xref: /openbmc/linux/arch/x86/kernel/head_32.S (revision 22246614) 
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.h>
15#include <asm/pgtable.h>
16#include <asm/desc.h>
17#include <asm/cache.h>
18#include <asm/thread_info.h>
19#include <asm/asm-offsets.h>
20#include <asm/setup.h>
21#include <asm/processor-flags.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 *  - one bit for each possible page, but only in low memory, which means
44 *     2^32/4096/8 = 128K worst case (4G/4G split.)
45 *  - enough space to map all low memory, which means
46 *     (2^32/4096) / 1024 pages (worst case, non PAE)
47 *     (2^32/4096) / 512 + 4 pages (worst case for PAE)
48 *  - a few pages for allocator use before the kernel pagetable has
49 *     been set up
50 *
51 * Modulo rounding, each megabyte assigned here requires a kilobyte of
52 * memory, which is currently unreclaimed.
53 *
54 * This should be a multiple of a page.
55 */
56LOW_PAGES = 1<<(32-PAGE_SHIFT_asm)
57
58/*
59 * To preserve the DMA pool in PAGEALLOC kernels, we'll allocate
60 * pagetables from above the 16MB DMA limit, so we'll have to set
61 * up pagetables 16MB more (worst-case):
62 */
63#ifdef CONFIG_DEBUG_PAGEALLOC
64LOW_PAGES = LOW_PAGES + 0x1000000
65#endif
66
67#if PTRS_PER_PMD > 1
68PAGE_TABLE_SIZE = (LOW_PAGES / PTRS_PER_PMD) + PTRS_PER_PGD
69#else
70PAGE_TABLE_SIZE = (LOW_PAGES / PTRS_PER_PGD)
71#endif
72BOOTBITMAP_SIZE = LOW_PAGES / 8
73ALLOCATOR_SLOP = 4
74
75INIT_MAP_BEYOND_END = BOOTBITMAP_SIZE + (PAGE_TABLE_SIZE + ALLOCATOR_SLOP)*PAGE_SIZE_asm
76
77/*
78 * 32-bit kernel entrypoint; only used by the boot CPU.  On entry,
79 * %esi points to the real-mode code as a 32-bit pointer.
80 * CS and DS must be 4 GB flat segments, but we don't depend on
81 * any particular GDT layout, because we load our own as soon as we
82 * can.
83 */
84.section .text.head,"ax",@progbits
85ENTRY(startup_32)
86	/* test KEEP_SEGMENTS flag to see if the bootloader is asking
87		us to not reload segments */
88	testb $(1<<6), BP_loadflags(%esi)
89	jnz 2f
90
91/*
92 * Set segments to known values.
93 */
94	lgdt pa(boot_gdt_descr)
95	movl $(__BOOT_DS),%eax
96	movl %eax,%ds
97	movl %eax,%es
98	movl %eax,%fs
99	movl %eax,%gs
1002:
101
102/*
103 * Clear BSS first so that there are no surprises...
104 */
105	cld
106	xorl %eax,%eax
107	movl $pa(__bss_start),%edi
108	movl $pa(__bss_stop),%ecx
109	subl %edi,%ecx
110	shrl $2,%ecx
111	rep ; stosl
112/*
113 * Copy bootup parameters out of the way.
114 * Note: %esi still has the pointer to the real-mode data.
115 * With the kexec as boot loader, parameter segment might be loaded beyond
116 * kernel image and might not even be addressable by early boot page tables.
117 * (kexec on panic case). Hence copy out the parameters before initializing
118 * page tables.
