/* SPDX-License-Identifier: GPL-2.0 */
/*
 *  linux/boot/head.S
 *
 *  Copyright (C) 1991, 1992, 1993  Linus Torvalds
 */

/*
 *  head.S contains the 32-bit startup code.
 *
 * NOTE!!! Startup happens at absolute address 0x00001000, which is also where
 * the page directory will exist. The startup code will be overwritten by
 * the page directory. [According to comments etc elsewhere on a compressed
 * kernel it will end up at 0x1000 + 1Mb I hope so as I assume this. - AC]
 *
 * Page 0 is deliberately kept safe, since System Management Mode code in
 * laptops may need to access the BIOS data stored there.  This is also
 * useful for future device drivers that either access the BIOS via VM86
 * mode.
 */

/*
 * High loaded stuff by Hans Lermen & Werner Almesberger, Feb. 1996
 */
        .text

#include <linux/init.h>
#include <linux/linkage.h>
#include <asm/segment.h>
#include <asm/page_types.h>
#include <asm/boot.h>
#include <asm/asm-offsets.h>
#include <asm/bootparam.h>

/*
 * These symbols needed to be marked as .hidden to prevent the BFD linker from
 * generating R_386_32 (rather than R_386_RELATIVE) relocations for them when
 * the 32-bit compressed kernel is linked as PIE. This is no longer necessary,
 * but it doesn't hurt to keep them .hidden.
 */
        .hidden _bss
        .hidden _ebss
        .hidden _end

        __HEAD
SYM_FUNC_START(startup_32)
        cld
        cli

/*
 * Calculate the delta between where we were compiled to run
 * at and where we were actually loaded at.  This can only be done
 * with a short local call on x86.  Nothing  else will tell us what
 * address we are running at.  The reserved chunk of the real-mode
 * data at 0x1e4 (defined as a scratch field) are used as the stack
 * for this calculation. Only 4 bytes are needed.
 */
        leal    (BP_scratch+4)(%esi), %esp
        call    1f
1:      popl    %edx
        addl    $_GLOBAL_OFFSET_TABLE_+(.-1b), %edx

        /* Load new GDT */
        leal    gdt@GOTOFF(%edx), %eax
        movl    %eax, 2(%eax)
        lgdt    (%eax)

        /* Load segment registers with our descriptors */
        movl    $__BOOT_DS, %eax
        movl    %eax, %ds
        movl    %eax, %es
        movl    %eax, %fs
        movl    %eax, %gs
        movl    %eax, %ss

/*
 * %edx contains the address we are loaded at by the boot loader (plus the
 * offset to the GOT).  The below code calculates %ebx to be the address where
 * we should move the kernel image temporarily for safe in-place decompression
 * (again, plus the offset to the GOT).
 *
 * %ebp is calculated to be the address that the kernel will be decompressed to.
 */

#ifdef CONFIG_RELOCATABLE
        leal    startup_32@GOTOFF(%edx), %ebx
        movl    BP_kernel_alignment(%esi), %eax
        decl    %eax
        addl    %eax, %ebx
        notl    %eax
        andl    %eax, %ebx
        cmpl    $LOAD_PHYSICAL_ADDR, %ebx
        jae     1f
#endif
        movl    $LOAD_PHYSICAL_ADDR, %ebx
1:

        movl    %ebx, %ebp      // Save the output address for later
        /* Target address to relocate to for decompression */
        addl    BP_init_size(%esi), %ebx
        subl    $_end@GOTOFF, %ebx

        /* Set up the stack */
        leal    boot_stack_end@GOTOFF(%ebx), %esp

        /* Zero EFLAGS */
        pushl   $0
        popfl

/*
 * Copy the compressed kernel to the end of our buffer
 * where decompression in place becomes safe.
 */
        pushl   %esi
        leal    (_bss@GOTOFF-4)(%edx), %esi
        leal    (_bss@GOTOFF-4)(%ebx), %edi
        movl    $(_bss - startup_32), %ecx
        shrl    $2, %ecx
        std
        rep     movsl
        cld
        popl    %esi

        /*
         * The GDT may get overwritten either during the copy we just did or
         * during extract_kernel below. To avoid any issues, repoint the GDTR
         * to the new copy of the GDT.
         */
        leal    gdt@GOTOFF(%ebx), %eax
        movl    %eax, 2(%eax)
        lgdt    (%eax)

/*
 * Jump to the relocated address.
 */
        leal    .Lrelocated@GOTOFF(%ebx), %eax
        jmp     *%eax
SYM_FUNC_END(startup_32)

        .text
SYM_FUNC_START_LOCAL_NOALIGN(.Lrelocated)

/*
 * Clear BSS (stack is currently empty)
 */
        xorl    %eax, %eax
        leal    _bss@GOTOFF(%ebx), %edi
        leal    _ebss@GOTOFF(%ebx), %ecx
        subl    %edi, %ecx
        shrl    $2, %ecx
        rep     stosl

/*
 * Do the extraction, and jump to the new kernel..
 */
        /* push arguments for extract_kernel: */

        pushl   %ebp                    /* output address */
        pushl   %esi                    /* real mode pointer */
        call    extract_kernel          /* returns kernel entry point in %eax */
        addl    $24, %esp

/*
 * Jump to the extracted kernel.
 */
        xorl    %ebx, %ebx
        jmp     *%eax
SYM_FUNC_END(.Lrelocated)

        .data
        .balign 8
SYM_DATA_START_LOCAL(gdt)
        .word   gdt_end - gdt - 1
        .long   0
        .word   0
        .quad   0x0000000000000000      /* Reserved */
        .quad   0x00cf9a000000ffff      /* __KERNEL_CS */
        .quad   0x00cf92000000ffff      /* __KERNEL_DS */
SYM_DATA_END_LABEL(gdt, SYM_L_LOCAL, gdt_end)

/*
 * Stack and heap for uncompression
 */
        .bss
        .balign 4
boot_stack:
        .fill BOOT_STACK_SIZE, 1, 0
boot_stack_end: