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(bios_start, 0x100000 - bios_start);
98}