xref: /linux/arch/x86/kernel/head_32.S (revision 5bdef865eb358b6f3760e25e591ae115e9eeddef) 
1/*
2 *
3 *  Copyright (C) 1991, 1992  Linus Torvalds
4 *
5 *  Enhanced CPU detection and feature setting code by Mike Jagdis
6 *  and Martin Mares, November 1997.
7 */
8
9.text
10#include <linux/threads.h>
11#include <linux/init.h>
12#include <linux/linkage.h>
13#include <asm/segment.h>
14#include <asm/page_types.h>
15#include <asm/pgtable_types.h>
16#include <asm/cache.h>
17#include <asm/thread_info.h>
18#include <asm/asm-offsets.h>
19#include <asm/setup.h>
20#include <asm/processor-flags.h>
21#include <asm/percpu.h>
22
23/* Physical address */
24#define pa(X) ((X) - __PAGE_OFFSET)
25
26/*
27 * References to members of the new_cpu_data structure.
28 */
29
30#define X86		new_cpu_data+CPUINFO_x86
31#define X86_VENDOR	new_cpu_data+CPUINFO_x86_vendor
32#define X86_MODEL	new_cpu_data+CPUINFO_x86_model
33#define X86_MASK	new_cpu_data+CPUINFO_x86_mask
34#define X86_HARD_MATH	new_cpu_data+CPUINFO_hard_math
35#define X86_CPUID	new_cpu_data+CPUINFO_cpuid_level
36#define X86_CAPABILITY	new_cpu_data+CPUINFO_x86_capability
37#define X86_VENDOR_ID	new_cpu_data+CPUINFO_x86_vendor_id
38
39/*
40 * This is how much memory in addition to the memory covered up to
41 * and including _end we need mapped initially.
42 * We need:
43 *     (KERNEL_IMAGE_SIZE/4096) / 1024 pages (worst case, non PAE)
44 *     (KERNEL_IMAGE_SIZE/4096) / 512 + 4 pages (worst case for PAE)
45 *
46 * Modulo rounding, each megabyte assigned here requires a kilobyte of
47 * memory, which is currently unreclaimed.
48 *
49 * This should be a multiple of a page.
50 *
51 * KERNEL_IMAGE_SIZE should be greater than pa(_end)
52 * and small than max_low_pfn, otherwise will waste some page table entries
53 */
54
55#if PTRS_PER_PMD > 1
56#define PAGE_TABLE_SIZE(pages) (((pages) / PTRS_PER_PMD) + PTRS_PER_PGD)
57#else
58#define PAGE_TABLE_SIZE(pages) ((pages) / PTRS_PER_PGD)
59#endif
60
61/* Enough space to fit pagetables for the low memory linear map */
62MAPPING_BEYOND_END = \
63	PAGE_TABLE_SIZE(((1<<32) - __PAGE_OFFSET) >> PAGE_SHIFT) << PAGE_SHIFT
64
65/*
66 * Worst-case size of the kernel mapping we need to make:
67 * the worst-case size of the kernel itself, plus the extra we need
68 * to map for the linear map.
69 */
70KERNEL_PAGES = (KERNEL_IMAGE_SIZE + MAPPING_BEYOND_END)>>PAGE_SHIFT
71
72INIT_MAP_SIZE = PAGE_TABLE_SIZE(KERNEL_PAGES) * PAGE_SIZE_asm
73RESERVE_BRK(pagetables, INIT_MAP_SIZE)
74
75/*
76 * 32-bit kernel entrypoint; only used by the boot CPU.  On entry,
77 * %esi points to the real-mode code as a 32-bit pointer.
78 * CS and DS must be 4 GB flat segments, but we don't depend on
79 * any particular GDT layout, because we load our own as soon as we
80 * can.
81 */
82.section .text.head,"ax",@progbits
83ENTRY(startup_32)
84	/* test KEEP_SEGMENTS flag to see if the bootloader is asking
85		us to not reload segments */
86	testb $(1<<6), BP_loadflags(%esi)
87	jnz 2f
88
89/*
90 * Set segments to known values.
91 */
92	lgdt pa(boot_gdt_descr)
93	movl $(__BOOT_DS),%eax
94	movl %eax,%ds
95	movl %eax,%es
96	movl %eax,%fs
97	movl %eax,%gs
982:
99
100/*
101 * Clear BSS first so that there are no surprises...
