/*
 * Copyright (c) 2011-2015, 2021-2022 Jonas 'Sortie' Termansen.
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 *
 * scheduler.cpp
 * Decides the order to execute threads in and switching between them.
 */

#include <sys/types.h>

#include <assert.h>
#include <string.h>
#include <timespec.h>

#if defined(__x86_64__)
#include <msr.h>
#endif

#include <sortix/clock.h>
#include <sortix/timespec.h>

#include <sortix/kernel/decl.h>
#include <sortix/kernel/interrupt.h>
#include <sortix/kernel/kernel.h>
#include <sortix/kernel/memorymanagement.h>
#include <sortix/kernel/process.h>
#include <sortix/kernel/random.h>
#include <sortix/kernel/registers.h>
#include <sortix/kernel/scheduler.h>
#include <sortix/kernel/signal.h>
#include <sortix/kernel/syscall.h>
#include <sortix/kernel/thread.h>
#include <sortix/kernel/time.h>

#if defined(__i386__) || defined(__x86_64__)
#include "x86-family/gdt.h"
#include "x86-family/float.h"
#endif

namespace Sortix {
namespace Scheduler {

static Thread* current_thread;

void SaveInterruptedContext(const struct interrupt_context* intctx,
                            struct thread_registers* registers)
{
#if defined(__i386__)
	registers->signal_pending = intctx->signal_pending;
	registers->kerrno = intctx->kerrno;
	registers->eax = intctx->eax;
	registers->ebx = intctx->ebx;
	registers->ecx = intctx->ecx;
	registers->edx = intctx->edx;
	registers->edi = intctx->edi;
	registers->esi = intctx->esi;
	registers->esp = intctx->esp;
	registers->ebp = intctx->ebp;
	registers->eip = intctx->eip;
	registers->eflags = intctx->eflags;
	registers->fsbase = (unsigned long) GDT::GetFSBase();
	registers->gsbase = (unsigned long) GDT::GetGSBase();
	asm ( "mov %%cr3, %0" : "=r"(registers->cr3) );
	registers->kernel_stack = GDT::GetKernelStack();
	registers->cs = intctx->cs;
	registers->ds = intctx->ds;
	registers->ss = intctx->ss;
	asm volatile ("fxsave (%0)" : : "r"(registers->fpuenv));
#elif defined(__x86_64__)
	registers->signal_pending = intctx->signal_pending;
	registers->kerrno = intctx->kerrno;
	registers->rax = intctx->rax;
	registers->rbx = intctx->rbx;
	registers->rcx = intctx->rcx;
	registers->rdx = intctx->rdx;
	registers->rdi = intctx->rdi;
	registers->rsi = intctx->rsi;
	registers->rsp = intctx->rsp;
	registers->rbp = intctx->rbp;
	registers->r8 = intctx->r8;
	registers->r9 = intctx->r9;
	registers->r10 = intctx->r10;
	registers->r11 = intctx->r11;
	registers->r12 = intctx->r12;
	registers->r13 = intctx->r13;
	registers->r14 = intctx->r14;
	registers->r15 = intctx->r15;
	registers->r15 = intctx->r15;
	registers->rip = intctx->rip;
	registers->rflags = intctx->rflags;
	registers->fsbase = (unsigned long) rdmsr(MSRID_FSBASE);
	registers->gsbase = (unsigned long) rdmsr(MSRID_GSBASE);
	asm ( "mov %%cr3, %0" : "=r"(registers->cr3) );
	registers->kernel_stack = GDT::GetKernelStack();
	registers->cs = intctx->cs;
	registers->ds = intctx->ds;
	registers->ss = intctx->ss;
	asm volatile ("fxsave (%0)" : : "r"(registers->fpuenv));
#else
#warning "You need to implement register saving"
#endif
}

