arch/x86/kernel/ebda.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  * This function reserves all conventional PC system BIOS related
  10  * firmware memory areas (some of which are data, some of which
  11  * are code), that must not be used by the kernel as available
  12  * RAM.
  13  *
  14  * The BIOS places the EBDA/XBDA at the top of conventional
  15  * memory, and usually decreases the reported amount of
  16  * conventional memory (int 0x12) too.
  17  *
  18  * This means that as a first approximation on most systems we can
  19  * guess the reserved BIOS area by looking at the low BIOS RAM size
  20  * value and assume that everything above that value (up to 1MB) is
  21  * reserved.
  22  *
  23  * But life in firmware country is not that simple:
  24  *
  25  * - This code also contains a quirk for Dell systems that neglect
  26  *   to reserve the EBDA area in the 'RAM size' value ...
  27  *
  28  * - The same quirk also avoids a problem with the AMD768MPX
  29  *   chipset: reserve a page before VGA to prevent PCI prefetch
  30  *   into it (errata #56). (Usually the page is reserved anyways,
  31  *   unless you have no PS/2 mouse plugged in.)
  32  *
  33  * - Plus paravirt systems don't have a reliable value in the
  34  *   'BIOS RAM size' pointer we can rely on, so we must quirk
  35  *   them too.
  36  *
  37  * Due to those various problems this function is deliberately
  38  * very conservative and tries to err on the side of reserving
  39  * too much, to not risk reserving too little.
  40  *
  41  * Losing a small amount of memory in the bottom megabyte is
  42  * rarely a problem, as long as we have enough memory to install
  43  * the SMP bootup trampoline which *must* be in this area.
  44  *
  45  * Using memory that is in use by the BIOS or by some DMA device
  46  * the BIOS didn't shut down *is* a big problem to the kernel,
  47  * obviously.
  48  */
  49
  50 #define BIOS_RAM_SIZE_KB_PTR    0x413
  51
  52 #define BIOS_START_MIN          0x20000U        /* 128K, less than this is insane */
  53 #define BIOS_START_MAX          0x9f000U        /* 640K, absolute maximum */
  54
  55 void __init reserve_bios_regions(void)
  56 {
  57         unsigned int bios_start, ebda_start;
  58
  59         /*
  60          * NOTE: In a paravirtual environment the BIOS reserved
  61          * area is absent. We'll just have to assume that the
  62          * paravirt case can handle memory setup correctly,
  63          * without our help.
  64          */
  65         if (!x86_platform.legacy.reserve_bios_regions)
  66                 return;
  67
  68         /*
  69          * BIOS RAM size is encoded in kilobytes, convert it
  70          * to bytes to get a first guess at where the BIOS
  71          * firmware area starts:
  72          */
  73         bios_start = *(unsigned short *)__va(BIOS_RAM_SIZE_KB_PTR);
  74         bios_start <<= 10;
  75
  76         /*
  77          * If bios_start is less than 128K, assume it is bogus
  78          * and bump it up to 640K.  Similarly, if bios_start is above 640K,
  79          * don't trust it.
  80          */
  81         if (bios_start < BIOS_START_MIN || bios_start > BIOS_START_MAX)
  82                 bios_start = BIOS_START_MAX;
  83
  84         /* Get the start address of the EBDA page: */
  85         ebda_start = get_bios_ebda();
  86
  87         /*
  88          * If the EBDA start address is sane and is below the BIOS region,
  89          * then also reserve everything from the EBDA start address up to
  90          * the BIOS region.
  91          */
  92         if (ebda_start >= BIOS_START_MIN && ebda_start < bios_start)
  93                 bios_start = ebda_start;
  94
  95         /* Reserve all memory between bios_start and the 1MB mark: */
  96         memblock_reserve(bios_start, 0x100000 - bios_start);
  97 }