/*
 *
 *  Copyright (C) 1991, 1992  Linus Torvalds
 *
 *  Enhanced CPU detection and feature setting code by Mike Jagdis
 *  and Martin Mares, November 1997.
 */

.text
#include <linux/threads.h>
#include <linux/init.h>
#include <linux/linkage.h>
#include <asm/segment.h>
#include <asm/page_types.h>
#include <asm/pgtable_types.h>
#include <asm/cache.h>
#include <asm/thread_info.h>
#include <asm/asm-offsets.h>
#include <asm/setup.h>
#include <asm/processor-flags.h>
#include <asm/msr-index.h>
#include <asm/cpufeature.h>
#include <asm/percpu.h>
#include <asm/nops.h>
#include <asm/bootparam.h>

/* Physical address */
#define pa(X) ((X) - __PAGE_OFFSET)

/*
 * References to members of the new_cpu_data structure.
 */

#define X86             new_cpu_data+CPUINFO_x86
#define X86_VENDOR      new_cpu_data+CPUINFO_x86_vendor
#define X86_MODEL       new_cpu_data+CPUINFO_x86_model
#define X86_MASK        new_cpu_data+CPUINFO_x86_mask
#define X86_HARD_MATH   new_cpu_data+CPUINFO_hard_math
#define X86_CPUID       new_cpu_data+CPUINFO_cpuid_level
#define X86_CAPABILITY  new_cpu_data+CPUINFO_x86_capability
#define X86_VENDOR_ID   new_cpu_data+CPUINFO_x86_vendor_id

/*
 * This is how much memory in addition to the memory covered up to
 * and including _end we need mapped initially.
 * We need:
 *     (KERNEL_IMAGE_SIZE/4096) / 1024 pages (worst case, non PAE)
 *     (KERNEL_IMAGE_SIZE/4096) / 512 + 4 pages (worst case for PAE)
 *
 * Modulo rounding, each megabyte assigned here requires a kilobyte of
 * memory, which is currently unreclaimed.
 *
 * This should be a multiple of a page.
 *
 * KERNEL_IMAGE_SIZE should be greater than pa(_end)
 * and small than max_low_pfn, otherwise will waste some page table entries
 */

#if PTRS_PER_PMD > 1
#define PAGE_TABLE_SIZE(pages) (((pages) / PTRS_PER_PMD) + PTRS_PER_PGD)
#else
#define PAGE_TABLE_SIZE(pages) ((pages) / PTRS_PER_PGD)
#endif

/*
 * Number of possible pages in the lowmem region.
 *
 * We shift 2 by 31 instead of 1 by 32 to the left in order to avoid a
 * gas warning about overflowing shift count when gas has been compiled
 * with only a host target support using a 32-bit type for internal
 * representation.
 */
LOWMEM_PAGES = (((2<<31) - __PAGE_OFFSET) >> PAGE_SHIFT)

/* Enough space to fit pagetables for the low memory linear map */
MAPPING_BEYOND_END = PAGE_TABLE_SIZE(LOWMEM_PAGES) << PAGE_SHIFT

/*
 * Worst-case size of the kernel mapping we need to make:
 * a relocatable kernel can live anywhere in lowmem, so we need to be able
 * to map all of lowmem.
 */
KERNEL_PAGES = LOWMEM_PAGES

INIT_MAP_SIZE = PAGE_TABLE_SIZE(KERNEL_PAGES) * PAGE_SIZE
RESERVE_BRK(pagetables, INIT_MAP_SIZE)

/*
 * 32-bit kernel entrypoint; only used by the boot CPU.  On entry,
 * %esi points to the real-mode code as a 32-bit pointer.
 * CS and DS must be 4 GB flat segments, but we don't depend on
 * any particular GDT layout, because we load our own as soon as we
 * can.
 */
__HEAD
ENTRY(startup_32)
        movl pa(stack_start),%ecx
        
        /* test KEEP_SEGMENTS flag to see if the bootloader is asking
                us to not reload segments */
        testb $KEEP_SEGMENTS, BP_loadflags(%esi)
        jnz 2f

/*
 * Set segments to known values.
 */
        lgdt pa(boot_gdt_descr)
        movl $(__BOOT_DS),%eax
        movl %eax,%ds
        movl %eax,%es
        movl %eax,%fs
        movl %eax,%gs
        movl %eax,%ss
2:
        leal -__PAGE_OFFSET(%ecx),%esp

