SchOS/kernel/arch/x86_64/mp.c

// Multiprocessor support
// Search memory for MP description structures.
// http://developer.intel.com/design/pentium/datashts/24201606.pdf

#include "types.h"
#include "defs.h"
#include "param.h"
#include "memlayout.h"
#include "mp.h"
#include "x86.h"
#include "mmu.h"
#include "proc.h"

struct cpu cpus[NCPU];
// Processors the firmware says exist.
int ncpu;

// Processors actually running, which is not the same number and must not be
// confused with it. Enumeration is cheap and startup is where the difficulty
// is: right now the APs are found and deliberately not started, so this stays
// at 1 while ncpu is 2, 4 or 16.
//
// /proc/cpuinfo reports *this* one, matching Linux, where cpuinfo lists online
// CPUs. Reporting ncpu there told a VM with four processors that it had four
// working ones, which was a comfortable thing to read and false.
int ncpu_online = 1;
uchar ioapicid;
// Where the I/O APIC's registers are. Every source that describes it -- the
// MADT, or this table -- says so, and ioapicinit() maps whatever they said.
uint64 ioapic_phys = IOAPIC_DEFAULT_PHYS;

static uchar
sum(uchar *addr, int len)
{
  int i, sum;

  sum = 0;
  for(i=0; i<len; i++)
    sum += addr[i];
  return sum;
}

// Look for an MP structure in the len bytes at addr.
static struct mp*
mpsearch1(addr_t a, int len)
{
  uchar *e, *p, *addr;
  addr = P2V(a);
  e = addr+len;
  for(p = addr; p < e; p += sizeof(struct mp))
    if(memcmp(p, "_MP_", 4) == 0 && sum(p, sizeof(struct mp)) == 0)
      return (struct mp*)p;
  return 0;
}

// Search for the MP Floating Pointer Structure, which according to the
// spec is in one of the following three locations:
// 1) in the first KB of the EBDA;
// 2) in the last KB of system base memory;
// 3) in the BIOS ROM between 0xE0000 and 0xFFFFF.
static struct mp*
mpsearch(void)
{
  uchar *bda;
  uint p;
  struct mp *mp;

  bda = (uchar *) P2V(0x400);
  if((p = ((bda[0x0F]<<8)| bda[0x0E]) << 4)){
    if((mp = mpsearch1(p, 1024)))
      return mp;
  } else {
    p = ((bda[0x14]<<8)|bda[0x13])*1024;
    if((mp = mpsearch1(p-1024, 1024)))
      return mp;
  }
  return mpsearch1(0xF0000, 0x10000);
}

// Search for an MP configuration table.  For now,
// don't accept the default configurations (physaddr == 0).
// Check for correct signature, calculate the checksum and,
// if correct, check the version.
// To do: check extended table checksum.
static struct mpconf*
mpconfig(struct mp **pmp)
{
  struct mpconf *conf;
  struct mp *mp;

  if((mp = mpsearch()) == 0 || mp->physaddr == 0)
    return 0;
  conf = (struct mpconf*) P2V((addr_t) mp->physaddr);
  if(memcmp(conf, "PCMP", 4) != 0)
    return 0;
  if(conf->version != 1 && conf->version != 4)
    return 0;
  if(sum((uchar*)conf, conf->length) != 0)
    return 0;
  *pmp = mp;
  return conf;
}

void
mpinit(void)
{
  uchar *p, *e;
  struct mp *mp;
  struct mpconf *conf;
  struct mpproc *mproc;
  struct mpioapic *ioapic;

  if((conf = mpconfig(&mp)) == 0) {
    cprintf("No other CPUs found.\n");
    return;
  }
  lapic = (uint*)P2V(conf->lapicaddr_p);
  for(p=(uchar*)(conf+1), e=(uchar*)conf+conf->length; p<e; ){
    switch(*p){
    case MPPROC:
      mproc = (struct mpproc*)p;
      // Bit 0 is "usable". A disabled entry is a processor that is not
      // there, and starting it means waiting for an answer that never comes.
      if((mproc->flags & 0x01) && ncpu < NCPU) {
        cpus[ncpu].apicid = mproc->apicid;  // apicid may differ from ncpu
        ncpu++;
      }
      p += sizeof(struct mpproc);
      continue;
    case MPIOAPIC:
      ioapic = (struct mpioapic*)p;
      if(ioapic_phys == IOAPIC_DEFAULT_PHYS || ioapic->addr_p == IOAPIC_DEFAULT_PHYS){
        ioapicid = ioapic->apicno;
        ioapic_phys = ioapic->addr_p;
      }
      p += sizeof(struct mpioapic);
      continue;
    case MPBUS:
    case MPIOINTR:
    case MPLINTR:
      p += 8;
      continue;
    default:
      panic("Major problem parsing mp config.");
      break;
    }
  }
  // "Seems we are" is doing a lot of work here, and the number is usually
  // wrong. This walks the *legacy Intel MP table* (1994), which modern firmware
  // fills in minimally or not at all -- QEMU/SeaBIOS emits exactly one
  // processor entry regardless of -smp, because it expects the OS to enumerate
  // CPUs from the ACPI MADT instead. Verified 2026-08-08: with -smp 4 the table
  // is 208 bytes and contains one type-0 entry, then buses, an I/O APIC and
  // interrupt assignments. So ncpu is 1 on QEMU no matter what, and SMP is
  // silently never exercised there.
  //
  // See docs/architecture/boot.md -- this is half of the SMP bug, and the
  // reason the other half (AP startup triple-faulting) cannot be reproduced
  // under QEMU until the MADT is parsed.
  cprintf("Seems we are SMP, ncpu = %d\n",ncpu);
  if(mp->imcrp){
    // Bochs doesn't support IMCR, so this doesn't run on Bochs.
    // But it would on real hardware.
    outb(0x22, 0x70);   // Select IMCR
    outb(0x23, inb(0x23) | 1);  // Mask external interrupts.
  }
}