realprot.S revision 1.13
1 /* $NetBSD: realprot.S,v 1.13 2025/03/05 22:21:11 andvar Exp $ */ 2 3 /*- 4 * Copyright (c) 2003 The NetBSD Foundation, Inc. 5 * All rights reserved. 6 * 7 * This code is derived from software contributed to The NetBSD Foundation 8 * by David Laight. 9 * 10 * Redistribution and use in source and binary forms, with or without 11 * modification, are permitted provided that the following conditions 12 * are met: 13 * 1. Redistributions of source code must retain the above copyright 14 * notice, this list of conditions and the following disclaimer. 15 * 2. Redistributions in binary form must reproduce the above copyright 16 * notice, this list of conditions and the following disclaimer in the 17 * documentation and/or other materials provided with the distribution. 18 * 19 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 20 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 21 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 22 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 23 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 24 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 25 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 26 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 27 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 28 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 29 * POSSIBILITY OF SUCH DAMAGE. 30 */ 31 32 /* 33 * Loosely based on code from stand/lib/libcrt/bootsect/start_bootsect.S 34 */ 35 36 #include <machine/asm.h> 37 #include <x86/specialreg.h> 38 39 .text 40 .align 16 41 gdt: 42 .word 0, 0 43 .byte 0, 0x00, 0x00, 0 44 45 /* kernel code segment */ 46 .globl flatcodeseg 47 flatcodeseg = . - gdt 48 .word 0xffff, 0 49 .byte 0, 0x9f, 0xcf, 0 50 51 /* kernel data segment */ 52 .globl flatdataseg 53 flatdataseg = . - gdt 54 .word 0xffff, 0 55 .byte 0, 0x93, 0xcf, 0 56 57 /* boot code segment, base will be patched */ 58 bootcodeseg = . - gdt 59 .word 0xffff, 0 60 .byte 0, 0x9e, 0x4f, 0 61 62 /* boot data segment, base will be patched */ 63 bootdataseg = . - gdt 64 .word 0xffff, 0 65 .byte 0, 0x92, 0xcf, 0 66 67 /* 16 bit real mode, base will be patched */ 68 bootrealseg = . - gdt 69 .word 0xffff, 0 70 .byte 0, 0x9e, 0x00, 0 71 72 /* limits (etc) for data segment in real mode */ 73 bootrealdata = . - gdt 74 .word 0xffff, 0 75 .byte 0, 0x92, 0x00, 0 76 gdtlen = . - gdt 77 78 .align 16 79 gdtarg: 80 .word gdtlen-1 /* limit */ 81 .long 0 /* physical addr, will be inserted */ 82 83 toreal: .word xreal /* off:seg address for indirect jump */ 84 ourseg: .word 0 /* real mode code and data segment */ 85 86 stkseg: .word 0 /* real mode stack segment */ 87 stkdif: .long 0 /* diff. between real and prot sp */ 88 89 .global gdt_fixup 90 gdt_fixup: 91 .code16 92 pushl %eax 93 pushl %edx 94 95 xorl %eax, %eax 96 mov %cs, %ax 97 mov %ax, ourseg 98 /* sort out stuff for %ss != %ds */ 99 xorl %edx, %edx 100 movw %ss, %dx 101 movw %dx, stkseg 102 subl %eax, %edx 103 shll $4, %edx 104 movl %edx, stkdif 105 106 /* fix up GDT entries for bootstrap */ 107 mov %ax, %dx 108 shll $4, %eax 109 shr $12, %dx 110 111 #define FIXUP(gdt_index) \ 112 movw %ax, gdt+gdt_index+2; \ 113 movb %dl, gdt+gdt_index+4 114 115 FIXUP(bootcodeseg) 116 FIXUP(bootrealseg) 117 FIXUP(bootdataseg) 118 119 /* fix up GDT pointer */ 120 addl $gdt, %eax 121 movl %eax, gdtarg+2 122 123 popl %edx 124 popl %eax 125 ret 126 127 /* 128 * real_to_prot() 129 * 130 * Switch CPU to 32bit protected mode to execute C. 131 * 132 * NB: Call with the 32bit calll instruction so that a 32 bit 133 * return address is pushed. 134 * 135 * All registers are preserved, %ss:%esp will point to the same 136 * place as %ss:%sp did, although the actual value of %esp might 137 * be changed. 138 * 139 * Interrupts are disabled while we are in 32bit mode to save us 140 * having to setup a different IDT. This code is only used during 141 * the boot process and it doesn't use any interrupts. 