/*
 * Clear BSS first so that there are no surprises...
 */
        cld
        xorl %eax,%eax
        movl $pa(__bss_start),%edi
        movl $pa(__bss_stop),%ecx
        subl %edi,%ecx
        shrl $2,%ecx
        rep ; stosl
/*
 * Copy bootup parameters out of the way.
 * Note: %esi still has the pointer to the real-mode data.
 * With the kexec as boot loader, parameter segment might be loaded beyond
 * kernel image and might not even be addressable by early boot page tables.
 * (kexec on panic case). Hence copy out the parameters before initializing
 * page tables.
 */
        movl $pa(boot_params),%edi
        movl $(PARAM_SIZE/4),%ecx
        cld
        rep
        movsl
        movl pa(boot_params) + NEW_CL_POINTER,%esi
        andl %esi,%esi
        jz 1f                   # No command line
        movl $pa(boot_command_line),%edi
        movl $(COMMAND_LINE_SIZE/4),%ecx
        rep
        movsl
1:

#ifdef CONFIG_OLPC
        /* save OFW's pgdir table for later use when calling into OFW */
        movl %cr3, %eax
        movl %eax, pa(olpc_ofw_pgd)
#endif

#ifdef CONFIG_MICROCODE
        /* Early load ucode on BSP. */
        call load_ucode_bsp
#endif

/*
 * Initialize page tables.  This creates a PDE and a set of page
 * tables, which are located immediately beyond __brk_base.  The variable
 * _brk_end is set up to point to the first "safe" location.
 * Mappings are created both at virtual address 0 (identity mapping)
 * and PAGE_OFFSET for up to _end.
 */
#ifdef CONFIG_X86_PAE

        /*
         * In PAE mode initial_page_table is statically defined to contain
         * enough entries to cover the VMSPLIT option (that is the top 1, 2 or 3
         * entries). The identity mapping is handled by pointing two PGD entries
         * to the first kernel PMD.
         *
         * Note the upper half of each PMD or PTE are always zero at this stage.
         */

#define KPMDS (((-__PAGE_OFFSET) >> 30) & 3) /* Number of kernel PMDs */

        xorl %ebx,%ebx                          /* %ebx is kept at zero */

        movl $pa(__brk_base), %edi
        movl $pa(initial_pg_pmd), %edx
        movl $PTE_IDENT_ATTR, %eax
10:
        leal PDE_IDENT_ATTR(%edi),%ecx          /* Create PMD entry */
        movl %ecx,(%edx)                        /* Store PMD entry */
                                                /* Upper half already zero */
        addl $8,%edx
        movl $512,%ecx
11:
        stosl
        xchgl %eax,%ebx
        stosl
        xchgl %eax,%ebx
        addl $0x1000,%eax
        loop 11b

        /*
         * End condition: we must map up to the end + MAPPING_BEYOND_END.
         */
        movl $pa(_end) + MAPPING_BEYOND_END + PTE_IDENT_ATTR, %ebp
        cmpl %ebp,%eax
        jb 10b
1:
        addl $__PAGE_OFFSET, %edi
        movl %edi, pa(_brk_end)
        shrl $12, %eax
        movl %eax, pa(max_pfn_mapped)

        /* Do early initialization of the fixmap area */
        movl $pa(initial_pg_fixmap)+PDE_IDENT_ATTR,%eax
        movl %eax,pa(initial_pg_pmd+0x1000*KPMDS-8)
#else   /* Not PAE */

page_pde_offset = (__PAGE_OFFSET >> 20);

        movl $pa(__brk_base), %edi
        movl $pa(initial_page_table), %edx
        movl $PTE_IDENT_ATTR, %eax
10:
        leal PDE_IDENT_ATTR(%edi),%ecx          /* Create PDE entry */
        movl %ecx,(%edx)                        /* Store identity PDE entry */
        movl %ecx,page_pde_offset(%edx)         /* Store kernel PDE entry */
        addl $4,%edx
        movl $1024, %ecx
11:
        stosl
        addl $0x1000,%eax
        loop 11b
        /*
         * End condition: we must map up to the end + MAPPING_BEYOND_END.
         */
        movl $pa(_end) + MAPPING_BEYOND_END + PTE_IDENT_ATTR, %ebp
        cmpl %ebp,%eax
        jb 10b
        addl $__PAGE_OFFSET, %edi
        movl %edi, pa(_brk_end)
        shrl $12, %eax
        movl %eax, pa(max_pfn_mapped)

