xv6/ioapic.c

// The I/O APIC manages hardware interrupts for an SMP system.
// http://www.intel.com/design/chipsets/datashts/29056601.pdf

#include "types.h"
#include "defs.h"
#include "traps.h"

#define IOAPIC  0xFEC00000   // Default physical address of IO APIC

#define REG_ID     0x00  // Register index: ID
#define REG_VER    0x01  // Register index: version
#define REG_TABLE  0x10  // Redirection table base

// The redirection table starts at REG_TABLE and uses
// two registers to configure each interrupt.  
// The first (low) register in a pair contains configuration bits.
// The second (high) register contains a bitmask telling which
// CPUs can serve that interrupt.
#define INT_DISABLED   0x00100000  // Interrupt disabled
#define INT_LEVEL      0x00008000  // Level-triggered (vs edge-)
#define INT_ACTIVELOW  0x00002000  // Active low (vs high)
#define INT_LOGICAL    0x00000800  // Destination is CPU id (vs APIC ID)

volatile struct ioapic *ioapic;

// IO APIC MMIO structure: write reg, then read or write data.
struct ioapic {
  uint reg;
  uint pad[3];
  uint data;
};

static uint
ioapic_read(int reg)
{
  ioapic->reg = reg;
  return ioapic->data;
}

static void
ioapic_write(int reg, uint data)
{
  ioapic->reg = reg;
  ioapic->data = data;
}

void
ioapic_init(void)
{
  int i, id, maxintr;

  if(!ismp)
    return;

  ioapic = (volatile struct ioapic*)IOAPIC;
  maxintr = (ioapic_read(REG_VER) >> 16) & 0xFF;
  id = ioapic_read(REG_ID) >> 24;
  if(id != ioapic_id)
    cprintf("ioapic_init: id isn't equal to ioapic_id; not a MP\n");

  // Mark all interrupts edge-triggered, active high, disabled,
  // and not routed to any CPUs.
  for(i = 0; i <= maxintr; i++){
    ioapic_write(REG_TABLE+2*i, INT_DISABLED | (IRQ_OFFSET + i));
    ioapic_write(REG_TABLE+2*i+1, 0);
  }
}

void
ioapic_enable(int irq, int cpunum)
{
  if(!ismp)
    return;

  // Mark interrupt edge-triggered, active high,
  // enabled, and routed to the given cpunum,
  // which happens to be that cpu's APIC ID.
  ioapic_write(REG_TABLE+2*irq, IRQ_OFFSET + irq);
  ioapic_write(REG_TABLE+2*irq+1, cpunum << 24);
}