void LoadInterruptedContext(struct interrupt_context* intctx,
                            const struct thread_registers* registers)
{
#if defined(__i386__)
	intctx->signal_pending = registers->signal_pending;
	intctx->kerrno = registers->kerrno;
	intctx->eax = registers->eax;
	intctx->ebx = registers->ebx;
	intctx->ecx = registers->ecx;
	intctx->edx = registers->edx;
	intctx->edi = registers->edi;
	intctx->esi = registers->esi;
	intctx->esp = registers->esp;
	intctx->ebp = registers->ebp;
	intctx->eip = registers->eip;
	intctx->eflags = registers->eflags;
	GDT::SetFSBase(registers->fsbase);
	GDT::SetGSBase(registers->gsbase);
	asm volatile ( "mov %0, %%cr3" : : "r"(registers->cr3) );
	GDT::SetKernelStack(registers->kernel_stack);
	intctx->cs = registers->cs;
	intctx->ds = registers->ds;
	intctx->ss = registers->ss;
	asm volatile ("fxrstor (%0)" : : "r"(registers->fpuenv));
#elif defined(__x86_64__)
	intctx->signal_pending = registers->signal_pending;
	intctx->kerrno = registers->kerrno;
	intctx->rax = registers->rax;
	intctx->rbx = registers->rbx;
	intctx->rcx = registers->rcx;
	intctx->rdx = registers->rdx;
	intctx->rdi = registers->rdi;
	intctx->rsi = registers->rsi;
	intctx->rsp = registers->rsp;
	intctx->rbp = registers->rbp;
	intctx->r8 = registers->r8;
	intctx->r9 = registers->r9;
	intctx->r10 = registers->r10;
	intctx->r11 = registers->r11;
	intctx->r12 = registers->r12;
	intctx->r13 = registers->r13;
	intctx->r14 = registers->r14;
	intctx->r15 = registers->r15;
	intctx->r15 = registers->r15;
	intctx->rip = registers->rip;
	intctx->rflags = registers->rflags;
	wrmsr(MSRID_FSBASE, registers->fsbase);
	wrmsr(MSRID_GSBASE, registers->gsbase);
	asm volatile ( "mov %0, %%cr3" : : "r"(registers->cr3) );
	GDT::SetKernelStack(registers->kernel_stack);
	intctx->cs = registers->cs;
	intctx->ds = registers->ds;
	intctx->ss = registers->ss;
	asm volatile ("fxrstor (%0)" : : "r"(registers->fpuenv));
#else
#warning "You need to implement register loading"
#endif
}

extern "C" void fake_interrupt(void);

// Pretend a particular interrupt arrived on another thread's stack. This
// assumes we're _not_ on current_thread's stack right now. This sets up the
// interrupt context such that the interrupt handler runs in that thread.
// The interrupt handler runs with premption enabled. This is used to deliver
// signals during context switches, as the signal handler needs to have
// preemption enabled.
static void FakeInterruptedContext(struct interrupt_context* intctx, int int_no)
{
#if defined(__i386__)
	uintptr_t stack = current_thread->kernel_stack_pos +
	                  current_thread->kernel_stack_size;
	stack -= sizeof(struct interrupt_context);
	struct interrupt_context* fakectx = (struct interrupt_context*) stack;
	memcpy(fakectx, intctx, sizeof(struct interrupt_context));
	fakectx->int_no = int_no;
	fakectx->err_code = 0;
	stack -= 4;
	stack &= 0xFFFFFFF0;
	intctx->signal_pending = intctx->signal_pending;
	intctx->kerrno = 0;
	intctx->cr2 = 0;
	intctx->ds = KDS | KRPL;
	intctx->edi = intctx->edi;
	intctx->esi = intctx->esi;
	intctx->ebp = intctx->signal_pending;
	intctx->ebx = (uintptr_t) fakectx;
	intctx->edx = intctx->edx;
	intctx->ecx = intctx->ecx;
	intctx->eax = intctx->eax;
	intctx->int_no = intctx->int_no;
	intctx->err_code = intctx->err_code;
	intctx->eip = (uintptr_t) fake_interrupt;
	intctx->cs = KCS | KRPL;
	intctx->eflags = FLAGS_RESERVED1 | FLAGS_INTERRUPT | FLAGS_ID;
	intctx->esp = stack;
	intctx->ss = KDS | KRPL;
#elif defined(__x86_64__)
	uintptr_t stack = current_thread->kernel_stack_pos +
	                  current_thread->kernel_stack_size;
	stack -= sizeof(struct interrupt_context);
	struct interrupt_context* fakectx = (struct interrupt_context*) stack;
	memcpy(fakectx, intctx, sizeof(struct interrupt_context));
	fakectx->int_no = int_no;
	fakectx->err_code = 0;
	stack &= 0xFFFFFFFFFFFFFFF0;
	intctx->signal_pending = intctx->signal_pending;
	intctx->kerrno = 0;
	intctx->cr2 = 0;
	intctx->ds = KDS | KRPL;
	intctx->rdi = (uintptr_t) fakectx;
	intctx->rsi = intctx->rsi;
	intctx->rbp = intctx->signal_pending;
	intctx->rbx = (uintptr_t) fakectx;
	intctx->rdx = intctx->rdx;
	intctx->rcx = intctx->rcx;
	intctx->rax = intctx->rax;
	intctx->r8 = intctx->r8;
	intctx->r9 = intctx->r9;
	intctx->r10 = intctx->r10;
	intctx->r11 = intctx->r11;
	intctx->r12 = intctx->r12;
	intctx->r13 = intctx->r13;
	intctx->r14 = intctx->r14;
	intctx->r15 = intctx->r15;
	intctx->int_no = intctx->int_no;
	intctx->err_code = intctx->err_code;
	intctx->rip = (uintptr_t) fake_interrupt;
	intctx->cs = KCS | KRPL;
	intctx->rflags = FLAGS_RESERVED1 | FLAGS_INTERRUPT | FLAGS_ID;
	intctx->rsp = stack;
	intctx->ss = KDS | KRPL;
#else
#warning "You need to implement faking an interrupt"
#endif
}