        /* Do early initialization of the fixmap area */
        movl $pa(initial_pg_fixmap)+PDE_IDENT_ATTR,%eax
        movl %eax,pa(initial_page_table+0xffc)
#endif

#ifdef CONFIG_PARAVIRT
        /* This is can only trip for a broken bootloader... */
        cmpw $0x207, pa(boot_params + BP_version)
        jb default_entry

        /* Paravirt-compatible boot parameters.  Look to see what architecture
                we're booting under. */
        movl pa(boot_params + BP_hardware_subarch), %eax
        cmpl $num_subarch_entries, %eax
        jae bad_subarch

        movl pa(subarch_entries)(,%eax,4), %eax
        subl $__PAGE_OFFSET, %eax
        jmp *%eax

bad_subarch:
WEAK(lguest_entry)
WEAK(xen_entry)
        /* Unknown implementation; there's really
           nothing we can do at this point. */
        ud2a

        __INITDATA

subarch_entries:
        .long default_entry             /* normal x86/PC */
        .long lguest_entry              /* lguest hypervisor */
        .long xen_entry                 /* Xen hypervisor */
        .long default_entry             /* Moorestown MID */
num_subarch_entries = (. - subarch_entries) / 4
.previous
#else
        jmp default_entry
#endif /* CONFIG_PARAVIRT */

#ifdef CONFIG_HOTPLUG_CPU
/*
 * Boot CPU0 entry point. It's called from play_dead(). Everything has been set
 * up already except stack. We just set up stack here. Then call
 * start_secondary().
 */
ENTRY(start_cpu0)
        movl stack_start, %ecx
        movl %ecx, %esp
        jmp  *(initial_code)
ENDPROC(start_cpu0)
#endif

/*
 * Non-boot CPU entry point; entered from trampoline.S
 * We can't lgdt here, because lgdt itself uses a data segment, but
 * we know the trampoline has already loaded the boot_gdt for us.
 *
 * If cpu hotplug is not supported then this code can go in init section
 * which will be freed later
 */
ENTRY(startup_32_smp)
        cld
        movl $(__BOOT_DS),%eax
        movl %eax,%ds
        movl %eax,%es
        movl %eax,%fs
        movl %eax,%gs
        movl pa(stack_start),%ecx
        movl %eax,%ss
        leal -__PAGE_OFFSET(%ecx),%esp

#ifdef CONFIG_MICROCODE
        /* Early load ucode on AP. */
        call load_ucode_ap
#endif

default_entry:
#define CR0_STATE       (X86_CR0_PE | X86_CR0_MP | X86_CR0_ET | \
                         X86_CR0_NE | X86_CR0_WP | X86_CR0_AM | \
                         X86_CR0_PG)
        movl $(CR0_STATE & ~X86_CR0_PG),%eax
        movl %eax,%cr0

/*
 * We want to start out with EFLAGS unambiguously cleared. Some BIOSes leave
 * bits like NT set. This would confuse the debugger if this code is traced. So
 * initialize them properly now before switching to protected mode. That means
 * DF in particular (even though we have cleared it earlier after copying the
 * command line) because GCC expects it.
 */
        pushl $0
        popfl

/*
 * New page tables may be in 4Mbyte page mode and may be using the global pages.
 *
 * NOTE! If we are on a 486 we may have no cr4 at all! Specifically, cr4 exists
 * if and only if CPUID exists and has flags other than the FPU flag set.
 */
        movl $-1,pa(X86_CPUID)          # preset CPUID level
        movl $X86_EFLAGS_ID,%ecx
        pushl %ecx
        popfl                           # set EFLAGS=ID
        pushfl
        popl %eax                       # get EFLAGS
        testl $X86_EFLAGS_ID,%eax       # did EFLAGS.ID remained set?
        jz enable_paging                # hw disallowed setting of ID bit
                                        # which means no CPUID and no CR4

        xorl %eax,%eax
        cpuid
        movl %eax,pa(X86_CPUID)         # save largest std CPUID function

        movl $1,%eax
        cpuid
        andl $~1,%edx                   # Ignore CPUID.FPU
        jz enable_paging                # No flags or only CPUID.FPU = no CR4

        movl pa(mmu_cr4_features),%eax
        movl %eax,%cr4

        testb $X86_CR4_PAE, %al         # check if PAE is enabled
        jz enable_paging

