LXR linux/arch/x86/kernel/head.c

   1#include <linux/kernel.h>
   2#include <linux/init.h>
   3#include <linux/memblock.h>
   4
   5#include <asm/setup.h>
   6#include <asm/bios_ebda.h>
   7
   8/*
   9 * The BIOS places the EBDA/XBDA at the top of conventional
  10 * memory, and usually decreases the reported amount of
  11 * conventional memory (int 0x12) too. This also contains a
  12 * workaround for Dell systems that neglect to reserve EBDA.
  13 * The same workaround also avoids a problem with the AMD768MPX
  14 * chipset: reserve a page before VGA to prevent PCI prefetch
  15 * into it (errata #56). Usually the page is reserved anyways,
  16 * unless you have no PS/2 mouse plugged in.
  17 *
  18 * This functions is deliberately very conservative.  Losing
  19 * memory in the bottom megabyte is rarely a problem, as long
  20 * as we have enough memory to install the trampoline.  Using
  21 * memory that is in use by the BIOS or by some DMA device
  22 * the BIOS didn't shut down *is* a big problem.
  23 */
  24
  25#define BIOS_LOWMEM_KILOBYTES   0x413
  26#define LOWMEM_CAP              0x9f000U        /* Absolute maximum */
  27#define INSANE_CUTOFF           0x20000U        /* Less than this = insane */
  28
  29void __init reserve_ebda_region(void)
  30{
  31        unsigned int lowmem, ebda_addr;
  32
  33        /*
  34         * To determine the position of the EBDA and the
  35         * end of conventional memory, we need to look at
  36         * the BIOS data area. In a paravirtual environment
  37         * that area is absent. We'll just have to assume
  38         * that the paravirt case can handle memory setup
  39         * correctly, without our help.
  40         */
  41        if (paravirt_enabled())
  42                return;
  43
  44        /* end of low (conventional) memory */
  45        lowmem = *(unsigned short *)__va(BIOS_LOWMEM_KILOBYTES);
  46        lowmem <<= 10;
  47
  48        /* start of EBDA area */
  49        ebda_addr = get_bios_ebda();
  50
  51        /*
  52         * Note: some old Dells seem to need 4k EBDA without
  53         * reporting so, so just consider the memory above 0x9f000
  54         * to be off limits (bugzilla 2990).
  55         */
  56
  57        /* If the EBDA address is below 128K, assume it is bogus */
  58        if (ebda_addr < INSANE_CUTOFF)
  59                ebda_addr = LOWMEM_CAP;
  60
  61        /* If lowmem is less than 128K, assume it is bogus */
  62        if (lowmem < INSANE_CUTOFF)
  63                lowmem = LOWMEM_CAP;
  64
  65        /* Use the lower of the lowmem and EBDA markers as the cutoff */
  66        lowmem = min(lowmem, ebda_addr);
  67        lowmem = min(lowmem, LOWMEM_CAP); /* Absolute cap */
  68
  69        /* reserve all memory between lowmem and the 1MB mark */
  70        memblock_reserve(lowmem, 0x100000 - lowmem);
  71}
  72