119 */
120	movl $pa(boot_params),%edi
121	movl $(PARAM_SIZE/4),%ecx
122	cld
123	rep
124	movsl
125	movl pa(boot_params) + NEW_CL_POINTER,%esi
126	andl %esi,%esi
127	jz 1f			# No comand line
128	movl $pa(boot_command_line),%edi
129	movl $(COMMAND_LINE_SIZE/4),%ecx
130	rep
131	movsl
1321:
133
134#ifdef CONFIG_PARAVIRT
135	/* This is can only trip for a broken bootloader... */
136	cmpw $0x207, pa(boot_params + BP_version)
137	jb default_entry
138
139	/* Paravirt-compatible boot parameters.  Look to see what architecture
140		we're booting under. */
141	movl pa(boot_params + BP_hardware_subarch), %eax
142	cmpl $num_subarch_entries, %eax
143	jae bad_subarch
144
145	movl pa(subarch_entries)(,%eax,4), %eax
146	subl $__PAGE_OFFSET, %eax
147	jmp *%eax
148
149bad_subarch:
150WEAK(lguest_entry)
151WEAK(xen_entry)
152	/* Unknown implementation; there's really
153	   nothing we can do at this point. */
154	ud2a
155
156	__INITDATA
157
158subarch_entries:
159	.long default_entry		/* normal x86/PC */
160	.long lguest_entry		/* lguest hypervisor */
161	.long xen_entry			/* Xen hypervisor */
162num_subarch_entries = (. - subarch_entries) / 4
163.previous
164#endif /* CONFIG_PARAVIRT */
165
166/*
167 * Initialize page tables.  This creates a PDE and a set of page
168 * tables, which are located immediately beyond _end.  The variable
169 * init_pg_tables_end is set up to point to the first "safe" location.
170 * Mappings are created both at virtual address 0 (identity mapping)
171 * and PAGE_OFFSET for up to _end+sizeof(page tables)+INIT_MAP_BEYOND_END.
172 *
173 * Note that the stack is not yet set up!
174 */
175#define PTE_ATTR	0x007		/* PRESENT+RW+USER */
176#define PDE_ATTR	0x067		/* PRESENT+RW+USER+DIRTY+ACCESSED */
177#define PGD_ATTR	0x001		/* PRESENT (no other attributes) */
178
179default_entry:
180#ifdef CONFIG_X86_PAE
181
182	/*
183	 * In PAE mode swapper_pg_dir is statically defined to contain enough
184	 * entries to cover the VMSPLIT option (that is the top 1, 2 or 3
185	 * entries). The identity mapping is handled by pointing two PGD
186	 * entries to the first kernel PMD.
187	 *
188	 * Note the upper half of each PMD or PTE are always zero at
189	 * this stage.
190	 */
191
192#define KPMDS ((0x100000000-__PAGE_OFFSET) >> 30) /* Number of kernel PMDs */
193
194	xorl %ebx,%ebx				/* %ebx is kept at zero */
195
196	movl $pa(pg0), %edi
197	movl $pa(swapper_pg_pmd), %edx
198	movl $PTE_ATTR, %eax
19910:
200	leal PDE_ATTR(%edi),%ecx		/* Create PMD entry */
201	movl %ecx,(%edx)			/* Store PMD entry */
202						/* Upper half already zero */
203	addl $8,%edx
204	movl $512,%ecx
20511:
206	stosl
207	xchgl %eax,%ebx
208	stosl
209	xchgl %eax,%ebx
210	addl $0x1000,%eax
211	loop 11b
212
213	/*
214	 * End condition: we must map up to and including INIT_MAP_BEYOND_END
215	 * bytes beyond the end of our own page tables.