        /* Check if extended functions are implemented */
        movl $0x80000000, %eax
        cpuid
        /* Value must be in the range 0x80000001 to 0x8000ffff */
        subl $0x80000001, %eax
        cmpl $(0x8000ffff-0x80000001), %eax
        ja enable_paging

        /* Clear bogus XD_DISABLE bits */
        call verify_cpu

        mov $0x80000001, %eax
        cpuid
        /* Execute Disable bit supported? */
        btl $(X86_FEATURE_NX & 31), %edx
        jnc enable_paging

        /* Setup EFER (Extended Feature Enable Register) */
        movl $MSR_EFER, %ecx
        rdmsr

        btsl $_EFER_NX, %eax
        /* Make changes effective */
        wrmsr

enable_paging:

/*
 * Enable paging
 */
        movl $pa(initial_page_table), %eax
        movl %eax,%cr3          /* set the page table pointer.. */
        movl $CR0_STATE,%eax
        movl %eax,%cr0          /* ..and set paging (PG) bit */
        ljmp $__BOOT_CS,$1f     /* Clear prefetch and normalize %eip */
1:
        /* Shift the stack pointer to a virtual address */
        addl $__PAGE_OFFSET, %esp

/*
 * start system 32-bit setup. We need to re-do some of the things done
 * in 16-bit mode for the "real" operations.
 */
        movl setup_once_ref,%eax
        andl %eax,%eax
        jz 1f                           # Did we do this already?
        call *%eax
1:

/*
 * Check if it is 486
 */
        movb $4,X86                     # at least 486
        cmpl $-1,X86_CPUID
        je is486

        /* get vendor info */
        xorl %eax,%eax                  # call CPUID with 0 -> return vendor ID
        cpuid
        movl %eax,X86_CPUID             # save CPUID level
        movl %ebx,X86_VENDOR_ID         # lo 4 chars
        movl %edx,X86_VENDOR_ID+4       # next 4 chars
        movl %ecx,X86_VENDOR_ID+8       # last 4 chars

        orl %eax,%eax                   # do we have processor info as well?
        je is486

        movl $1,%eax            # Use the CPUID instruction to get CPU type
        cpuid
        movb %al,%cl            # save reg for future use
        andb $0x0f,%ah          # mask processor family
        movb %ah,X86
        andb $0xf0,%al          # mask model
        shrb $4,%al
        movb %al,X86_MODEL
        andb $0x0f,%cl          # mask mask revision
        movb %cl,X86_MASK
        movl %edx,X86_CAPABILITY

is486:
        movl $0x50022,%ecx      # set AM, WP, NE and MP
        movl %cr0,%eax
        andl $0x80000011,%eax   # Save PG,PE,ET
        orl %ecx,%eax
        movl %eax,%cr0

        lgdt early_gdt_descr
        lidt idt_descr
        ljmp $(__KERNEL_CS),$1f
1:      movl $(__KERNEL_DS),%eax        # reload all the segment registers
        movl %eax,%ss                   # after changing gdt.

        movl $(__USER_DS),%eax          # DS/ES contains default USER segment
        movl %eax,%ds
        movl %eax,%es

        movl $(__KERNEL_PERCPU), %eax
        movl %eax,%fs                   # set this cpu's percpu

        movl $(__KERNEL_STACK_CANARY),%eax
        movl %eax,%gs

        xorl %eax,%eax                  # Clear LDT
        lldt %ax

        pushl $0                # fake return address for unwinder
        jmp *(initial_code)

#include "verify_cpu.S"

/*
 *  setup_once
 *
 *  The setup work we only want to run on the BSP.
 *
 *  Warning: %esi is live across this function.
 */
__INIT
setup_once:
        /*
         * Set up a idt with 256 interrupt gates that push zero if there
         * is no error code and then jump to early_idt_handler_common.
         * It doesn't actually load the idt - that needs to be done on
         * each CPU. Interrupts are enabled elsewhere, when we can be
         * relatively sure everything is ok.
         */

        movl $idt_table,%edi
        movl $early_idt_handler_array,%eax
        movl $NUM_EXCEPTION_VECTORS,%ecx
1:
        movl %eax,(%edi)
        movl %eax,4(%edi)
        /* interrupt gate, dpl=0, present */
        movl $(0x8E000000 + __KERNEL_CS),2(%edi)
        addl $EARLY_IDT_HANDLER_SIZE,%eax
        addl $8,%edi
        loop 1b

        movl $256 - NUM_EXCEPTION_VECTORS,%ecx
        movl $ignore_int,%edx
        movl $(__KERNEL_CS << 16),%eax
        movw %dx,%ax            /* selector = 0x0010 = cs */
        movw $0x8E00,%dx        /* interrupt gate - dpl=0, present */
2:
        movl %eax,(%edi)
        movl %edx,4(%edi)
        addl $8,%edi
        loop 2b