static void SwitchRegisters(struct interrupt_context* intctx,
                            Thread* prev,
                            Thread* next)
{
	if ( prev == next )
		return;

	Random::Mix(Random::SOURCE_PREEMPTION, intctx, sizeof(*intctx));
	SaveInterruptedContext(intctx, &prev->registers);
	if ( !prev->registers.cr3 )
		Log::PrintF("Thread %p had cr3=0x%zx\n", prev, prev->registers.cr3);
	if ( !next->registers.cr3 )
		Log::PrintF("Thread %p has cr3=0x%zx\n", next, next->registers.cr3);
	LoadInterruptedContext(intctx, &next->registers);

	current_thread = next;
}

static Thread* idle_thread;
static Thread* first_runnable_thread;
static Thread* true_current_thread;

static void SwitchThread(struct interrupt_context* intctx,
                         Thread* old_thread,
                         Thread* new_thread)
{
	assert(new_thread->state == ThreadState::RUNNABLE ||
	       new_thread == idle_thread);
	SwitchRegisters(intctx, old_thread, new_thread);
	if ( intctx->signal_pending && InUserspace(intctx) )
	{
		// Become the thread for real and run the signal handler.
		if ( old_thread == new_thread )
		{
			// We're already this thread, so run the signal handler.
			Interrupt::Enable();
			assert(Interrupt::IsEnabled());
			Signal::DispatchHandler(intctx, NULL);
		}
		else
		{
			// We need to transfer execution to the correct stack. We know the
			// the thread is in user-space and isn't using its kernel stack, and
			// we know we're not using the stack right now.
			FakeInterruptedContext(intctx, 130);
		}
	}
}

static Thread* FindRunnableThreadWithSystemTid(uintptr_t system_tid)
{
	Thread* begun_thread = first_runnable_thread;
	if ( !begun_thread )
		return NULL;
	Thread* iter = begun_thread;
	do
	{
		if ( iter->system_tid == system_tid )
			return iter;
		iter = iter->scheduler_list_next;
	} while ( iter != begun_thread );
	return NULL;
}

static Thread* PopNextThread(bool yielded)
{
	Thread* result;

	uintptr_t yield_to_tid = current_thread->yield_to_tid;
	if ( yielded && yield_to_tid != 0 )
	{
		if ( (result = FindRunnableThreadWithSystemTid(yield_to_tid)) )
			return result;
	}

	if ( first_runnable_thread )
	{
		result = first_runnable_thread;
		first_runnable_thread = first_runnable_thread->scheduler_list_next;
	}
	else
	{
		result = idle_thread;
	}

	return result;
}

void SwitchTo(struct interrupt_context* intctx, Thread* new_thread)
{
	Thread* old_thread = CurrentThread();
	if ( new_thread == old_thread )
		return;
	if ( new_thread->state != ThreadState::RUNNABLE )
		return;
	if ( old_thread != idle_thread )
		first_runnable_thread = old_thread;
	true_current_thread = new_thread;
	SwitchThread(intctx, old_thread, new_thread);
}

static void RealSwitch(struct interrupt_context* intctx, bool yielded)
{
	Thread* old_thread = CurrentThread();
	Thread* new_thread = PopNextThread(yielded);
	true_current_thread = new_thread;
	SwitchThread(intctx, old_thread, new_thread);
}