216	 */
217	leal (INIT_MAP_BEYOND_END+PTE_ATTR)(%edi),%ebp
218	cmpl %ebp,%eax
219	jb 10b
2201:
221	movl %edi,pa(init_pg_tables_end)
222
223	/* Do early initialization of the fixmap area */
224	movl $pa(swapper_pg_fixmap)+PDE_ATTR,%eax
225	movl %eax,pa(swapper_pg_pmd+0x1000*KPMDS-8)
226#else	/* Not PAE */
227
228page_pde_offset = (__PAGE_OFFSET >> 20);
229
230	movl $pa(pg0), %edi
231	movl $pa(swapper_pg_dir), %edx
232	movl $PTE_ATTR, %eax
23310:
234	leal PDE_ATTR(%edi),%ecx		/* Create PDE entry */
235	movl %ecx,(%edx)			/* Store identity PDE entry */
236	movl %ecx,page_pde_offset(%edx)		/* Store kernel PDE entry */
237	addl $4,%edx
238	movl $1024, %ecx
23911:
240	stosl
241	addl $0x1000,%eax
242	loop 11b
243	/*
244	 * End condition: we must map up to and including INIT_MAP_BEYOND_END
245	 * bytes beyond the end of our own page tables; the +0x007 is
246	 * the attribute bits
247	 */
248	leal (INIT_MAP_BEYOND_END+PTE_ATTR)(%edi),%ebp
249	cmpl %ebp,%eax
250	jb 10b
251	movl %edi,pa(init_pg_tables_end)
252
253	/* Do early initialization of the fixmap area */
254	movl $pa(swapper_pg_fixmap)+PDE_ATTR,%eax
255	movl %eax,pa(swapper_pg_dir+0xffc)
256#endif
257	jmp 3f
258/*
259 * Non-boot CPU entry point; entered from trampoline.S
260 * We can't lgdt here, because lgdt itself uses a data segment, but
261 * we know the trampoline has already loaded the boot_gdt for us.
262 *
263 * If cpu hotplug is not supported then this code can go in init section
264 * which will be freed later
265 */
266
267#ifndef CONFIG_HOTPLUG_CPU
268.section .init.text,"ax",@progbits
269#endif
270
271#ifdef CONFIG_SMP
272ENTRY(startup_32_smp)
273	cld
274	movl $(__BOOT_DS),%eax
275	movl %eax,%ds
276	movl %eax,%es
277	movl %eax,%fs
278	movl %eax,%gs
279#endif /* CONFIG_SMP */
2803:
281
282/*
283 *	New page tables may be in 4Mbyte page mode and may
284 *	be using the global pages.
285 *
286 *	NOTE! If we are on a 486 we may have no cr4 at all!
287 *	So we do not try to touch it unless we really have
288 *	some bits in it to set.  This won't work if the BSP
289 *	implements cr4 but this AP does not -- very unlikely
290 *	but be warned!  The same applies to the pse feature
291 *	if not equally supported. --macro
292 *
293 *	NOTE! We have to correct for the fact that we're
294 *	not yet offset PAGE_OFFSET..
295 */
296#define cr4_bits pa(mmu_cr4_features)
297	movl cr4_bits,%edx
298	andl %edx,%edx
299	jz 6f
300	movl %cr4,%eax		# Turn on paging options (PSE,PAE,..)
301	orl %edx,%eax
302	movl %eax,%cr4
303
304	btl $5, %eax		# check if PAE is enabled
305	jnc 6f
306
307	/* Check if extended functions are implemented */
308	movl $0x80000000, %eax
309	cpuid
310	cmpl $0x80000000, %eax
311	jbe 6f
312	mov $0x80000001, %eax
313	cpuid
314	/* Execute Disable bit supported? */
315	btl $20, %edx
316	jnc 6f
317
318	/* Setup EFER (Extended Feature Enable Register) */
319	movl $0xc0000080, %ecx
320	rdmsr
321
322	btsl $11, %eax
323	/* Make changes effective */
324	wrmsr
325
3266:
327
328/*
329 * Enable paging
330 */
331	movl $pa(swapper_pg_dir),%eax
332	movl %eax,%cr3		/* set the page table pointer.. */
333	movl %cr0,%eax
334	orl  $X86_CR0_PG,%eax
335	movl %eax,%cr0		/* ..and set paging (PG) bit */
336	ljmp $__BOOT_CS,$1f	/* Clear prefetch and normalize %eip */
3371:
338	/* Set up the stack pointer */
339	lss stack_start,%esp
340
341/*
342 * Initialize eflags.  Some BIOS's leave bits like NT set.  This would
343 * confuse the debugger if this code is traced.
344 * XXX - best to initialize before switching to protected mode.
345 */
346	pushl $0
347	popfl
348
349#ifdef CONFIG_SMP
350	cmpb $0, ready
351	jz  1f				/* Initial CPU cleans BSS */
352	jmp checkCPUtype
3531:
354#endif /* CONFIG_SMP */
355
356/*
357 * start system 32-bit setup. We need to re-do some of the things done
358 * in 16-bit mode for the "real" operations.