102 */
103	cld
104	xorl %eax,%eax
105	movl $pa(__bss_start),%edi
106	movl $pa(__bss_stop),%ecx
107	subl %edi,%ecx
108	shrl $2,%ecx
109	rep ; stosl
110/*
111 * Copy bootup parameters out of the way.
112 * Note: %esi still has the pointer to the real-mode data.
113 * With the kexec as boot loader, parameter segment might be loaded beyond
114 * kernel image and might not even be addressable by early boot page tables.
115 * (kexec on panic case). Hence copy out the parameters before initializing
116 * page tables.
117 */
118	movl $pa(boot_params),%edi
119	movl $(PARAM_SIZE/4),%ecx
120	cld
121	rep
122	movsl
123	movl pa(boot_params) + NEW_CL_POINTER,%esi
124	andl %esi,%esi
125	jz 1f			# No comand line
126	movl $pa(boot_command_line),%edi
127	movl $(COMMAND_LINE_SIZE/4),%ecx
128	rep
129	movsl
1301:
131
132#ifdef CONFIG_PARAVIRT
133	/* This is can only trip for a broken bootloader... */
134	cmpw $0x207, pa(boot_params + BP_version)
135	jb default_entry
136
137	/* Paravirt-compatible boot parameters.  Look to see what architecture
138		we're booting under. */
139	movl pa(boot_params + BP_hardware_subarch), %eax
140	cmpl $num_subarch_entries, %eax
141	jae bad_subarch
142
143	movl pa(subarch_entries)(,%eax,4), %eax
144	subl $__PAGE_OFFSET, %eax
145	jmp *%eax
146
147bad_subarch:
148WEAK(lguest_entry)
149WEAK(xen_entry)
150	/* Unknown implementation; there's really
151	   nothing we can do at this point. */
152	ud2a
153
154	__INITDATA
155
156subarch_entries:
157	.long default_entry		/* normal x86/PC */
158	.long lguest_entry		/* lguest hypervisor */
159	.long xen_entry			/* Xen hypervisor */
160num_subarch_entries = (. - subarch_entries) / 4
161.previous
162#endif /* CONFIG_PARAVIRT */
163
164/*
165 * Initialize page tables.  This creates a PDE and a set of page
166 * tables, which are located immediately beyond __brk_base.  The variable
167 * _brk_end is set up to point to the first "safe" location.
168 * Mappings are created both at virtual address 0 (identity mapping)
169 * and PAGE_OFFSET for up to _end.
170 *
171 * Note that the stack is not yet set up!
172 */
173default_entry:
174#ifdef CONFIG_X86_PAE
175
176	/*
177	 * In PAE mode swapper_pg_dir is statically defined to contain enough
178	 * entries to cover the VMSPLIT option (that is the top 1, 2 or 3
179	 * entries). The identity mapping is handled by pointing two PGD
180	 * entries to the first kernel PMD.
181	 *
182	 * Note the upper half of each PMD or PTE are always zero at
183	 * this stage.
184	 */
185
186#define KPMDS (((-__PAGE_OFFSET) >> 30) & 3) /* Number of kernel PMDs */
187
188	xorl %ebx,%ebx				/* %ebx is kept at zero */
189
190	movl $pa(__brk_base), %edi
191	movl $pa(swapper_pg_pmd), %edx
192	movl $PTE_IDENT_ATTR, %eax
19310:
194	leal PDE_IDENT_ATTR(%edi),%ecx		/* Create PMD entry */
195	movl %ecx,(%edx)			/* Store PMD entry */
196						/* Upper half already zero */
197	addl $8,%edx
198	movl $512,%ecx
19911:
200	stosl
201	xchgl %eax,%ebx
202	stosl
203	xchgl %eax,%ebx
204	addl $0x1000,%eax
205	loop 11b
206
207	/*
208	 * End condition: we must map up to the end + MAPPING_BEYOND_END.
