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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 : 11 : void __init reserve_bios_regions(void) 57 : : { 58 : 11 : 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 [ + - ]: 11 : 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 : 11 : bios_start = *(unsigned short *)__va(BIOS_RAM_SIZE_KB_PTR); 75 : 11 : 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 [ + - ]: 11 : if (bios_start < BIOS_START_MIN || bios_start > BIOS_START_MAX) 83 : 11 : bios_start = BIOS_START_MAX; 84 : : 85 : : /* Get the start address of the EBDA page: */ 86 [ - + ]: 11 : 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 [ - + ]: 11 : 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 : 11 : memblock_reserve(bios_start, 0x100000 - bios_start); 98 : : }