// SPDX-License-Identifier: GPL-2.0-only
/* -*- linux-c -*- ------------------------------------------------------- *
 *
 *   Copyright (C) 1991, 1992 Linus Torvalds
 *   Copyright 2007 rPath, Inc. - All Rights Reserved
 *
 * ----------------------------------------------------------------------- */

/*
 * Prepare the machine for transition to protected mode.
 */

#include "boot.h"
#include <asm/desc_defs.h>
#include <asm/segment.h>

/*
 * Invoke the realmode switch hook if present; otherwise
 * disable all interrupts.
 */
static void realmode_switch_hook(void)
{
        if (boot_params.hdr.realmode_swtch) {
                asm volatile("lcallw *%0"
                             : : "m" (boot_params.hdr.realmode_swtch)
                             : "eax", "ebx", "ecx", "edx");
        } else {
                asm volatile("cli");
                outb(0x80, 0x70); /* Disable NMI */
                io_delay();
        }
}

/*
 * Disable all interrupts at the legacy PIC.
 */
static void mask_all_interrupts(void)
{
        outb(0xff, 0xa1);       /* Mask all interrupts on the secondary PIC */
        io_delay();
        outb(0xfb, 0x21);       /* Mask all but cascade on the primary PIC */
        io_delay();
}

/*
 * Reset IGNNE# if asserted in the FPU.
 */
static void reset_coprocessor(void)
{
        outb(0, 0xf0);
        io_delay();
        outb(0, 0xf1);
        io_delay();
}

/*
 * Set up the GDT
 */

struct gdt_ptr {
        u16 len;
        u32 ptr;
} __attribute__((packed));

static void setup_gdt(void)
{
        /* There are machines which are known to not boot with the GDT
           being 8-byte unaligned.  Intel recommends 16 byte alignment. */
        static const u64 boot_gdt[] __attribute__((aligned(16))) = {
                /* CS: code, read/execute, 4 GB, base 0 */
                [GDT_ENTRY_BOOT_CS] = GDT_ENTRY(DESC_CODE32, 0, 0xfffff),
                /* DS: data, read/write, 4 GB, base 0 */
                [GDT_ENTRY_BOOT_DS] = GDT_ENTRY(DESC_DATA32, 0, 0xfffff),
                /* TSS: 32-bit tss, 104 bytes, base 4096 */
                /* We only have a TSS here to keep Intel VT happy;
                   we don't actually use it for anything. */
                [GDT_ENTRY_BOOT_TSS] = GDT_ENTRY(DESC_TSS32, 4096, 103),
        };
        /* Xen HVM incorrectly stores a pointer to the gdt_ptr, instead
           of the gdt_ptr contents.  Thus, make it static so it will
           stay in memory, at least long enough that we switch to the
           proper kernel GDT. */
        static struct gdt_ptr gdt;

        gdt.len = sizeof(boot_gdt)-1;
        gdt.ptr = (u32)&boot_gdt + (ds() << 4);

        asm volatile("lgdtl %0" : : "m" (gdt));
}

/*
 * Set up the IDT
 */
static void setup_idt(void)
{
        static const struct gdt_ptr null_idt = {0, 0};
        asm volatile("lidtl %0" : : "m" (null_idt));
}

/*
 * Actual invocation sequence
 */
void go_to_protected_mode(void)
{
        /* Hook before leaving real mode, also disables interrupts */
        realmode_switch_hook();

        /* Enable the A20 gate */
        if (enable_a20()) {
                puts("A20 gate not responding, unable to boot...\n");
                die();
        }

        /* Reset coprocessor (IGNNE#) */
        reset_coprocessor();

        /* Mask all interrupts in the PIC */
        mask_all_interrupts();

        /* Actual transition to protected mode... */
        setup_idt();
        setup_gdt();
        protected_mode_jump(boot_params.hdr.code32_start,
                            (u32)&boot_params + (ds() << 4));
}