95 lines
2.8 KiB
C
95 lines
2.8 KiB
C
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/*
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* Copyright (c) 2018 Intel Corporation
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*
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* SPDX-License-Identifier: Apache-2.0
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*/
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#include "serial.h"
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#include "x86_64-hw.h"
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#include "shared-page.h"
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/*
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* 16 bit boot stub. This code gets copied into a low memory page and
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* used as the bootstrap code for SMP processors, which always start
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* in real mode. It is compiled with gcc's -m16 switch, which is a
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* wrapper around the assembler's .code16gcc directive which cleverly
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* takes 32 bit assembly and "fixes" it with appropriate address size
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* prefixes to run in real mode on a 386.
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*
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* It is just code! We have the .text segment and NOTHING ELSE. No
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* static or global variables can be used, nor const read-only data.
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* Neither is the linker run, so nothing can be relocated and all
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* symbolic references need to be to addresses within this file. In
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* fact, any relocations that do sneak in will be left at zero at
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* runtime!
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*/
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__asm__(" cli\n"
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" xor %ax, %ax\n"
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" mov %ax, %ss\n"
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" mov %ax, %ds\n"
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" mov $80000, %esp\n" /* FIXME: put stack someplace officiallerish */
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" jmp _start16\n");
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void _start16(void)
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{
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#ifdef XUK_DEBUG
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serial_putc('1'); serial_putc('6'); serial_putc('\n');
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#endif
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/* First, serialize on a simple spinlock. Note there's a
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* theoretical flaw here in that we are on a shared stack with the
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* other CPUs here and we don't *technically* know that "oldlock"
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* does not get written to the (clobberable!) stack memory. But
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* in practice the compiler does the right thing here and we spin
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* in registers until exiting the loop, at which point we are the
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* only users of the stack, and thus safe.
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*/
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int oldlock;
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do {
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__asm__ volatile("pause; mov $1, %%eax; xchg %%eax, (%1)"
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: "=a"(oldlock) : "m"(_shared.smpinit_lock));
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} while (oldlock);
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/* Put a red banner at the top of the screen to announce our
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* presence
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*/
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volatile unsigned short *vga = (unsigned short *)0xb8000;
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for (int i = 0; i < 240; i++)
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vga[i] = 0xcc20;
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/* Spin again waiting on the BSP processor to give us a stack. We
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* won't use it until the entry code of stub32, but we want to
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* make sure it's there before we jump.
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*/
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while (!_shared.smpinit_stack) {
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}
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/* Load the GDT the CPU0 already prepared for us */
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__asm__ volatile ("lgdtw (%0)\n" : : "r"(_shared.gdt16_addr));
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/* Enter protected mode by setting the bottom bit of CR0 */
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int cr0;
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__asm__ volatile ("mov %%cr0, %0\n" : "=r"(cr0));
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cr0 |= 1;
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__asm__ volatile ("mov %0, %%cr0\n" : : "r"(cr0));
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/* Set up data and stack segments */
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short ds = GDT_SELECTOR(2);
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__asm__ volatile ("mov %0, %%ds; mov %0, %%ss" : : "r"(ds));
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/* Far jump to the 32 bit entry point, passing a cookie in EAX to
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* tell it what we're doing
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*/
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int magic = BOOT_MAGIC_STUB16;
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__asm__ volatile ("ljmpl $0x8,$0x100000" : : "a"(magic));
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while (1) {
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__asm__("hlt");
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}
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}
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