1 // SPDX-License-Identifier: GPL-2.0-only
2 /* -*- linux-c -*- ------------------------------------------------------- *
3 *
4 * Copyright (C) 1991, 1992 Linus Torvalds
5 * Copyright 2007 rPath, Inc. - All Rights Reserved
6 *
7 * ----------------------------------------------------------------------- */
8
9 /*
10 * Prepare the machine for transition to protected mode.
11 */
12
13 #include "boot.h"
14 #include <asm/desc_defs.h>
15 #include <asm/segment.h>
16
17 /*
18 * Invoke the realmode switch hook if present; otherwise
19 * disable all interrupts.
20 */
21 static void realmode_switch_hook(void)
22 {
23 if (boot_params.hdr.realmode_swtch) {
24 asm volatile("lcallw *%0"
25 : : "m" (boot_params.hdr.realmode_swtch)
26 : "eax", "ebx", "ecx", "edx");
27 } else {
28 asm volatile("cli");
29 outb(0x80, 0x70); /* Disable NMI */
30 io_delay();
31 }
32 }
33
34 /*
35 * Disable all interrupts at the legacy PIC.
36 */
37 static void mask_all_interrupts(void)
38 {
39 outb(0xff, 0xa1); /* Mask all interrupts on the secondary PIC */
40 io_delay();
41 outb(0xfb, 0x21); /* Mask all but cascade on the primary PIC */
42 io_delay();
43 }
44
45 /*
46 * Reset IGNNE# if asserted in the FPU.
47 */
48 static void reset_coprocessor(void)
49 {
50 outb(0, 0xf0);
51 io_delay();
52 outb(0, 0xf1);
53 io_delay();
54 }
55
56 /*
57 * Set up the GDT
58 */
59
60 struct gdt_ptr {
61 u16 len;
62 u32 ptr;
63 } __attribute__((packed));
64
65 static void setup_gdt(void)
66 {
67 /* There are machines which are known to not boot with the GDT
68 being 8-byte unaligned. Intel recommends 16 byte alignment. */
69 static const u64 boot_gdt[] __attribute__((aligned(16))) = {
70 /* CS: code, read/execute, 4 GB, base 0 */
71 [GDT_ENTRY_BOOT_CS] = GDT_ENTRY(DESC_CODE32, 0, 0xfffff),
72 /* DS: data, read/write, 4 GB, base 0 */
73 [GDT_ENTRY_BOOT_DS] = GDT_ENTRY(DESC_DATA32, 0, 0xfffff),
74 /* TSS: 32-bit tss, 104 bytes, base 4096 */
75 /* We only have a TSS here to keep Intel VT happy;
76 we don't actually use it for anything. */
77 [GDT_ENTRY_BOOT_TSS] = GDT_ENTRY(DESC_TSS32, 4096, 103),
78 };
79 /* Xen HVM incorrectly stores a pointer to the gdt_ptr, instead
80 of the gdt_ptr contents. Thus, make it static so it will
81 stay in memory, at least long enough that we switch to the
82 proper kernel GDT. */
83 static struct gdt_ptr gdt;
84
85 gdt.len = sizeof(boot_gdt)-1;
86 gdt.ptr = (u32)&boot_gdt + (ds() << 4);
87
88 asm volatile("lgdtl %0" : : "m" (gdt));
89 }
90
91 /*
92 * Set up the IDT
93 */
94 static void setup_idt(void)
95 {
96 static const struct gdt_ptr null_idt = {0, 0};
97 asm volatile("lidtl %0" : : "m" (null_idt));
98 }
99
100 /*
101 * Actual invocation sequence
102 */
103 void go_to_protected_mode(void)
104 {
105 /* Hook before leaving real mode, also disables interrupts */
106 realmode_switch_hook();
107
108 /* Enable the A20 gate */
109 if (enable_a20()) {
110 puts("A20 gate not responding, unable to boot...\n");
111 die();
112 }
113
114 /* Reset coprocessor (IGNNE#) */
115 reset_coprocessor();
116
117 /* Mask all interrupts in the PIC */
118 mask_all_interrupts();
119
120 /* Actual transition to protected mode... */
121 setup_idt();
122 setup_gdt();
123 protected_mode_jump(boot_params.hdr.code32_start,
124 (u32)&boot_params + (ds() << 4));
125 }
126
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