LXR linux/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
  56void __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}
  99