209	 */
210	movl $pa(_end) + MAPPING_BEYOND_END + PTE_IDENT_ATTR, %ebp
211	cmpl %ebp,%eax
212	jb 10b
2131:
214	addl $__PAGE_OFFSET, %edi
215	movl %edi, pa(_brk_end)
216	shrl $12, %eax
217	movl %eax, pa(max_pfn_mapped)
218
219	/* Do early initialization of the fixmap area */
220	movl $pa(swapper_pg_fixmap)+PDE_IDENT_ATTR,%eax
221	movl %eax,pa(swapper_pg_pmd+0x1000*KPMDS-8)
222#else	/* Not PAE */
223
224page_pde_offset = (__PAGE_OFFSET >> 20);
225
226	movl $pa(__brk_base), %edi
227	movl $pa(swapper_pg_dir), %edx
228	movl $PTE_IDENT_ATTR, %eax
22910:
230	leal PDE_IDENT_ATTR(%edi),%ecx		/* Create PDE entry */
231	movl %ecx,(%edx)			/* Store identity PDE entry */
232	movl %ecx,page_pde_offset(%edx)		/* Store kernel PDE entry */
233	addl $4,%edx
234	movl $1024, %ecx
23511:
236	stosl
237	addl $0x1000,%eax
238	loop 11b
239	/*
240	 * End condition: we must map up to the end + MAPPING_BEYOND_END.
241	 */
242	movl $pa(_end) + MAPPING_BEYOND_END + PTE_IDENT_ATTR, %ebp
243	cmpl %ebp,%eax
244	jb 10b
245	addl $__PAGE_OFFSET, %edi
246	movl %edi, pa(_brk_end)
247	shrl $12, %eax
248	movl %eax, pa(max_pfn_mapped)
249
250	/* Do early initialization of the fixmap area */
251	movl $pa(swapper_pg_fixmap)+PDE_IDENT_ATTR,%eax
252	movl %eax,pa(swapper_pg_dir+0xffc)
253#endif
254	jmp 3f
255/*
256 * Non-boot CPU entry point; entered from trampoline.S
257 * We can't lgdt here, because lgdt itself uses a data segment, but
258 * we know the trampoline has already loaded the boot_gdt for us.
259 *
260 * If cpu hotplug is not supported then this code can go in init section
261 * which will be freed later
262 */
263
264#ifndef CONFIG_HOTPLUG_CPU
265.section .init.text,"ax",@progbits
266#endif
267
268#ifdef CONFIG_SMP
269ENTRY(startup_32_smp)
270	cld
271	movl $(__BOOT_DS),%eax
272	movl %eax,%ds
273	movl %eax,%es
274	movl %eax,%fs
275	movl %eax,%gs
276#endif /* CONFIG_SMP */
2773:
278
279/*
280 *	New page tables may be in 4Mbyte page mode and may
281 *	be using the global pages.
282 *
283 *	NOTE! If we are on a 486 we may have no cr4 at all!
284 *	So we do not try to touch it unless we really have
285 *	some bits in it to set.  This won't work if the BSP
286 *	implements cr4 but this AP does not -- very unlikely
287 *	but be warned!  The same applies to the pse feature
288 *	if not equally supported. --macro
289 *
290 *	NOTE! We have to correct for the fact that we're
291 *	not yet offset PAGE_OFFSET..
292 */
293#define cr4_bits pa(mmu_cr4_features)
294	movl cr4_bits,%edx
295	andl %edx,%edx
296	jz 6f
297	movl %cr4,%eax		# Turn on paging options (PSE,PAE,..)
298	orl %edx,%eax
299	movl %eax,%cr4
300
301	btl $5, %eax		# check if PAE is enabled
302	jnc 6f
303
304	/* Check if extended functions are implemented */
305	movl $0x80000000, %eax
306	cpuid
307	cmpl $0x80000000, %eax
308	jbe 6f
309	mov $0x80000001, %eax
310	cpuid
311	/* Execute Disable bit supported? */
312	btl $20, %edx
313	jnc 6f
314
315	/* Setup EFER (Extended Feature Enable Register) */
316	movl $0xc0000080, %ecx
317	rdmsr
318
319	btsl $11, %eax
320	/* Make changes effective */
321	wrmsr
322
3236:
324
325/*
326 * Enable paging
327 */
328	movl $pa(swapper_pg_dir),%eax
329	movl %eax,%cr3		/* set the page table pointer.. */
330	movl %cr0,%eax
331	orl  $X86_CR0_PG,%eax
332	movl %eax,%cr0		/* ..and set paging (PG) bit */
333	ljmp $__BOOT_CS,$1f	/* Clear prefetch and normalize %eip */
3341:
335	/* Set up the stack pointer */
336	lss stack_start,%esp
337
338/*
339 * Initialize eflags.  Some BIOS's leave bits like NT set.  This would
340 * confuse the debugger if this code is traced.
341 * XXX - best to initialize before switching to protected mode.
