Re: [RFC v2 06/16] luo: luo_subsystems: add subsystem registration

[Pratyush Yadav <pratyush@kernel.org>]

On Thu, May 15 2025, Pasha Tatashin wrote:

> Introduce the framework for kernel subsystems (e.g., KVM, IOMMU, device
> drivers) to register with LUO and participate in the live update process
> via callbacks.
>
> Subsystem Registration:
> - Defines struct liveupdate_subsystem in linux/liveupdate.h,
>   which subsystems use to provide their name and optional callbacks
>   (prepare, freeze, cancel, finish). The callbacks accept
>   a u64 *data intended for passing state/handles.
> - Exports liveupdate_register_subsystem() and
>   liveupdate_unregister_subsystem() API functions.
> - Adds drivers/misc/liveupdate/luo_subsystems.c to manage a list
>   of registered subsystems.
>   Registration/unregistration is restricted to
>   specific LUO states (NORMAL/UPDATED).
>
> Callback Framework:
> - The main luo_core.c state transition functions
>   now delegate to new luo_do_subsystems_*_calls() functions
>   defined in luo_subsystems.c.
> - These new functions are intended to iterate through the registered
>   subsystems and invoke their corresponding callbacks.
>
> FDT Integration:
> - Adds a /subsystems subnode within the main LUO FDT created in
>   luo_core.c. This node has its own compatibility string
>   (subsystems-v1).
> - luo_subsystems_fdt_setup() populates this node by adding a
>   property for each registered subsystem, using the subsystem's
>   name.
>   Currently, these properties are initialized with a placeholder
>   u64 value (0).
> - luo_subsystems_startup() is called from luo_core.c on boot to
>   find and validate the /subsystems node in the FDT received via
>   KHO. It panics if the node is missing or incompatible.
> - Adds a stub API function liveupdate_get_subsystem_data() intended
>   for subsystems to retrieve their persisted u64 data from the FDT
>       in the new kernel.
>
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
[...]
> +/**
> + * liveupdate_unregister_subsystem - Unregister a kernel subsystem handler from
> + * LUO
> + * @h: Pointer to the same liveupdate_subsystem structure that was used during
> + * registration.
> + *
> + * Unregisters a previously registered subsystem handler. Typically called
> + * during module exit or subsystem teardown. LUO removes the structure from its
> + * internal list; the caller is responsible for any necessary memory cleanup
> + * of the structure itself.
> + *
> + * Return: 0 on success, negative error code otherwise.
> + * -EINVAL if h is NULL.
> + * -ENOENT if the specified handler @h is not found in the registration list.
> + * -EBUSY if LUO is not in the NORMAL state.
> + */
> +int liveupdate_unregister_subsystem(struct liveupdate_subsystem *h)
> +{
> +	struct liveupdate_subsystem *iter;
> +	bool found = false;
> +	int ret = 0;
> +
> +	luo_state_read_enter();
> +	if (!liveupdate_state_normal() && !liveupdate_state_updated()) {
> +		luo_state_read_exit();
> +		return -EBUSY;
> +	}
> +
> +	mutex_lock(&luo_subsystem_list_mutex);
> +	list_for_each_entry(iter, &luo_subsystems_list, list) {
> +		if (iter == h) {
> +			found = true;

Nit: you don't actually need the found variable. You can do the same
check that list_for_each_entry() uses, which is to call
list_entry_is_head().

> +			break;
> +		}
> +	}
> +
> +	if (found) {
> +		list_del_init(&h->list);
> +	} else {
> +		pr_warn("Subsystem handler '%s' not found for unregistration.\n",
> +			h->name);
> +		ret = -ENOENT;
> +	}
> +
> +	mutex_unlock(&luo_subsystem_list_mutex);
> +	luo_state_read_exit();
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(liveupdate_unregister_subsystem);
[...]

-- 
Regards,
Pratyush Yadav