1 #include <linux/kernel.h>
2 #include <linux/init.h>
3 #include <linux/memblock.h>
4
5 #include <asm/setup.h>
6 #include <asm/bios_ebda.h>
7
8 /*
9 * The BIOS places the EBDA/XBDA at the top of conventional
10 * memory, and usually decreases the reported amount of
11 * conventional memory (int 0x12) too. This also contains a
12 * workaround for Dell systems that neglect to reserve EBDA.
13 * The same workaround also avoids a problem with the AMD768MPX
14 * chipset: reserve a page before VGA to prevent PCI prefetch
15 * into it (errata #56). Usually the page is reserved anyways,
16 * unless you have no PS/2 mouse plugged in.
17 *
18 * This functions is deliberately very conservative. Losing
19 * memory in the bottom megabyte is rarely a problem, as long
20 * as we have enough memory to install the trampoline. Using
21 * memory that is in use by the BIOS or by some DMA device
22 * the BIOS didn't shut down *is* a big problem.
23 */
24
25 #define BIOS_LOWMEM_KILOBYTES 0x413
26 #define LOWMEM_CAP 0x9f000U /* Absolute maximum */
27 #define INSANE_CUTOFF 0x20000U /* Less than this = insane */
28
reserve_ebda_region(void)29 void __init reserve_ebda_region(void)
30 {
31 unsigned int lowmem, ebda_addr;
32
33 /*
34 * To determine the position of the EBDA and the
35 * end of conventional memory, we need to look at
36 * the BIOS data area. In a paravirtual environment
37 * that area is absent. We'll just have to assume
38 * that the paravirt case can handle memory setup
39 * correctly, without our help.
40 */
41 if (paravirt_enabled())
42 return;
43
44 /* end of low (conventional) memory */
45 lowmem = *(unsigned short *)__va(BIOS_LOWMEM_KILOBYTES);
46 lowmem <<= 10;
47
48 /* start of EBDA area */
49 ebda_addr = get_bios_ebda();
50
51 /*
52 * Note: some old Dells seem to need 4k EBDA without
53 * reporting so, so just consider the memory above 0x9f000
54 * to be off limits (bugzilla 2990).
55 */
56
57 /* If the EBDA address is below 128K, assume it is bogus */
58 if (ebda_addr < INSANE_CUTOFF)
59 ebda_addr = LOWMEM_CAP;
60
61 /* If lowmem is less than 128K, assume it is bogus */
62 if (lowmem < INSANE_CUTOFF)
63 lowmem = LOWMEM_CAP;
64
65 /* Use the lower of the lowmem and EBDA markers as the cutoff */
66 lowmem = min(lowmem, ebda_addr);
67 lowmem = min(lowmem, LOWMEM_CAP); /* Absolute cap */
68
69 /* reserve all memory between lowmem and the 1MB mark */
70 memblock_reserve(lowmem, 0x100000 - lowmem);
71 }
72