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
  29 void __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 }