359 */
360	call setup_idt
361
362checkCPUtype:
363
364	movl $-1,X86_CPUID		#  -1 for no CPUID initially
365
366/* check if it is 486 or 386. */
367/*
368 * XXX - this does a lot of unnecessary setup.  Alignment checks don't
369 * apply at our cpl of 0 and the stack ought to be aligned already, and
370 * we don't need to preserve eflags.
371 */
372
373	movb $3,X86		# at least 386
374	pushfl			# push EFLAGS
375	popl %eax		# get EFLAGS
376	movl %eax,%ecx		# save original EFLAGS
377	xorl $0x240000,%eax	# flip AC and ID bits in EFLAGS
378	pushl %eax		# copy to EFLAGS
379	popfl			# set EFLAGS
380	pushfl			# get new EFLAGS
381	popl %eax		# put it in eax
382	xorl %ecx,%eax		# change in flags
383	pushl %ecx		# restore original EFLAGS
384	popfl
385	testl $0x40000,%eax	# check if AC bit changed
386	je is386
387
388	movb $4,X86		# at least 486
389	testl $0x200000,%eax	# check if ID bit changed
390	je is486
391
392	/* get vendor info */
393	xorl %eax,%eax			# call CPUID with 0 -> return vendor ID
394	cpuid
395	movl %eax,X86_CPUID		# save CPUID level
396	movl %ebx,X86_VENDOR_ID		# lo 4 chars
397	movl %edx,X86_VENDOR_ID+4	# next 4 chars
398	movl %ecx,X86_VENDOR_ID+8	# last 4 chars
399
400	orl %eax,%eax			# do we have processor info as well?
401	je is486
402
403	movl $1,%eax		# Use the CPUID instruction to get CPU type
404	cpuid
405	movb %al,%cl		# save reg for future use
406	andb $0x0f,%ah		# mask processor family
407	movb %ah,X86
408	andb $0xf0,%al		# mask model
409	shrb $4,%al
410	movb %al,X86_MODEL
411	andb $0x0f,%cl		# mask mask revision
412	movb %cl,X86_MASK
413	movl %edx,X86_CAPABILITY
414
415is486:	movl $0x50022,%ecx	# set AM, WP, NE and MP
416	jmp 2f
417
418is386:	movl $2,%ecx		# set MP
4192:	movl %cr0,%eax
420	andl $0x80000011,%eax	# Save PG,PE,ET
421	orl %ecx,%eax
422	movl %eax,%cr0
423
424	call check_x87
425	lgdt early_gdt_descr
426	lidt idt_descr
427	ljmp $(__KERNEL_CS),$1f
4281:	movl $(__KERNEL_DS),%eax	# reload all the segment registers
429	movl %eax,%ss			# after changing gdt.
430	movl %eax,%fs			# gets reset once there's real percpu
431
432	movl $(__USER_DS),%eax		# DS/ES contains default USER segment
433	movl %eax,%ds
434	movl %eax,%es
435
436	xorl %eax,%eax			# Clear GS and LDT
437	movl %eax,%gs
438	lldt %ax
439
440	cld			# gcc2 wants the direction flag cleared at all times
441	pushl $0		# fake return address for unwinder
442#ifdef CONFIG_SMP
443	movb ready, %cl
444	movb $1, ready
445	cmpb $0,%cl		# the first CPU calls start_kernel
446	je   1f
447	movl $(__KERNEL_PERCPU), %eax
448	movl %eax,%fs		# set this cpu's percpu
449	jmp initialize_secondary # all other CPUs call initialize_secondary
4501:
451#endif /* CONFIG_SMP */
452	jmp i386_start_kernel
453
454/*
455 * We depend on ET to be correct. This checks for 287/387.
