = MYDEV_GET_CLASS(obj);
    MyDevState *mydev = MYDEV(obj);
    /* call parent class enter phase */
    if (myclass->parent_phases.enter) {
        myclass->parent_phases.enter(obj, type);
    }
    /* initialize local state only */
    mydev->var = 0;
}

static void mydev_reset_hold(Object *obj, ResetType type)
{
    MyDevClass *myclass = MYDEV_GET_CLASS(obj);
    MyDevState *mydev = MYDEV(obj);
    /* call parent class hold phase */
    if (myclass->parent_phases.hold) {
        myclass->parent_phases.hold(obj, type);
    }
    /* set an IO */
    qemu_set_irq(mydev->irq, 1);
}

static void mydev_reset_exit(Object *obj, ResetType type)
{
    MyDevClass *myclass = MYDEV_GET_CLASS(obj);
    MyDevState *mydev = MYDEV(obj);
    /* call parent class exit phase */
    if (myclass->parent_phases.exit) {
        myclass->parent_phases.exit(obj, type);
    }
    /* clear an IO */
    qemu_set_irq(mydev->irq, 0);
}

typedef struct MyDevClass {
    MyParentClass parent_class;
    /* to store eventual parent reset methods */
    ResettablePhases parent_phases;
} MyDevClass;

static void mydev_class_init(ObjectClass *class, void *data)
{
    MyDevClass *myclass = MYDEV_CLASS(class);
    ResettableClass *rc = RESETTABLE_CLASS(class);
    resettable_class_set_parent_phases(rc,
                                       mydev_reset_enter,
                                       mydev_reset_hold,
                                       mydev_reset_exit,
                                       &myclass->parent_phases);
}

In the above example, we override all three phases. It is possible to override only some of them by passing NULL instead of a function pointer to resettable_class_set_parent_phases(). For example, the following will only override the enter phase and leave hold and exit untouched:

resettable_class_set_parent_phases(rc, mydev_reset_enter, NULL, NULL,
                                   &myclass->parent_phases);

This is equivalent to providing a trivial implementation of the hold and exit phases which does nothing but call the parent class’s implementation of the phase.

Polling the reset state

Resettable interface provides the resettable_is_in_reset() function. This function returns true if the object parameter is currently under reset.

An object is under reset from the beginning of the enter phase (before either its children or its own enter method is called) to the exit phase. During enter and hold phase only, the function will return that the object is in reset. The state is changed after the exit is propagated to its children and just before calling the object’s own exit method.

This function may be used if the object behavior has to be adapted while in reset state. For example if a device has an irq input, it will probably need to ignore it while in reset; then it can for example check the reset state at the beginning of the irq callback.

Note that until migration of the reset state is supported, an object should not be left in reset. So apart from being currently executing one of the reset phases, the only cases when this function will return true is if an external interaction (like changing an io) is made during hold or exit phase of another object in the same reset group.

Helpers device_is_in_reset() and bus_is_in_reset() are also provided for devices and buses and should be preferred.

Base class handling of reset

This section documents parts of the reset mechanism that you only need to know about if you are extending it to work with a new base class other than DeviceClass or BusClass, or maintaining the existing code in those classes. Most people can ignore it.

Methods to implement

There are two other methods that need to exist in a class implementing the interface: get_state() and child_foreach().

get_state() is simple. resettable is an interface and, as a consequence, does not have any class state structure. But in order to factorize the code, we need one. This method must return a pointer to ResettableState structure. The structure must be allocated by the base class; preferably it should be located inside the object instance structure.

child_foreach() is more complex. It should execute the given callback on every reset child of the given resettable object. All children must be resettable too. Additional parameters (a reset type and an opaque pointer) must be passed to the callback too.

In DeviceClass and BusClass the ResettableState is located DeviceState and BusState structure. child_foreach() is implemented to follow the bus hierarchy; for a bus, it calls the function on every child device; for a device, it calls the function on every bus child. When we reset the main system bus, we reset the whole machine bus tree.

Changing a resettable parent

One thing which should be taken care of by the base class is handling reset hierarchy changes.

The reset hierarchy is supposed to be static and built during machine creation. But there are actually some exceptions. To cope with this, the resettable API provides resettable_change_parent(). This function allows to set, update or remove the parent of a resettable object after machine creation is done. As parameters, it takes the object being moved, the old parent if any and the new parent if any.

This function can be used at any time when not in a reset operation. During a reset operation it must be used only in hold phase. Using it in enter or exit phase is an error. Also it should not be used during machine creation, although it is harmless to do so: the function is a no-op as long as old and new parent are NULL or not in reset.

There is currently 2 cases where this function is used:

device hotplug; it means a new device is introduced on a live bus.
hot bus change; it means an existing live device is added, moved or removed in the bus hierarchy. At the moment, it occurs only in the raspi machines for changing the sdbus used by sd card.

Reset of the complete system

Reset of the complete system is a little complicated. The typical flow is:

Code which wishes to reset the entire system does so by calling qemu_system_reset_request(). This schedules a reset, but the reset will happen asynchronously after the function returns. That makes this safe to call from, for example, device models.
The function which is called to make the reset happen is qemu_system_reset(). Generally only core system code should call this directly.
qemu_system_reset() calls the MachineClass::reset method of the current machine, if it has one. That method must call qemu_devices_reset(). If the machine has no reset method, qemu_system_reset() calls qemu_devices_reset() directly.
qemu_devices_reset() performs a reset of the system, using the three-phase mechanism listed above. It resets all objects that were registered with it using qemu_register_resettable(). It also calls all the functions registered with it using qemu_register_reset(). Those functions are called during the “hold” phase of this reset.
The most important object that this reset resets is the ‘sysbus’ bus. The sysbus bus is the root of the qbus tree. This means that all devices on the sysbus are reset, and all their child buses, and all the devices on those child buses.
Devices which are not on the qbus tree are not automatically reset! (The most obvious example of this is CPU objects, but anything that directly inherits from TYPE_OBJECT or TYPE_DEVICE rather than from TYPE_SYS_BUS_DEVICE or some other plugs-into-a-bus type will be in this category.) You need to therefore arrange for these to be reset in some other way (e.g. using qemu_register_resettable() or qemu_register_reset()).