342 */
343	pushl $0
344	popfl
345
346#ifdef CONFIG_SMP
347	cmpb $0, ready
348	jz  1f				/* Initial CPU cleans BSS */
349	jmp checkCPUtype
3501:
351#endif /* CONFIG_SMP */
352
353/*
354 * start system 32-bit setup. We need to re-do some of the things done
355 * in 16-bit mode for the "real" operations.
356 */
357	call setup_idt
358
359checkCPUtype:
360
361	movl $-1,X86_CPUID		#  -1 for no CPUID initially
362
363/* check if it is 486 or 386. */
364/*
365 * XXX - this does a lot of unnecessary setup.  Alignment checks don't
366 * apply at our cpl of 0 and the stack ought to be aligned already, and
367 * we don't need to preserve eflags.
368 */
369
370	movb $3,X86		# at least 386
371	pushfl			# push EFLAGS
372	popl %eax		# get EFLAGS
373	movl %eax,%ecx		# save original EFLAGS
374	xorl $0x240000,%eax	# flip AC and ID bits in EFLAGS
375	pushl %eax		# copy to EFLAGS
376	popfl			# set EFLAGS
377	pushfl			# get new EFLAGS
378	popl %eax		# put it in eax
379	xorl %ecx,%eax		# change in flags
380	pushl %ecx		# restore original EFLAGS
381	popfl
382	testl $0x40000,%eax	# check if AC bit changed
383	je is386
384
385	movb $4,X86		# at least 486
386	testl $0x200000,%eax	# check if ID bit changed
387	je is486
388
389	/* get vendor info */
390	xorl %eax,%eax			# call CPUID with 0 -> return vendor ID
391	cpuid
392	movl %eax,X86_CPUID		# save CPUID level
393	movl %ebx,X86_VENDOR_ID		# lo 4 chars
394	movl %edx,X86_VENDOR_ID+4	# next 4 chars
395	movl %ecx,X86_VENDOR_ID+8	# last 4 chars
396
397	orl %eax,%eax			# do we have processor info as well?
398	je is486
399
400	movl $1,%eax		# Use the CPUID instruction to get CPU type
401	cpuid
402	movb %al,%cl		# save reg for future use
403	andb $0x0f,%ah		# mask processor family
404	movb %ah,X86
405	andb $0xf0,%al		# mask model
406	shrb $4,%al
407	movb %al,X86_MODEL
408	andb $0x0f,%cl		# mask mask revision
409	movb %cl,X86_MASK
410	movl %edx,X86_CAPABILITY
411
412is486:	movl $0x50022,%ecx	# set AM, WP, NE and MP
413	jmp 2f
414
415is386:	movl $2,%ecx		# set MP
4162:	movl %cr0,%eax
417	andl $0x80000011,%eax	# Save PG,PE,ET
418	orl %ecx,%eax
419	movl %eax,%cr0
420
421	call check_x87
422	lgdt early_gdt_descr
423	lidt idt_descr
424	ljmp $(__KERNEL_CS),$1f
4251:	movl $(__KERNEL_DS),%eax	# reload all the segment registers
426	movl %eax,%ss			# after changing gdt.
427
428	movl $(__USER_DS),%eax		# DS/ES contains default USER segment
429	movl %eax,%ds
430	movl %eax,%es
431
432	movl $(__KERNEL_PERCPU), %eax
433	movl %eax,%fs			# set this cpu's percpu
434
435#ifdef CONFIG_CC_STACKPROTECTOR
436	/*
437	 * The linker can't handle this by relocation.  Manually set
438	 * base address in stack canary segment descriptor.
439	 */
440	cmpb $0,ready
441	jne 1f
442	movl $per_cpu__gdt_page,%eax
443	movl $per_cpu__stack_canary,%ecx
444	subl $20, %ecx
445	movw %cx, 8 * GDT_ENTRY_STACK_CANARY + 2(%eax)
446	shrl $16, %ecx
447	movb %cl, 8 * GDT_ENTRY_STACK_CANARY + 4(%eax)
448	movb %ch, 8 * GDT_ENTRY_STACK_CANARY + 7(%eax)
4491:
450#endif
451	movl $(__KERNEL_STACK_CANARY),%eax
452	movl %eax,%gs
453
454	xorl %eax,%eax			# Clear LDT
455	lldt %ax
456
457	cld			# gcc2 wants the direction flag cleared at all times
458	pushl $0		# fake return address for unwinder
459#ifdef CONFIG_SMP
460	movb ready, %cl
461	movb $1, ready
462	cmpb $0,%cl		# the first CPU calls start_kernel
463	je   1f
464	movl (stack_start), %esp
4651:
466#endif /* CONFIG_SMP */
467	jmp *(initial_code)
468
469/*
470 * We depend on ET to be correct. This checks for 287/387.
