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/segment.h>
15
16 /*
17 * Invoke the realmode switch hook if present; otherwise
18 * disable all interrupts.
19 */
20 static void realmode_switch_hook(void)
21 {
22 if (boot_params.hdr.realmode_swtch) {
23 asm volatile("lcallw *%0"
24 : : "m" (boot_params.hdr.realmode_swtch)
25 : "eax", "ebx", "ecx", "edx");
26 } else {
27 asm volatile("cli");
28 outb(0x80, 0x70); /* Disable NMI */
29 io_delay();
30 }
31 }
32
33 /*
34 * Disable all interrupts at the legacy PIC.
35 */
36 static void mask_all_interrupts(void)
37 {
38 outb(0xff, 0xa1); /* Mask all interrupts on the secondary PIC */
39 io_delay();
40 outb(0xfb, 0x21); /* Mask all but cascade on the primary PIC */
41 io_delay();
42 }
43
44 /*
45 * Reset IGNNE# if asserted in the FPU.
46 */
47 static void reset_coprocessor(void)
48 {
49 outb(0, 0xf0);
50 io_delay();
51 outb(0, 0xf1);
52 io_delay();
53 }
54
55 /*
56 * Set up the GDT
57 */
58
59 struct gdt_ptr {
60 u16 len;
61 u32 ptr;
62 } __attribute__((packed));
63
64 static void setup_gdt(void)
65 {
66 /* There are machines which are known to not boot with the GDT
67 being 8-byte unaligned. Intel recommends 16 byte alignment. */
68 static const u64 boot_gdt[] __attribute__((aligned(16))) = {
69 /* CS: code, read/execute, 4 GB, base 0 */
70 [GDT_ENTRY_BOOT_CS] = GDT_ENTRY(0xc09b, 0, 0xfffff),
71 /* DS: data, read/write, 4 GB, base 0 */
72 [GDT_ENTRY_BOOT_DS] = GDT_ENTRY(0xc093, 0, 0xfffff),
73 /* TSS: 32-bit tss, 104 bytes, base 4096 */
74 /* We only have a TSS here to keep Intel VT happy;
75 we don't actually use it for anything. */
76 [GDT_ENTRY_BOOT_TSS] = GDT_ENTRY(0x0089, 4096, 103),
77 };
78 /* Xen HVM incorrectly stores a pointer to the gdt_ptr, instead
79 of the gdt_ptr contents. Thus, make it static so it will
80 stay in memory, at least long enough that we switch to the
81 proper kernel GDT. */
82 static struct gdt_ptr gdt;
83
84 gdt.len = sizeof(boot_gdt)-1;
85 gdt.ptr = (u32)&boot_gdt + (ds() << 4);
86
87 asm volatile("lgdtl %0" : : "m" (gdt));
88 }
89
90 /*
91 * Set up the IDT
92 */
93 static void setup_idt(void)
94 {
95 static const struct gdt_ptr null_idt = {0, 0};
96 asm volatile("lidtl %0" : : "m" (null_idt));
97 }
98
99 /*
100 * Actual invocation sequence
101 */
102 void go_to_protected_mode(void)
103 {
104 /* Hook before leaving real mode, also disables interrupts */
105 realmode_switch_hook();
106
107 /* Enable the A20 gate */
108 if (enable_a20()) {
109 puts("A20 gate not responding, unable to boot...\n");
110 die();
111 }
112
113 /* Reset coprocessor (IGNNE#) */
114 reset_coprocessor();
115
116 /* Mask all interrupts in the PIC */
117 mask_all_interrupts();
118
119 /* Actual transition to protected mode... */
120 setup_idt();
121 setup_gdt();
122 protected_mode_jump(boot_params.hdr.code32_start,
123 (u32)&boot_params + (ds() << 4));
124 }