142 */ 143 ENTRY(real_to_prot) 144 .code16 145 pushl %eax 146 cli 147 148 lgdt %cs:gdtarg /* Global descriptor table */ 149 150 movl %cr0, %eax 151 or $CR0_PE, %ax 152 movl %eax, %cr0 /* Enter 'protected mode' */ 153 154 ljmp $bootcodeseg, $1f /* Jump into a 32bit segment */ 155 1: 156 157 .code32 158 /* Set all the segment registers to map the same area as the code */ 159 mov $bootdataseg, %eax 160 mov %ax, %ds 161 mov %ax, %es 162 mov %ax, %ss 163 addl stkdif, %esp /* Allow for real %ss != %ds */ 164 165 popl %eax 166 ret 167 168 /* 169 * prot_to_real() 170 * 171 * Switch CPU back to 16bit real mode in order to call system bios functions. 172 * 173 * All registers are preserved, except that %sp may be changed so that 174 * %ss:%sp points to the same memory. 175 * Note that %ebp is preserved and will not reference the correct part 176 * of the stack. 177 * 178 * Interrupts are enabled while in real mode. 179 * 180 * Based on the description in section 14.5 of the 80386 Programmer's 181 * reference book. 182 */ 183 /* 184 * EPIA_HACK 185 * 186 * VIA C3 processors (Eden, Samuel 2) don't seem to correctly switch back to 187 * executing 16 bit code after the switch to real mode and subsequent jump. 188 * 189 * It is speculated that the CPU is prefetching and decoding branch 190 * targets and not invalidating this buffer on the long jump. 191 * Further investigation indicates that the caching of return addresses 192 * is most likely the problem. 193 * 194 * Previous versions just used some extra call/ret and a few NOPs, these 195 * only helped a bit, but booting compressed kernels would still fail. 196 * 197 * Trashing the return address stack (by doing 'call' without matched 'ret') 198 * Seems to fix things completely. 1 iteration isn't enough, 16 is plenty. 199 */ 200 ENTRY(prot_to_real) 201 .code32 202 pushl %eax 203 #ifdef EPIA_HACK 204 push %ecx 205 push $0x10 206 pop %ecx 207 1: call trash_return_cache 208 loop 1b 209 pop %ecx 210 #endif 211 212 /* 213 * Load the segment registers while still in protected mode. 214 * Otherwise the control bits don't get changed. 215 * The correct base addresses are loaded later. 216 */ 217 movw $bootrealdata, %ax 218 movw %ax, %ds 219 movw %ax, %es 220 movw %ax, %ss 221 222 /* 223 * Load %cs with a segment that has the correct attributes for 224 * 16bit operation. 225 */ 226 ljmp $bootrealseg, $1f 227 1: 228 229 .code16 230 movl %cr0, %eax 231 and $~CR0_PE, %eax 232 movl %eax, %cr0 /* Disable protected mode */ 233 234 /* Jump far indirect to load real mode %cs */ 235 ljmp *%cs:toreal 236 xreal: 237 /* 238 * CPU is now in real mode, load the other segment registers 239 * with their correct base addresses. 240 */ 241 mov %cs, %ax 242 mov %ax, %ds 243 mov %ax, %es 244 /* 245 * If stack was above 64k, 16bit %ss needs to be different from 246 * 32bit %ss (and the other segment registers). 247 */ 248 mov stkseg, %ax 249 mov %ax, %ss 250 subl stkdif, %esp 251 252 /* Check we are returning to an address below 64k */ 253 push %bp 254 movw %sp, %bp 255 movw 2/*bp*/ + 4/*eax*/ + 2(%bp), %ax /* high bits ret addr */ 256 test %ax, %ax 257 jne 1f 258 pop %bp 259 260 sti 261 popl %eax 262 retl 263 264 1: movw $3f, %si 265 call message 266 movl 2/*bp*/ + 4/*eax*/(%bp), %eax /* return address */ 267 call dump_eax 268 int $0x18 269 2: sti 270 hlt 271 jmp 2b 272 3: .asciz "prot_to_real can't return to " 273 274 .global dump_eax_buff 275 dump_eax_buff: 276 . = . + 16 277 278 #ifdef EPIA_HACK 279 trash_return_cache: 280 .code32 281 pop %eax 282 jmp *%eax 283 #endif 284 285 /* vtophys(void *) 286 * convert boot time 'linear' address to a physical one 287 */ 288 289 ENTRY(vtophys) 290 .code32 291 xorl %eax, %eax 292 movw ourseg, %ax 293 shll $4, %eax 294 addl 4(%esp), %eax 295 ret 296
Indexes created Mon Sep 29 21:09:56 GMT 2025