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(base: bios_start, size: 0x100000 - bios_start); |
98 | } |
99 | |