/* Copyright (C) 2005  David Decotigny
   Copyright (C) 2000-2004, The KOS team

   Initially taken from SOS
*/

#include "assert.h"
#include "klibc.h"
#include "segment.h"

#include "cpu_context.h"

/**
 * Here is the definition of a CPU context for IA32 processors. This
 * is a Matos/SOS convention, not a specification given by the IA32
 * spec. However there is a strong constraint related to the x86
 * interrupt handling specification: the top of the stack MUST be
 * compatible with the 'iret' instruction, ie there must be the
 * err_code (might be 0), eip, cs and eflags of the destination
 * context in that order (see Intel x86 specs vol 3, figure 5-4).
 *
 * @note IMPORTANT: This definition MUST be consistent with the way
 * the registers are stored on the stack in
 * irq_wrappers.S/exception_wrappers.S !!! Hence the constraint above.
 */
struct cpu_state {
  /* (Lower addresses) */

  /* These are Matos/SOS convention */
  uint16_t gs;
  uint16_t fs;
  uint16_t es;
  uint16_t ds;
  uint16_t cpl0_ss;           /* This is ALWAYS the Stack Segment of the
                 Kernel context (CPL0) of the interrupted
                 thread, even for a user thread */
  uint16_t alignment_padding; /* unused */
  uint32_t eax;
  uint32_t ebx;
  uint32_t ecx;
  uint32_t edx;
  uint32_t esi;
  uint32_t edi;
  uint32_t ebp;

  /* MUST NEVER CHANGE (dependent on the IA32 iret instruction) */
  uint32_t error_code;
  vaddr_t eip;
  uint32_t cs; /* 32bits according to the specs ! However, the CS
        register is really 16bits long */
  uint32_t eflags;

  /* (Higher addresses) */
} __attribute__((packed));

/**
 * The CS value pushed on the stack by the CPU upon interrupt, and
 * needed by the iret instruction, is 32bits long while the real CPU
 * CS register is 16bits only: this macro simply retrieves the CPU
 * "CS" register value from the CS value pushed on the stack by the
 * CPU upon interrupt.
 *
 * The remaining 16bits pushed by the CPU should be considered
 * "reserved" and architecture dependent. IMHO, the specs don't say
 * anything about them. Considering that some architectures generate
 * non-zero values for these 16bits (at least Cyrix), we'd better
 * ignore them.
 */
#define GET_CPU_CS_REGISTER_VALUE(pushed_ui32_cs_value) ((pushed_ui32_cs_value)&0xffff)

/**
 * Structure of an interrupted Kernel thread's context
 */
struct cpu_kstate {
  struct cpu_state regs;
} __attribute__((packed));

/**
 * THE main operation of a kernel thread. This routine calls the
 * kernel thread function start_func and calls exit_func when
 * start_func returns.
 */
static void core_routine(cpu_kstate_function_arg1_t *start_func, void *start_arg,
                         cpu_kstate_function_arg1_t *exit_func, void *exit_arg)
    __attribute__((noreturn));

static void core_routine(cpu_kstate_function_arg1_t *start_func, void *start_arg,
                         cpu_kstate_function_arg1_t *exit_func, void *exit_arg)
{
  start_func(start_arg);
  exit_func(exit_arg);

  assert(!"The exit function of the thread should NOT return !");
  for (;;)
    ;
}

int cpu_kstate_init(struct cpu_state **ctxt, cpu_kstate_function_arg1_t *start_func,
                    vaddr_t start_arg, vaddr_t stack_bottom, size_t stack_size,
                    cpu_kstate_function_arg1_t *exit_func, vaddr_t exit_arg)
{
  /* We are initializing a Kernel thread's context */
  struct cpu_kstate *kctxt;

  /* This is a critical internal function, so that it is assumed that
     the caller knows what he does: we legitimally assume that values
     for ctxt, start_func, stack_* and exit_func are allways VALID ! */

  /* Setup the stack.
   *
   * On x86, the stack goes downward. Each frame is configured this
   * way (higher addresses first):
   *
   *  - (optional unused space. As of gcc 3.3, this space is 24 bytes)
   *  - arg n
   *  - arg n-1
   *  - ...
   *  - arg 1
   *  - return instruction address: The address the function returns to
   *    once finished
   *  - local variables
   *
   * The remaining of the code should be read from the end upward to
   * understand how the processor will handle it.
   */

  vaddr_t tmp_vaddr = stack_bottom + stack_size;
  uint32_t *stack   = (uint32_t *)tmp_vaddr;