void Switch(struct interrupt_context* intctx)
{
	RealSwitch(intctx, false);
}

void InterruptYieldCPU(struct interrupt_context* intctx, void* /*user*/)
{
	bool wait = false;
	switch ( current_thread->yield_operation )
	{
	case YIELD_OPERATION_NONE: RealSwitch(intctx, true); return;
	case YIELD_OPERATION_WAIT_FUTEX:
		wait = !current_thread->futex_woken && !current_thread->timer_woken;
		break;
	case YIELD_OPERATION_WAIT_FUTEX_SIGNAL:
		wait = !current_thread->futex_woken && !current_thread->timer_woken &&
		       !asm_signal_is_pending;
		break;
	case YIELD_OPERATION_WAIT_KUTEX:
		wait = !current_thread->kutex_woken && !current_thread->timer_woken;
		break;
	case YIELD_OPERATION_WAIT_KUTEX_SIGNAL:
		wait = !current_thread->kutex_woken && !current_thread->timer_woken &&
		       !asm_signal_is_pending;
		break;
	}
	if ( wait )
	{
		SetThreadState(current_thread, ThreadState::FUTEX_WAITING);
		RealSwitch(intctx, false);
	}
}

void ThreadExitCPU(struct interrupt_context* intctx, void* /*user*/)
{
	SetThreadState(current_thread, ThreadState::DEAD);
	RealSwitch(intctx, false);
}

// The idle thread serves no purpose except being an infinite loop that does
// nothing, which is only run when the system has nothing to do.
void SetIdleThread(Thread* thread)
{
	assert(!idle_thread);
	idle_thread = thread;
	SetThreadState(thread, ThreadState::NONE);
	current_thread = thread;
	true_current_thread = thread;
}

Process* GetKernelProcess()
{
	if ( !idle_thread )
		return NULL;
	return idle_thread->process;
}

void SetThreadState(Thread* thread, ThreadState state, bool wake_only)
{
	bool was_enabled = Interrupt::SetEnabled(false);

	// Avoid transitioning to the RUNNABLE state from unintended states.
	if ( wake_only && thread->state != ThreadState::FUTEX_WAITING )
		state = thread->state;

	// Remove the thread from the list of runnable threads.
	if ( thread->state == ThreadState::RUNNABLE &&
	     state != ThreadState::RUNNABLE )
	{
		if ( thread == first_runnable_thread )
			first_runnable_thread = thread->scheduler_list_next;
		if ( thread == first_runnable_thread )
			first_runnable_thread = NULL;
		assert(thread->scheduler_list_prev);
		assert(thread->scheduler_list_next);
		thread->scheduler_list_prev->scheduler_list_next = thread->scheduler_list_next;
		thread->scheduler_list_next->scheduler_list_prev = thread->scheduler_list_prev;
		thread->scheduler_list_prev = NULL;
		thread->scheduler_list_next = NULL;
	}

	// Insert the thread into the scheduler's carousel linked list.
	if ( thread->state != ThreadState::RUNNABLE &&
	     state == ThreadState::RUNNABLE )
	{
		if ( first_runnable_thread == NULL )
			first_runnable_thread = thread;
		thread->scheduler_list_prev = first_runnable_thread->scheduler_list_prev;
		thread->scheduler_list_next = first_runnable_thread;
		first_runnable_thread->scheduler_list_prev = thread;
		thread->scheduler_list_prev->scheduler_list_next = thread;
	}

	thread->state = state;

	assert(thread->state != ThreadState::RUNNABLE || thread->scheduler_list_prev);
	assert(thread->state != ThreadState::RUNNABLE || thread->scheduler_list_next);

	Interrupt::SetEnabled(was_enabled);
}

void SetSignalPending(Thread* thread, unsigned long is_pending)
{
	thread->registers.signal_pending = is_pending;
	if ( is_pending )
		Scheduler::SetThreadState(thread, ThreadState::RUNNABLE, true);
}

ThreadState GetThreadState(Thread* thread)
{
	return thread->state;
}

} // namespace Scheduler
} // namespace Sortix

namespace Sortix {

int sys_sched_yield(void)
{
	return kthread_yield(), 0;
}

} // namespace Sortix

namespace Sortix {
namespace Scheduler {

void ScheduleTrueThread()
{
	bool was_enabled = Interrupt::SetEnabled(false);
	if ( true_current_thread != current_thread )
	{
		current_thread->yield_to_tid = 0;
		first_runnable_thread = true_current_thread;
		kthread_yield();
	}
	Interrupt::SetEnabled(was_enabled);
}

} // namespace Scheduler
} // namespace Sortix

namespace Sortix {

Thread* CurrentThread()
{
	return Scheduler::current_thread;
}

Process* CurrentProcess()
{
	return CurrentThread()->process;
}

} // namespace Sortix
