SchOS/kernel/arch/x86_64/e820.c

// Find out how much memory this machine actually has.
//
// PHYSTOP used to be a constant: 0xE000000, 224 MB, inherited from xv6 and
// never revisited. That is not a limit on how much memory the kernel *uses* --
// it is an assertion that exactly that much exists. Boot a machine with less
// and kinit1() walks off the end of RAM into nothing; boot one with more and
// every byte above 224 MB is invisible. Measured before this file existed:
// SchOS booted at 224 MB and triple faulted at 192 MB, for no reason at all.
//
// The firmware has known the answer the whole time. boot/bios/stage2.S asks it with
// INT 0x15 / E820 in real mode -- the same interface Linux's boot stub and GRUB
// use -- and leaves the result in low memory. stage2main.c already parses it to
// print detected RAM on the splash. The kernel simply never read it.

#include "types.h"
#include "defs.h"
#include "memlayout.h"
#include "mmu.h"
#include "dma.h"   // DMA_ARENA_BYTES: the top of PHYSTOP must be able to hold it

// Set before kinit1() and never again. Starts at the old constant so that a
// crash between entry and e820_init() behaves the way it always did rather
// than in some new way.
uint64 phystop = PHYSTOP_FALLBACK;

// Where stage2 left the map. Must match E820_COUNT_ADDR / E820_MAP_ADDR in
// boot/bios/stage2main.c, which are themselves relative to VBE_INFO_ADDR.
#define E820_COUNT_ADDR (0x30000 + 0x200)
#define E820_MAP_ADDR   (0x30000 + 0x208)

#define E820_USABLE     1
#define E820_MAXENTRIES MEMREGION_MAX

// Every usable region, page aligned inwards and clipped to PHYSTOP_MAX.
// kinit1() frees these; see include/memlayout.h for why a list and not just a
// top address.
struct memregion mem_usable[MEMREGION_MAX];
int nmem_usable;

// The fallback path must still hand kalloc something, or the kernel comes up
// with an empty free list and dies somewhere far less obvious than here. One
// region spanning what the old constant asserted reproduces the previous
// behaviour exactly.
static void
assume_fallback(void)
{
  mem_usable[0].base = EXTMEM;
  mem_usable[0].end  = PHYSTOP_FALLBACK;
  nmem_usable = 1;
}

struct e820entry {
  uint64 base;
  uint64 length;
  uint32 type;
  uint32 acpi_ext;
} __attribute__((packed));

void
e820_init(void)
{
  uint32 count = *(uint32 *)P2V(E820_COUNT_ADDR);
  struct e820entry *e = (struct e820entry *)P2V(E820_MAP_ADDR);

  // A count of zero means the firmware declined; anything absurd means we are
  // reading something that is not a memory map. Either way the constant is the
  // only honest answer left.
  if (count == 0 || count > E820_MAXENTRIES) {
    cprintf("e820: no usable map (count %d), assuming %d MB\n",
            count, (int)(PHYSTOP_FALLBACK / (1024 * 1024)));
    assume_fallback();
    return;
  }

  // Collect every usable region rather than just the one containing the
  // kernel.
  //
  // A memory map is not one run of RAM: there are reserved holes in it -- ACPI
  // tables, firmware scratch, the legacy hole below 1 MB. The old code took the
  // run containing the kernel and stopped, because kalloc walked a single range
  // and taking the global maximum would have stepped over a reserved region and
  // put firmware tables on the free list. That was the right answer for the
  // wrong shape: under OVMF the kernel's run ends at the ACPI NVS region at
  // 8 MB, and every byte above it was lost. kalloc now frees region by region,
  // so the holes are skipped instead of being either freed or fatal.
  uint64 largest = 0;
  uint64 top = 0;
  nmem_usable = 0;
  for (uint32 i = 0; i < count; i++) {
    if (e[i].type != E820_USABLE)
      continue;

    // Align inwards: a region is only usable where a whole page fits.
    uint64 base = (e[i].base + PGSIZE - 1) & ~(uint64)(PGSIZE - 1);
    uint64 end  = (e[i].base + e[i].length) & ~(uint64)(PGSIZE - 1);

    // Above PHYSTOP_MAX the direct map is device memory, not RAM (see below).
    if (end > PHYSTOP_MAX)
      end = PHYSTOP_MAX;
    if (base >= end)
      continue;

    mem_usable[nmem_usable].base = base;
    mem_usable[nmem_usable].end  = end;
    nmem_usable++;

    // phystop comes from the largest region, not the highest. It has to be a
    // region that can hold the DMA arena, which is carved off the top of
    // PHYSTOP and must be physically contiguous; the largest is the only one
    // that is certain to have room. A small usable region above it is left
    // unfreed rather than raising the bound past a hole -- kfree() rejects
    // anything at or above PHYSTOP, and that check is worth more than the
    // stray page it costs.
    if (end - base > largest) {
      largest = end - base;
      top = end;
    }
  }

  if (nmem_usable == 0) {
    cprintf("e820: no usable regions in %d entries, assuming %d MB\n",
            count, (int)(PHYSTOP_FALLBACK / (1024 * 1024)));
    assume_fallback();
    return;
  }

  // The PHYSTOP_MAX ceiling is applied per region above, when each one is
  // clipped. kvmalloc() maps physical 0-4 GB with four 1 GB pages and marks the
  // top one uncached because that is where PCI BARs and the framebuffer live,
  // so 3 GB is the ceiling for anything the kernel will treat as RAM: above it
  // the direct map either does not exist or is device memory that would be
  // catastrophically slow to allocate from and is not ours to hand out.

  // Below this the kernel image, its page tables and the DMA arena do not fit,
  // and continuing produces a failure much harder to read than this line.
  if (top < PHYSTOP_MIN) {
    cprintf("e820: only %d MB usable; SchOS needs at least %d MB\n",
            (int)(top / (1024 * 1024)), (int)(PHYSTOP_MIN / (1024 * 1024)));
    // Left at the fallback deliberately rather than lowered: a machine this
    // small is going to fail, and it should fail having said why.
    assume_fallback();
    return;
  }

  // dma_init() carves [PHYSTOP - DMA_ARENA_BYTES, PHYSTOP) and needs it
  // physically contiguous, so the region that set `top` must be able to hold
  // it. Picking the largest region makes this true on any machine worth
  // booting; checking says so out loud if it ever is not.
  if (largest < DMA_ARENA_BYTES) {
    cprintf("e820: largest usable region is %d MB, too small for the %d MB "
            "DMA arena\n", (int)(largest / (1024 * 1024)),
            (int)(DMA_ARENA_BYTES / (1024 * 1024)));
    assume_fallback();
    return;
  }

  phystop = top;

  // Report what is actually usable, not phystop: with a fragmented map those
  // are different numbers, and printing the top address as though it were the
  // amount of RAM is how the 8 MB under OVMF went unnoticed for so long.
  uint64 usable = 0;
  for (int i = 0; i < nmem_usable; i++)
    usable += mem_usable[i].end - mem_usable[i].base;

  cprintf("e820: %d MB usable in %d region%s, top %d MB (%d entries)\n",
          (int)(usable / (1024 * 1024)), nmem_usable,
          nmem_usable == 1 ? "" : "s",
          (int)(phystop / (1024 * 1024)), count);
}