456 */
457check_x87:
458	movb $0,X86_HARD_MATH
459	clts
460	fninit
461	fstsw %ax
462	cmpb $0,%al
463	je 1f
464	movl %cr0,%eax		/* no coprocessor: have to set bits */
465	xorl $4,%eax		/* set EM */
466	movl %eax,%cr0
467	ret
468	ALIGN
4691:	movb $1,X86_HARD_MATH
470	.byte 0xDB,0xE4		/* fsetpm for 287, ignored by 387 */
471	ret
472
473/*
474 *  setup_idt
475 *
476 *  sets up a idt with 256 entries pointing to
477 *  ignore_int, interrupt gates. It doesn't actually load
478 *  idt - that can be done only after paging has been enabled
479 *  and the kernel moved to PAGE_OFFSET. Interrupts
480 *  are enabled elsewhere, when we can be relatively
481 *  sure everything is ok.
482 *
483 *  Warning: %esi is live across this function.
484 */
485setup_idt:
486	lea ignore_int,%edx
487	movl $(__KERNEL_CS << 16),%eax
488	movw %dx,%ax		/* selector = 0x0010 = cs */
489	movw $0x8E00,%dx	/* interrupt gate - dpl=0, present */
490
491	lea idt_table,%edi
492	mov $256,%ecx
493rp_sidt:
494	movl %eax,(%edi)
495	movl %edx,4(%edi)
496	addl $8,%edi
497	dec %ecx
498	jne rp_sidt
499
500.macro	set_early_handler handler,trapno
501	lea \handler,%edx
502	movl $(__KERNEL_CS << 16),%eax
503	movw %dx,%ax
504	movw $0x8E00,%dx	/* interrupt gate - dpl=0, present */
505	lea idt_table,%edi
506	movl %eax,8*\trapno(%edi)
507	movl %edx,8*\trapno+4(%edi)
508.endm
509
510	set_early_handler handler=early_divide_err,trapno=0
511	set_early_handler handler=early_illegal_opcode,trapno=6
512	set_early_handler handler=early_protection_fault,trapno=13
513	set_early_handler handler=early_page_fault,trapno=14
514
515	ret
516
517early_divide_err:
518	xor %edx,%edx
519	pushl $0	/* fake errcode */
520	jmp early_fault
521
522early_illegal_opcode:
523	movl $6,%edx
524	pushl $0	/* fake errcode */
525	jmp early_fault
526
527early_protection_fault:
528	movl $13,%edx
529	jmp early_fault
530
531early_page_fault:
532	movl $14,%edx
533	jmp early_fault
534
535early_fault:
536	cld
537#ifdef CONFIG_PRINTK
538	pusha
539	movl $(__KERNEL_DS),%eax
540	movl %eax,%ds
541	movl %eax,%es
542	cmpl $2,early_recursion_flag
543	je hlt_loop
544	incl early_recursion_flag
545	movl %cr2,%eax
546	pushl %eax
547	pushl %edx		/* trapno */
548	pushl $fault_msg
549#ifdef CONFIG_EARLY_PRINTK
550	call early_printk
551#else
552	call printk
553#endif
554#endif
555	call dump_stack
556hlt_loop:
557	hlt
558	jmp hlt_loop
559
560/* This is the default interrupt "handler" :-) */
561	ALIGN
562ignore_int:
563	cld
564#ifdef CONFIG_PRINTK
565	pushl %eax
566	pushl %ecx
567	pushl %edx
568	pushl %es
569	pushl %ds
570	movl $(__KERNEL_DS),%eax
571	movl %eax,%ds
572	movl %eax,%es
573	cmpl $2,early_recursion_flag
574	je hlt_loop
575	incl early_recursion_flag
576	pushl 16(%esp)
577	pushl 24(%esp)
578	pushl 32(%esp)
579	pushl 40(%esp)
580	pushl $int_msg
581#ifdef CONFIG_EARLY_PRINTK
582	call early_printk
583#else
584	call printk
585#endif
586	addl $(5*4),%esp
587	popl %ds
588	popl %es
589	popl %edx
590	popl %ecx
591	popl %eax
592#endif
593	iret
594
595.section .text
596/*
597 * Real beginning of normal "text" segment
598 */
599ENTRY(stext)
600ENTRY(_stext)
601
602/*
603 * BSS section
604 */
605.section ".bss.page_aligned","wa"
606	.align PAGE_SIZE_asm
607#ifdef CONFIG_X86_PAE
608swapper_pg_pmd:
609	.fill 1024*KPMDS,4,0
610#else
611ENTRY(swapper_pg_dir)
612	.fill 1024,4,0
613#endif
614swapper_pg_fixmap:
615	.fill 1024,4,0
616ENTRY(empty_zero_page)
617	.fill 4096,1,0
618/*
619 * This starts the data section.
