arch/x86/kernel/ebda.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 #include <linux/kernel.h>
   3 #include <linux/init.h>
   4 #include <linux/memblock.h>
   5
   6 #include <asm/setup.h>
   7 #include <asm/bios_ebda.h>
   8
   9 /*
  10  * This function reserves all conventional PC system BIOS related
  11  * firmware memory areas (some of which are data, some of which
  12  * are code), that must not be used by the kernel as available
  13  * RAM.
  14  *
  15  * The BIOS places the EBDA/XBDA at the top of conventional
  16  * memory, and usually decreases the reported amount of
  17  * conventional memory (int 0x12) too.
  18  *
  19  * This means that as a first approximation on most systems we can
  20  * guess the reserved BIOS area by looking at the low BIOS RAM size
  21  * value and assume that everything above that value (up to 1MB) is
  22  * reserved.
  23  *
  24  * But life in firmware country is not that simple:
  25  *
  26  * - This code also contains a quirk for Dell systems that neglect
  27  *   to reserve the EBDA area in the 'RAM size' value ...
  28  *
  29  * - The same quirk also avoids a problem with the AMD768MPX
  30  *   chipset: reserve a page before VGA to prevent PCI prefetch
  31  *   into it (errata #56). (Usually the page is reserved anyways,
  32  *   unless you have no PS/2 mouse plugged in.)
  33  *
  34  * - Plus paravirt systems don't have a reliable value in the
  35  *   'BIOS RAM size' pointer we can rely on, so we must quirk
  36  *   them too.
  37  *
  38  * Due to those various problems this function is deliberately
  39  * very conservative and tries to err on the side of reserving
  40  * too much, to not risk reserving too little.
  41  *
  42  * Losing a small amount of memory in the bottom megabyte is
  43  * rarely a problem, as long as we have enough memory to install
  44  * the SMP bootup trampoline which *must* be in this area.
  45  *
  46  * Using memory that is in use by the BIOS or by some DMA device
  47  * the BIOS didn't shut down *is* a big problem to the kernel,
  48  * obviously.
  49  */
  50
  51 #define BIOS_RAM_SIZE_KB_PTR    0x413
  52
  53 #define BIOS_START_MIN          0x20000U        /* 128K, less than this is insane */
  54 #define BIOS_START_MAX          0x9f000U        /* 640K, absolute maximum */
  55
  56 void __init reserve_bios_regions(void)
  57 {
  58         unsigned int bios_start, ebda_start;
  59
  60         /*
  61          * NOTE: In a paravirtual environment the BIOS reserved
  62          * area is absent. We'll just have to assume that the
  63          * paravirt case can handle memory setup correctly,
  64          * without our help.
  65          */
  66         if (!x86_platform.legacy.reserve_bios_regions)
  67                 return;
  68
  69         /*
  70          * BIOS RAM size is encoded in kilobytes, convert it
  71          * to bytes to get a first guess at where the BIOS
  72          * firmware area starts:
  73          */
  74         bios_start = *(unsigned short *)__va(BIOS_RAM_SIZE_KB_PTR);
  75         bios_start <<= 10;
  76
  77         /*
  78          * If bios_start is less than 128K, assume it is bogus
  79          * and bump it up to 640K.  Similarly, if bios_start is above 640K,
  80          * don't trust it.
  81          */
  82         if (bios_start < BIOS_START_MIN || bios_start > BIOS_START_MAX)
  83                 bios_start = BIOS_START_MAX;
  84
  85         /* Get the start address of the EBDA page: */
  86         ebda_start = get_bios_ebda();
  87
  88         /*
  89          * If the EBDA start address is sane and is below the BIOS region,
  90          * then also reserve everything from the EBDA start address up to
  91          * the BIOS region.
  92          */
  93         if (ebda_start >= BIOS_START_MIN && ebda_start < bios_start)
  94                 bios_start = ebda_start;
  95
  96         /* Reserve all memory between bios_start and the 1MB mark: */
  97         memblock_reserve(bios_start, 0x100000 - bios_start);
  98 }