#ifdef CONFIG_CC_STACKPROTECTOR
        /*
         * Configure the stack canary. The linker can't handle this by
         * relocation.  Manually set base address in stack canary
         * segment descriptor.
         */
        movl $gdt_page,%eax
        movl $stack_canary,%ecx
        movw %cx, 8 * GDT_ENTRY_STACK_CANARY + 2(%eax)
        shrl $16, %ecx
        movb %cl, 8 * GDT_ENTRY_STACK_CANARY + 4(%eax)
        movb %ch, 8 * GDT_ENTRY_STACK_CANARY + 7(%eax)
#endif

        andl $0,setup_once_ref  /* Once is enough, thanks */
        ret

ENTRY(early_idt_handler_array)
        # 36(%esp) %eflags
        # 32(%esp) %cs
        # 28(%esp) %eip
        # 24(%rsp) error code
        i = 0
        .rept NUM_EXCEPTION_VECTORS
        .ifeq (EXCEPTION_ERRCODE_MASK >> i) & 1
        pushl $0                # Dummy error code, to make stack frame uniform
        .endif
        pushl $i                # 20(%esp) Vector number
        jmp early_idt_handler_common
        i = i + 1
        .fill early_idt_handler_array + i*EARLY_IDT_HANDLER_SIZE - ., 1, 0xcc
        .endr
ENDPROC(early_idt_handler_array)
        
early_idt_handler_common:
        /*
         * The stack is the hardware frame, an error code or zero, and the
         * vector number.
         */
        cld

        cmpl $2,(%esp)          # X86_TRAP_NMI
        je .Lis_nmi             # Ignore NMI

        cmpl $2,%ss:early_recursion_flag
        je hlt_loop
        incl %ss:early_recursion_flag

        push %eax               # 16(%esp)
        push %ecx               # 12(%esp)
        push %edx               #  8(%esp)
        push %ds                #  4(%esp)
        push %es                #  0(%esp)
        movl $(__KERNEL_DS),%eax
        movl %eax,%ds
        movl %eax,%es

        cmpl $(__KERNEL_CS),32(%esp)
        jne 10f

        leal 28(%esp),%eax      # Pointer to %eip
        call early_fixup_exception
        andl %eax,%eax
        jnz ex_entry            /* found an exception entry */

10:
#ifdef CONFIG_PRINTK
        xorl %eax,%eax
        movw %ax,2(%esp)        /* clean up the segment values on some cpus */
        movw %ax,6(%esp)
        movw %ax,34(%esp)
        leal  40(%esp),%eax
        pushl %eax              /* %esp before the exception */
        pushl %ebx
        pushl %ebp
        pushl %esi
        pushl %edi
        movl %cr2,%eax
        pushl %eax
        pushl (20+6*4)(%esp)    /* trapno */
        pushl $fault_msg
        call printk
#endif
        call dump_stack
hlt_loop:
        hlt
        jmp hlt_loop

ex_entry:
        pop %es
        pop %ds
        pop %edx
        pop %ecx
        pop %eax
        decl %ss:early_recursion_flag
.Lis_nmi:
        addl $8,%esp            /* drop vector number and error code */
        iret
ENDPROC(early_idt_handler_common)

/* This is the default interrupt "handler" :-) */
        ALIGN
ignore_int:
        cld
#ifdef CONFIG_PRINTK
        pushl %eax
        pushl %ecx
        pushl %edx
        pushl %es
        pushl %ds
        movl $(__KERNEL_DS),%eax
        movl %eax,%ds
        movl %eax,%es
        cmpl $2,early_recursion_flag
        je hlt_loop
        incl early_recursion_flag
        pushl 16(%esp)
        pushl 24(%esp)
        pushl 32(%esp)
        pushl 40(%esp)
        pushl $int_msg
        call printk

        call dump_stack

        addl $(5*4),%esp
        popl %ds
        popl %es
        popl %edx
        popl %ecx
        popl %eax
#endif
        iret
ENDPROC(ignore_int)
__INITDATA
        .align 4
early_recursion_flag:
        .long 0

