Line data    Source code

       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           1 : void __init reserve_bios_regions(void)
      57             : {
      58           1 :         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           1 :         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           1 :         bios_start = *(unsigned short *)__va(BIOS_RAM_SIZE_KB_PTR);
      75           1 :         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           1 :         if (bios_start < BIOS_START_MIN || bios_start > BIOS_START_MAX)
      83           1 :                 bios_start = BIOS_START_MAX;
      84             : 
      85             :         /* Get the start address of the EBDA page: */
      86           1 :         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           1 :         if (ebda_start >= BIOS_START_MIN && ebda_start < bios_start)
      94           0 :                 bios_start = ebda_start;
      95             : 
      96             :         /* Reserve all memory between bios_start and the 1MB mark: */
      97           1 :         memblock_reserve(bios_start, 0x100000 - bios_start);
      98             : }