471 */
472check_x87:
473	movb $0,X86_HARD_MATH
474	clts
475	fninit
476	fstsw %ax
477	cmpb $0,%al
478	je 1f
479	movl %cr0,%eax		/* no coprocessor: have to set bits */
480	xorl $4,%eax		/* set EM */
481	movl %eax,%cr0
482	ret
483	ALIGN
4841:	movb $1,X86_HARD_MATH
485	.byte 0xDB,0xE4		/* fsetpm for 287, ignored by 387 */
486	ret
487
488/*
489 *  setup_idt
490 *
491 *  sets up a idt with 256 entries pointing to
492 *  ignore_int, interrupt gates. It doesn't actually load
493 *  idt - that can be done only after paging has been enabled
494 *  and the kernel moved to PAGE_OFFSET. Interrupts
495 *  are enabled elsewhere, when we can be relatively
496 *  sure everything is ok.
497 *
498 *  Warning: %esi is live across this function.
499 */
500setup_idt:
501	lea ignore_int,%edx
502	movl $(__KERNEL_CS << 16),%eax
503	movw %dx,%ax		/* selector = 0x0010 = cs */
504	movw $0x8E00,%dx	/* interrupt gate - dpl=0, present */
505
506	lea idt_table,%edi
507	mov $256,%ecx
508rp_sidt:
509	movl %eax,(%edi)
510	movl %edx,4(%edi)
511	addl $8,%edi
512	dec %ecx
513	jne rp_sidt
514
515.macro	set_early_handler handler,trapno
516	lea \handler,%edx
517	movl $(__KERNEL_CS << 16),%eax
518	movw %dx,%ax
519	movw $0x8E00,%dx	/* interrupt gate - dpl=0, present */
520	lea idt_table,%edi
521	movl %eax,8*\trapno(%edi)
522	movl %edx,8*\trapno+4(%edi)
523.endm
524
525	set_early_handler handler=early_divide_err,trapno=0
526	set_early_handler handler=early_illegal_opcode,trapno=6
527	set_early_handler handler=early_protection_fault,trapno=13
528	set_early_handler handler=early_page_fault,trapno=14
529
530	ret
531
532early_divide_err:
533	xor %edx,%edx
534	pushl $0	/* fake errcode */
535	jmp early_fault
536
537early_illegal_opcode:
538	movl $6,%edx
539	pushl $0	/* fake errcode */
540	jmp early_fault
541
542early_protection_fault:
543	movl $13,%edx
544	jmp early_fault
545
546early_page_fault:
547	movl $14,%edx
548	jmp early_fault
549
550early_fault:
551	cld
552#ifdef CONFIG_PRINTK
553	pusha
554	movl $(__KERNEL_DS),%eax
555	movl %eax,%ds
556	movl %eax,%es
557	cmpl $2,early_recursion_flag
558	je hlt_loop
559	incl early_recursion_flag
560	movl %cr2,%eax
561	pushl %eax
562	pushl %edx		/* trapno */
563	pushl $fault_msg
564	call printk
565#endif
566	call dump_stack
567hlt_loop:
568	hlt
569	jmp hlt_loop
570
571/* This is the default interrupt "handler" :-) */
572	ALIGN
573ignore_int:
574	cld
575#ifdef CONFIG_PRINTK
576	pushl %eax
577	pushl %ecx
578	pushl %edx
579	pushl %es
580	pushl %ds
581	movl $(__KERNEL_DS),%eax
582	movl %eax,%ds
583	movl %eax,%es
584	cmpl $2,early_recursion_flag
585	je hlt_loop
586	incl early_recursion_flag
587	pushl 16(%esp)
588	pushl 24(%esp)
589	pushl 32(%esp)
590	pushl 40(%esp)
591	pushl $int_msg
592	call printk
593
594	call dump_stack
595
596	addl $(5*4),%esp
597	popl %ds
598	popl %es
599	popl %edx
600	popl %ecx
601	popl %eax
602#endif
603	iret
604
605.section .cpuinit.data,"wa"
606.align 4
607ENTRY(initial_code)
608	.long i386_start_kernel
609
610/*
611 * BSS section
612 */
613.section ".bss.page_aligned","wa"
614	.align PAGE_SIZE_asm
615#ifdef CONFIG_X86_PAE
616swapper_pg_pmd:
617	.fill 1024*KPMDS,4,0
618#else
619ENTRY(swapper_pg_dir)
620	.fill 1024,4,0
621#endif
622swapper_pg_fixmap:
623	.fill 1024,4,0
624ENTRY(empty_zero_page)
625	.fill 4096,1,0
626
627/*
628 * This starts the data section.