620 */
621#ifdef CONFIG_X86_PAE
622.section ".data.page_aligned","wa"
623	/* Page-aligned for the benefit of paravirt? */
624	.align PAGE_SIZE_asm
625ENTRY(swapper_pg_dir)
626	.long	pa(swapper_pg_pmd+PGD_ATTR),0		/* low identity map */
627# if KPMDS == 3
628	.long	pa(swapper_pg_pmd+PGD_ATTR),0
629	.long	pa(swapper_pg_pmd+PGD_ATTR+0x1000),0
630	.long	pa(swapper_pg_pmd+PGD_ATTR+0x2000),0
631# elif KPMDS == 2
632	.long	0,0
633	.long	pa(swapper_pg_pmd+PGD_ATTR),0
634	.long	pa(swapper_pg_pmd+PGD_ATTR+0x1000),0
635# elif KPMDS == 1
636	.long	0,0
637	.long	0,0
638	.long	pa(swapper_pg_pmd+PGD_ATTR),0
639# else
640#  error "Kernel PMDs should be 1, 2 or 3"
641# endif
642	.align PAGE_SIZE_asm		/* needs to be page-sized too */
643#endif
644
645.data
646ENTRY(stack_start)
647	.long init_thread_union+THREAD_SIZE
648	.long __BOOT_DS
649
650ready:	.byte 0
651
652early_recursion_flag:
653	.long 0
654
655int_msg:
656	.asciz "Unknown interrupt or fault at EIP %p %p %p\n"
657
658fault_msg:
659/* fault info: */
660	.ascii "BUG: Int %d: CR2 %p\n"
661/* pusha regs: */
662	.ascii "     EDI %p  ESI %p  EBP %p  ESP %p\n"
663	.ascii "     EBX %p  EDX %p  ECX %p  EAX %p\n"
664/* fault frame: */
665	.ascii "     err %p  EIP %p   CS %p  flg %p\n"
666	.ascii "Stack: %p %p %p %p %p %p %p %p\n"
667	.ascii "       %p %p %p %p %p %p %p %p\n"
668	.asciz "       %p %p %p %p %p %p %p %p\n"
669
670#include "../../x86/xen/xen-head.S"
671
672/*
673 * The IDT and GDT 'descriptors' are a strange 48-bit object
674 * only used by the lidt and lgdt instructions. They are not
675 * like usual segment descriptors - they consist of a 16-bit
676 * segment size, and 32-bit linear address value:
677 */
678
679.globl boot_gdt_descr
680.globl idt_descr
681
682	ALIGN
683# early boot GDT descriptor (must use 1:1 address mapping)
684	.word 0				# 32 bit align gdt_desc.address
685boot_gdt_descr:
686	.word __BOOT_DS+7
687	.long boot_gdt - __PAGE_OFFSET
688
689	.word 0				# 32-bit align idt_desc.address
690idt_descr:
691	.word IDT_ENTRIES*8-1		# idt contains 256 entries
692	.long idt_table
693
694# boot GDT descriptor (later on used by CPU#0):
695	.word 0				# 32 bit align gdt_desc.address
696ENTRY(early_gdt_descr)
697	.word GDT_ENTRIES*8-1
698	.long per_cpu__gdt_page		/* Overwritten for secondary CPUs */
699
700/*
701 * The boot_gdt must mirror the equivalent in setup.S and is
702 * used only for booting.
703 */
704	.align L1_CACHE_BYTES
705ENTRY(boot_gdt)
706	.fill GDT_ENTRY_BOOT_CS,8,0
707	.quad 0x00cf9a000000ffff	/* kernel 4GB code at 0x00000000 */
708	.quad 0x00cf92000000ffff	/* kernel 4GB data at 0x00000000 */
709