/*
 * Copyright 2024 The Hafnium Authors.
 *
 * Use of this source code is governed by a BSD-style
 * license that can be found in the LICENSE file or at
 * https://opensource.org/licenses/BSD-3-Clause.
 */

#include "hf/timer_mgmt.h"

#include "hf/arch/timer.h"

#include "hf/api.h"
#include "hf/check.h"
#include "hf/cpu.h"
#include "hf/std.h"
#include "hf/vcpu.h"

static void timer_list_add_vcpu(struct cpu *cpu, struct vcpu *vcpu)
{
	struct timer_pending_vcpu_list *timer_list;

	assert(vcpu != NULL && cpu != NULL);

	timer_list = &cpu->pending_timer_vcpus_list;
	sl_lock(&cpu->lock);

	/* Add the vCPU's timer entry if not already part of any list. */
	if (list_empty(&vcpu->timer_node)) {
		/* `root_entry` is also the tail of the timer list. */
		list_prepend(&timer_list->root_entry, &vcpu->timer_node);
	}

	sl_unlock(&cpu->lock);
}

static void timer_list_remove_vcpu(struct cpu *cpu, struct vcpu *vcpu)
{
	assert(vcpu != NULL && cpu != NULL);

	sl_lock(&cpu->lock);
	list_remove(&vcpu->timer_node);
	sl_unlock(&cpu->lock);
}

/**
 * Depending on the state of the vCPU's arch timer, either track or untrack it
 * through the timer list on current CPU.
 */
void timer_vcpu_manage(struct vcpu *vcpu)
{
	assert(vcpu != NULL);

	if (arch_timer_enabled(&vcpu->regs)) {
		/*
		 * Add it to the list maintained by partition manager for this
		 * CPU.
		 */
		timer_list_add_vcpu(vcpu->cpu, vcpu);
	} else {
		timer_list_remove_vcpu(vcpu->cpu, vcpu);
	}
}

/**
 * A vCPU's timer entry is the last entry in the list if it's `next` field
 * points to `root_entry` of the list.
 */
static inline bool timer_is_list_end(struct vcpu *vcpu,
				     struct timer_pending_vcpu_list *timer_list)
{
	return (vcpu->timer_node.next == &timer_list->root_entry);
}

/**
 * Find the vCPU with the nearest timer deadline, being tracked by partition
 * manager, on current CPU.
 */
struct vcpu *timer_find_vcpu_nearest_deadline(struct cpu *cpu)
{
	struct vcpu *vcpu_with_deadline = NULL;
	struct vcpu *it_vcpu = NULL;
	struct timer_pending_vcpu_list *timer_list;
	uint64_t near_deadline = UINT64_MAX;
	struct list_entry *next_timer_entry;

	assert(cpu != NULL);

	timer_list = &cpu->pending_timer_vcpus_list;
	sl_lock(&cpu->lock);

	if (list_empty(&timer_list->root_entry)) {
		goto out;
	}

	next_timer_entry = timer_list->root_entry.next;

	/* Iterate to find the vCPU with nearest deadline. */
	do {
		uint64_t expiry_ns;

		/* vCPU iterator. */
		it_vcpu =
			CONTAINER_OF(next_timer_entry, struct vcpu, timer_node);
		assert(arch_timer_enabled(&it_vcpu->regs));

		expiry_ns = arch_timer_remaining_ns(&it_vcpu->regs);

		if (expiry_ns < near_deadline) {
			near_deadline = expiry_ns;
			vcpu_with_deadline = it_vcpu;
		}

		/* Look at the next entry in the list. */
		next_timer_entry = it_vcpu->timer_node.next;
	} while (!timer_is_list_end(it_vcpu, timer_list));

out:
	sl_unlock(&cpu->lock);
	return vcpu_with_deadline;
}

/**
 * Find the vCPU whose timer deadline has expired and needs to be resumed at
 * the earliest.
 */
struct vcpu *timer_find_target_vcpu(struct vcpu *current)
{
	struct vcpu *target_vcpu;

	/*
	 * There are three possible scenarios here when execution was brought
	 * back from NWd to SPMC as soon as host timer expired:
	 * 1. The vCPU that was being tracked by SPMC on this CPUx has expired
	 * timer. This will be the target vCPU.
	 *
	 * However, it is likely that this vCPU could have been migrated by
	 * NWd driver to another CPU(lets say CPUy). The S-EL2 host timer on
	 * CPUy will take care of signaling the virtual timer interrupt
	 * eventually.
	 *
	 * 2. If there is another vCPU with expired timer in the list maintained
	 *    by SPMC on present CPUx, SPMC will pick that vCPU to be
	 *    target_vcpu.
	 * 3. If none of the vCPUs have expired timer, simply resume the normal
	 *    world i.e., target_vcpu will be NULL.
	 */
	if (current->vm->id == HF_OTHER_WORLD_ID) {
		target_vcpu = timer_find_vcpu_nearest_deadline(current->cpu);
		if (target_vcpu != NULL) {
			if (arch_timer_remaining_ns(&target_vcpu->regs) == 0) {
				/*
				 * SPMC will either signal or queue the virtual
				 * timer interrupt to the target vCPU. No
				 * need to track this vcpu anymore.
				 */
				timer_list_remove_vcpu(current->cpu,
						       target_vcpu);
			} else {
				target_vcpu = NULL;
			}
		}
	} else {
		target_vcpu = current;
	}

	return target_vcpu;
}

void timer_migrate_to_other_cpu(struct cpu *to_cpu,
				struct vcpu_locked migrate_vcpu_locked)
{
	struct cpu *from_cpu;
	struct vcpu *migrate_vcpu;

	assert(to_cpu != NULL);

	migrate_vcpu = migrate_vcpu_locked.vcpu;
	from_cpu = migrate_vcpu->cpu;

	if (from_cpu != NULL && (to_cpu != from_cpu)) {
		if (!list_empty(&migrate_vcpu->timer_node)) {
			assert(arch_timer_enabled(&migrate_vcpu->regs));

			/*
			 * Remove vcpu from timer list maintained by SPMC for
			 * old CPU.
			 */
			timer_list_remove_vcpu(from_cpu, migrate_vcpu);

			/*
			 * Add vcpu to timer list maintained by SPMC for new
			 * CPU.
			 */
			timer_list_add_vcpu(to_cpu, migrate_vcpu);
		}
	}
}