629 */
630#ifdef CONFIG_X86_PAE
631.section ".data.page_aligned","wa"
632	/* Page-aligned for the benefit of paravirt? */
633	.align PAGE_SIZE_asm
634ENTRY(swapper_pg_dir)
635	.long	pa(swapper_pg_pmd+PGD_IDENT_ATTR),0	/* low identity map */
636# if KPMDS == 3
637	.long	pa(swapper_pg_pmd+PGD_IDENT_ATTR),0
638	.long	pa(swapper_pg_pmd+PGD_IDENT_ATTR+0x1000),0
639	.long	pa(swapper_pg_pmd+PGD_IDENT_ATTR+0x2000),0
640# elif KPMDS == 2
641	.long	0,0
642	.long	pa(swapper_pg_pmd+PGD_IDENT_ATTR),0
643	.long	pa(swapper_pg_pmd+PGD_IDENT_ATTR+0x1000),0
644# elif KPMDS == 1
645	.long	0,0
646	.long	0,0
647	.long	pa(swapper_pg_pmd+PGD_IDENT_ATTR),0
648# else
649#  error "Kernel PMDs should be 1, 2 or 3"
650# endif
651	.align PAGE_SIZE_asm		/* needs to be page-sized too */
652#endif
653
654.data
655ENTRY(stack_start)
656	.long init_thread_union+THREAD_SIZE
657	.long __BOOT_DS
658
659ready:	.byte 0
660
661early_recursion_flag:
662	.long 0
663
664int_msg:
665	.asciz "Unknown interrupt or fault at: %p %p %p\n"
666
667fault_msg:
668/* fault info: */
669	.ascii "BUG: Int %d: CR2 %p\n"
670/* pusha regs: */
671	.ascii "     EDI %p  ESI %p  EBP %p  ESP %p\n"
672	.ascii "     EBX %p  EDX %p  ECX %p  EAX %p\n"
673/* fault frame: */
674	.ascii "     err %p  EIP %p   CS %p  flg %p\n"
675	.ascii "Stack: %p %p %p %p %p %p %p %p\n"
676	.ascii "       %p %p %p %p %p %p %p %p\n"
677	.asciz "       %p %p %p %p %p %p %p %p\n"
678
679#include "../../x86/xen/xen-head.S"
680
681/*
682 * The IDT and GDT 'descriptors' are a strange 48-bit object
683 * only used by the lidt and lgdt instructions. They are not
684 * like usual segment descriptors - they consist of a 16-bit
685 * segment size, and 32-bit linear address value:
686 */
687
688.globl boot_gdt_descr
689.globl idt_descr
690
691	ALIGN
692# early boot GDT descriptor (must use 1:1 address mapping)
693	.word 0				# 32 bit align gdt_desc.address
694boot_gdt_descr:
695	.word __BOOT_DS+7
696	.long boot_gdt - __PAGE_OFFSET
697
698	.word 0				# 32-bit align idt_desc.address
699idt_descr:
700	.word IDT_ENTRIES*8-1		# idt contains 256 entries
701	.long idt_table
702
703# boot GDT descriptor (later on used by CPU#0):
704	.word 0				# 32 bit align gdt_desc.address
705ENTRY(early_gdt_descr)
706	.word GDT_ENTRIES*8-1
707	.long per_cpu__gdt_page		/* Overwritten for secondary CPUs */
708
709/*
710 * The boot_gdt must mirror the equivalent in setup.S and is
711 * used only for booting.
712 */
713	.align L1_CACHE_BYTES
714ENTRY(boot_gdt)
715	.fill GDT_ENTRY_BOOT_CS,8,0
716	.quad 0x00cf9a000000ffff	/* kernel 4GB code at 0x00000000 */
717	.quad 0x00cf92000000ffff	/* kernel 4GB data at 0x00000000 */
718