d0f7acf9eb
An x86-64 operating system built from xv6, now largely rewritten: SMP, a copy-on-write root filesystem (SchFS), PCI/USB/virtio drivers, a TTF rasteriser, and a Linux-numbered syscall ABI. No history before this point -- the tree was never under version control. This commit exists so the rename that follows has something to undo to.
105 lines
4.1 KiB
C
105 lines
4.1 KiB
C
// Find out how much memory this machine actually has.
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//
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// PHYSTOP used to be a constant: 0xE000000, 224 MB, inherited from xv6 and
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// never revisited. That is not a limit on how much memory the kernel *uses* --
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// it is an assertion that exactly that much exists. Boot a machine with less
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// and kinit1() walks off the end of RAM into nothing; boot one with more and
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// every byte above 224 MB is invisible. Measured before this file existed:
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// Schunix booted at 224 MB and triple faulted at 192 MB, for no reason at all.
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//
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// The firmware has known the answer the whole time. boot/stage2.S asks it with
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// INT 0x15 / E820 in real mode -- the same interface Linux's boot stub and GRUB
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// use -- and leaves the result in low memory. stage2main.c already parses it to
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// print detected RAM on the splash. The kernel simply never read it.
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#include "types.h"
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#include "defs.h"
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#include "memlayout.h"
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#include "mmu.h"
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// Set before kinit1() and never again. Starts at the old constant so that a
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// crash between entry and e820_init() behaves the way it always did rather
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// than in some new way.
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uint64 phystop = PHYSTOP_FALLBACK;
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// Where stage2 left the map. Must match E820_COUNT_ADDR / E820_MAP_ADDR in
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// boot/stage2main.c, which are themselves relative to VBE_INFO_ADDR.
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#define E820_COUNT_ADDR (0x30000 + 0x200)
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#define E820_MAP_ADDR (0x30000 + 0x208)
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#define E820_USABLE 1
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#define E820_MAXENTRIES 32
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struct e820entry {
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uint64 base;
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uint64 length;
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uint32 type;
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uint32 acpi_ext;
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} __attribute__((packed));
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void
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e820_init(void)
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{
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uint32 count = *(uint32 *)P2V(E820_COUNT_ADDR);
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struct e820entry *e = (struct e820entry *)P2V(E820_MAP_ADDR);
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// A count of zero means the firmware declined; anything absurd means we are
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// reading something that is not a memory map. Either way the constant is the
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// only honest answer left.
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if (count == 0 || count > E820_MAXENTRIES) {
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cprintf("e820: no usable map (count %d), assuming %d MB\n",
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count, (int)(PHYSTOP_FALLBACK / (1024 * 1024)));
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return;
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}
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// The end of the usable region that *contains the kernel*, not the highest
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// usable address anywhere.
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//
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// This distinction matters and is easy to get wrong. A memory map is not one
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// run of RAM: there are reserved holes in it -- ACPI tables, firmware
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// scratch, the legacy hole below 1 MB -- and kalloc's freerange() walks
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// straight from the end of the kernel image to phystop handing out every page
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// between. Taking the global maximum would step over a reserved region and
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// put firmware tables on the free list, which fails later, elsewhere, and
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// looks like anything but a memory map problem.
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uint64 top = 0;
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for (uint32 i = 0; i < count; i++) {
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if (e[i].type != E820_USABLE)
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continue;
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uint64 base = e[i].base, end = base + e[i].length;
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if (base <= EXTMEM && EXTMEM < end) {
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top = end;
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break;
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}
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}
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if (top == 0) {
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cprintf("e820: no usable region covers 0x%x, assuming %d MB\n",
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EXTMEM, (int)(PHYSTOP_FALLBACK / (1024 * 1024)));
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return;
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}
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// kvmalloc() maps physical 0-4 GB with four 1 GB pages, and marks the top one
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// uncached because that is where PCI BARs and the framebuffer live. So 3 GB
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// is the ceiling for anything the kernel will treat as RAM: above it the
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// direct map either does not exist or is device memory that would be
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// catastrophically slow to allocate from and is not ours to hand out.
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if (top > PHYSTOP_MAX)
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top = PHYSTOP_MAX;
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top &= ~(uint64)(PGSIZE - 1);
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// Below this the kernel image, its page tables and the DMA arena do not fit,
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// and continuing produces a failure much harder to read than this line.
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if (top < PHYSTOP_MIN) {
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cprintf("e820: only %d MB usable; Schunix needs at least %d MB\n",
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(int)(top / (1024 * 1024)), (int)(PHYSTOP_MIN / (1024 * 1024)));
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// Left at the fallback deliberately rather than lowered: a machine this
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// small is going to fail, and it should fail having said why.
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return;
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}
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phystop = top;
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cprintf("e820: %d MB usable (%d entries)\n",
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(int)(phystop / (1024 * 1024)), count);
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}
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