  /* If needed, poison the stack */
#ifdef CPU_STATE_DETECT_UNINIT_KERNEL_VARS
  memset((void *)stack_bottom, CPU_STATE_STACK_POISON, stack_size);
#elif defined(CPU_STATE_DETECT_KERNEL_STACK_OVERFLOW)
  cpu_state_prepare_detect_kernel_stack_overflow(stack_bottom, stack_size);
#endif

  /* Simulate a call to the core_routine() function: prepare its
     arguments */
  *(--stack) = exit_arg;
  *(--stack) = (uint32_t)exit_func;
  *(--stack) = start_arg;
  *(--stack) = (uint32_t)start_func;
  *(--stack) = 0; /* Return address of core_routine => force page fault */

  /*
   * Setup the initial context structure, so that the CPU will execute
   * the function core_routine() once this new context has been
   * restored on CPU
   */

  /* Compute the base address of the structure, which must be located
     below the previous elements */
  tmp_vaddr = ((vaddr_t)stack) - sizeof(struct cpu_kstate);
  kctxt     = (struct cpu_kstate *)tmp_vaddr;

  /* Initialize the CPU context structure */
  memset(kctxt, 0x0, sizeof(struct cpu_kstate));

  /* Tell the CPU context structure that the first instruction to
     execute will be that of the core_routine() function */
  kctxt->regs.eip = (uint32_t)core_routine;

  /* Setup the segment registers */
  kctxt->regs.cs      = BUILD_SEGMENT_REG_VALUE(0, FALSE, SEG_KCODE); /* Code */
  kctxt->regs.ds      = BUILD_SEGMENT_REG_VALUE(0, FALSE, SEG_KDATA); /* Data */
  kctxt->regs.es      = BUILD_SEGMENT_REG_VALUE(0, FALSE, SEG_KDATA); /* Data */
  kctxt->regs.cpl0_ss = BUILD_SEGMENT_REG_VALUE(0, FALSE, SEG_KDATA); /* Stack */
  /* fs and gs unused for the moment. */

  /* The newly created context is initially interruptible */
  kctxt->regs.eflags = (1 << 9); /* set IF bit */

  /* Finally, update the generic kernel/user thread context */
  *ctxt = (struct cpu_state *)kctxt;

  return 0;
}

#if defined(CPU_STATE_DETECT_KERNEL_STACK_OVERFLOW)
void cpu_state_prepare_detect_kernel_stack_overflow(const struct cpu_state *ctxt,
                                                    vaddr_t stack_bottom, size_t stack_size)
{
  (void)ctxt;
  size_t poison_size = CPU_STATE_DETECT_KERNEL_STACK_OVERFLOW;
  if (poison_size > stack_size)
    poison_size = stack_size;

  memset((void *)stack_bottom, CPU_STATE_STACK_POISON, poison_size);
}

void cpu_state_detect_kernel_stack_overflow(const struct cpu_state *ctxt, vaddr_t stack_bottom,
                                            size_t stack_size)
{
  unsigned char *c;
  size_t i;

  /* On Matos/SOS, "ctxt" corresponds to the address of the esp register of
     the saved context in Kernel mode (always, even for the interrupted
     context of a user thread). Here we make sure that this stack
     pointer is within the allowed stack area */
  assert(((vaddr_t)ctxt) >= stack_bottom);
  assert(((vaddr_t)ctxt) + sizeof(struct cpu_kstate) <= stack_bottom + stack_size);

  /* Check that the bottom of the stack has not been altered */
  for (c = (unsigned char *)stack_bottom, i = 0;
       (i < CPU_STATE_DETECT_KERNEL_STACK_OVERFLOW) && (i < stack_size); c++, i++) {
    assert(CPU_STATE_STACK_POISON == *c);
  }
}
#endif

/* =======================================================================
 * Public Accessor functions
 */

vaddr_t cpu_context_get_PC(const struct cpu_state *ctxt)
{
  assert(NULL != ctxt);

  /* This is the PC of the interrupted context (ie kernel or user
     context). */
  return ctxt->eip;
}

vaddr_t cpu_context_get_SP(const struct cpu_state *ctxt)
{
  assert(NULL != ctxt);

  /* On Matos/SOS, "ctxt" corresponds to the address of the esp register of
     the saved context in Kernel mode (always, even for the interrupted
     context of a user thread). */
  return (vaddr_t)ctxt;
}

void cpu_context_dump(const struct cpu_state *ctxt)
{
  printf("CPU: eip=%x esp=%x eflags=%x cs=%x ds=%x ss=%x err=%x", (unsigned)ctxt->eip,
         (unsigned)ctxt, (unsigned)ctxt->eflags, (unsigned)GET_CPU_CS_REGISTER_VALUE(ctxt->cs),
         (unsigned)ctxt->ds, (unsigned)ctxt->cpl0_ss, (unsigned)ctxt->error_code);
}