__REFDATA
        .align 4
ENTRY(initial_code)
        .long i386_start_kernel
ENTRY(setup_once_ref)
        .long setup_once

/*
 * BSS section
 */
__PAGE_ALIGNED_BSS
        .align PAGE_SIZE
#ifdef CONFIG_X86_PAE
initial_pg_pmd:
        .fill 1024*KPMDS,4,0
#else
ENTRY(initial_page_table)
        .fill 1024,4,0
#endif
initial_pg_fixmap:
        .fill 1024,4,0
ENTRY(empty_zero_page)
        .fill 4096,1,0
ENTRY(swapper_pg_dir)
        .fill 1024,4,0

/*
 * This starts the data section.
 */
#ifdef CONFIG_X86_PAE
__PAGE_ALIGNED_DATA
        /* Page-aligned for the benefit of paravirt? */
        .align PAGE_SIZE
ENTRY(initial_page_table)
        .long   pa(initial_pg_pmd+PGD_IDENT_ATTR),0     /* low identity map */
# if KPMDS == 3
        .long   pa(initial_pg_pmd+PGD_IDENT_ATTR),0
        .long   pa(initial_pg_pmd+PGD_IDENT_ATTR+0x1000),0
        .long   pa(initial_pg_pmd+PGD_IDENT_ATTR+0x2000),0
# elif KPMDS == 2
        .long   0,0
        .long   pa(initial_pg_pmd+PGD_IDENT_ATTR),0
        .long   pa(initial_pg_pmd+PGD_IDENT_ATTR+0x1000),0
# elif KPMDS == 1
        .long   0,0
        .long   0,0
        .long   pa(initial_pg_pmd+PGD_IDENT_ATTR),0
# else
#  error "Kernel PMDs should be 1, 2 or 3"
# endif
        .align PAGE_SIZE                /* needs to be page-sized too */
#endif

.data
.balign 4
ENTRY(stack_start)
        .long init_thread_union+THREAD_SIZE

__INITRODATA
int_msg:
        .asciz "Unknown interrupt or fault at: %p %p %p\n"

fault_msg:
/* fault info: */
        .ascii "BUG: Int %d: CR2 %p\n"
/* regs pushed in early_idt_handler: */
        .ascii "     EDI %p  ESI %p  EBP %p  EBX %p\n"
        .ascii "     ESP %p   ES %p   DS %p\n"
        .ascii "     EDX %p  ECX %p  EAX %p\n"
/* fault frame: */
        .ascii "     vec %p  err %p  EIP %p   CS %p  flg %p\n"
        .ascii "Stack: %p %p %p %p %p %p %p %p\n"
        .ascii "       %p %p %p %p %p %p %p %p\n"
        .asciz "       %p %p %p %p %p %p %p %p\n"

#include "../../x86/xen/xen-head.S"

/*
 * The IDT and GDT 'descriptors' are a strange 48-bit object
 * only used by the lidt and lgdt instructions. They are not
 * like usual segment descriptors - they consist of a 16-bit
 * segment size, and 32-bit linear address value:
 */

        .data
.globl boot_gdt_descr
.globl idt_descr

        ALIGN
# early boot GDT descriptor (must use 1:1 address mapping)
        .word 0                         # 32 bit align gdt_desc.address
boot_gdt_descr:
        .word __BOOT_DS+7
        .long boot_gdt - __PAGE_OFFSET

        .word 0                         # 32-bit align idt_desc.address
idt_descr:
        .word IDT_ENTRIES*8-1           # idt contains 256 entries
        .long idt_table

# boot GDT descriptor (later on used by CPU#0):
        .word 0                         # 32 bit align gdt_desc.address
ENTRY(early_gdt_descr)
        .word GDT_ENTRIES*8-1
        .long gdt_page                  /* Overwritten for secondary CPUs */

/*
 * The boot_gdt must mirror the equivalent in setup.S and is
 * used only for booting.
 */
        .align L1_CACHE_BYTES
ENTRY(boot_gdt)
        .fill GDT_ENTRY_BOOT_CS,8,0
        .quad 0x00cf9a000000ffff        /* kernel 4GB code at 0x00000000 */
        .quad 0x00cf92000000ffff        /* kernel 4GB data at 0x00000000 */