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