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 } 99