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 */
21static 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 */
37static 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 */
48static 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
60struct gdt_ptr {
61	u16 len;
62	u32 ptr;
63} __attribute__((packed));
64
65static 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 */
94static 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 */
103void 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