aboutsummaryrefslogtreecommitdiff
path: root/drivers/lguest/page_tables.c
blob: 04b22128a47421cac65a6dd73480ee56c87ded1d (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
/*P:700
 * The pagetable code, on the other hand, still shows the scars of
 * previous encounters.  It's functional, and as neat as it can be in the
 * circumstances, but be wary, for these things are subtle and break easily.
 * The Guest provides a virtual to physical mapping, but we can neither trust
 * it nor use it: we verify and convert it here then point the CPU to the
 * converted Guest pages when running the Guest.
:*/

/* Copyright (C) Rusty Russell IBM Corporation 2006.
 * GPL v2 and any later version */
#include <linux/mm.h>
#include <linux/gfp.h>
#include <linux/types.h>
#include <linux/spinlock.h>
#include <linux/random.h>
#include <linux/percpu.h>
#include <asm/tlbflush.h>
#include <asm/uaccess.h>
#include <asm/bootparam.h>
#include "lg.h"

/*M:008
 * We hold reference to pages, which prevents them from being swapped.
 * It'd be nice to have a callback in the "struct mm_struct" when Linux wants
 * to swap out.  If we had this, and a shrinker callback to trim PTE pages, we
 * could probably consider launching Guests as non-root.
:*/

/*H:300
 * The Page Table Code
 *
 * We use two-level page tables for the Guest, or three-level with PAE.  If
 * you're not entirely comfortable with virtual addresses, physical addresses
 * and page tables then I recommend you review arch/x86/lguest/boot.c's "Page
 * Table Handling" (with diagrams!).
 *
 * The Guest keeps page tables, but we maintain the actual ones here: these are
 * called "shadow" page tables.  Which is a very Guest-centric name: these are
 * the real page tables the CPU uses, although we keep them up to date to
 * reflect the Guest's.  (See what I mean about weird naming?  Since when do
 * shadows reflect anything?)
 *
 * Anyway, this is the most complicated part of the Host code.  There are seven
 * parts to this:
 *  (i) Looking up a page table entry when the Guest faults,
 *  (ii) Making sure the Guest stack is mapped,
 *  (iii) Setting up a page table entry when the Guest tells us one has changed,
 *  (iv) Switching page tables,
 *  (v) Flushing (throwing away) page tables,
 *  (vi) Mapping the Switcher when the Guest is about to run,
 *  (vii) Setting up the page tables initially.
:*/

/*
 * The Switcher uses the complete top PTE page.  That's 1024 PTE entries (4MB)
 * or 512 PTE entries with PAE (2MB).
 */
#define SWITCHER_PGD_INDEX (PTRS_PER_PGD - 1)

/*
 * For PAE we need the PMD index as well. We use the last 2MB, so we
 * will need the last pmd entry of the last pmd page.
 */
#ifdef CONFIG_X86_PAE
#define SWITCHER_PMD_INDEX 	(PTRS_PER_PMD - 1)
#define RESERVE_MEM 		2U
#define CHECK_GPGD_MASK		_PAGE_PRESENT
#else
#define RESERVE_MEM 		4U
#define CHECK_GPGD_MASK		_PAGE_TABLE
#endif

/*
 * We actually need a separate PTE page for each CPU.  Remember that after the
 * Switcher code itself comes two pages for each CPU, and we don't want this
 * CPU's guest to see the pages of any other CPU.
 */
static DEFINE_PER_CPU(pte_t *, switcher_pte_pages);
#define switcher_pte_page(cpu) per_cpu(switcher_pte_pages, cpu)

/*H:320
 * The page table code is curly enough to need helper functions to keep it
 * clear and clean.  The kernel itself provides many of them; one advantage
 * of insisting that the Guest and Host use the same CONFIG_PAE setting.
 *
 * There are two functions which return pointers to the shadow (aka "real")
 * page tables.
 *
 * spgd_addr() takes the virtual address and returns a pointer to the top-level
 * page directory entry (PGD) for that address.  Since we keep track of several
 * page tables, the "i" argument tells us which one we're interested in (it's
 * usually the current one).
 */
static pgd_t *spgd_addr(struct lg_cpu *cpu, u32 i, unsigned long vaddr)
{
	unsigned int index = pgd_index(vaddr);

#ifndef CONFIG_X86_PAE
	/* We kill any Guest trying to touch the Switcher addresses. */
	if (index >= SWITCHER_PGD_INDEX) {
		kill_guest(cpu, "attempt to access switcher pages");
		index = 0;
	}
#endif
	/* Return a pointer index'th pgd entry for the i'th page table. */
	return &cpu->lg->pgdirs[i].pgdir[index];
}

#ifdef CONFIG_X86_PAE
/*
 * This routine then takes the PGD entry given above, which contains the
 * address of the PMD page.  It then returns a pointer to the PMD entry for the
 * given address.
 */
static pmd_t *spmd_addr(struct lg_cpu *cpu, pgd_t spgd, unsigned long vaddr)
{
	unsigned int index = pmd_index(vaddr);
	pmd_t *page;

	/* We kill any Guest trying to touch the Switcher addresses. */
	if (pgd_index(vaddr) == SWITCHER_PGD_INDEX &&
					index >= SWITCHER_PMD_INDEX) {
		kill_guest(cpu, "attempt to access switcher pages");
		index = 0;
	}

	/* You should never call this if the PGD entry wasn't valid */
	BUG_ON(!(pgd_flags(spgd) & _PAGE_PRESENT));
	page = __va(pgd_pfn(spgd) << PAGE_SHIFT);

	return &page[index];
}
#endif

/*
 * This routine then takes the page directory entry returned above, which
 * contains the address of the page table entry (PTE) page.  It then returns a
 * pointer to the PTE entry for the given address.
 */
static pte_t *spte_addr(struct lg_cpu *cpu, pgd_t spgd, unsigned long vaddr)
{
#ifdef CONFIG_X86_PAE
	pmd_t *pmd = spmd_addr(cpu, spgd, vaddr);
	pte_t *page = __va(pmd_pfn(*pmd) << PAGE_SHIFT);

	/* You should never call this if the PMD entry wasn't valid */
	BUG_ON(!(pmd_flags(*pmd) & _PAGE_PRESENT));
#else
	pte_t *page = __va(pgd_pfn(spgd) << PAGE_SHIFT);
	/* You should never call this if the PGD entry wasn't valid */
	BUG_ON(!(pgd_flags(spgd) & _PAGE_PRESENT));
#endif

	return &page[pte_index(vaddr)];
}

/*
 * These functions are just like the above two, except they access the Guest
 * page tables.  Hence they return a Guest address.
 */
static unsigned long gpgd_addr(struct lg_cpu *cpu, unsigned long vaddr)
{
	unsigned int index = vaddr >> (PGDIR_SHIFT);
	return cpu->lg->pgdirs[cpu->cpu_pgd].gpgdir + index * sizeof(pgd_t);
}

#ifdef CONFIG_X86_PAE
/* Follow the PGD to the PMD. */
static unsigned long gpmd_addr(pgd_t gpgd, unsigned long vaddr)
{
	unsigned long gpage = pgd_pfn(gpgd) << PAGE_SHIFT;
	BUG_ON(!(pgd_flags(gpgd) & _PAGE_PRESENT));
	return gpage + pmd_index(vaddr) * sizeof(pmd_t);
}

/* Follow the PMD to the PTE. */
static unsigned long gpte_addr(struct lg_cpu *cpu,
			       pmd_t gpmd, unsigned long vaddr)
{
	unsigned long gpage = pmd_pfn(gpmd) << PAGE_SHIFT;

	BUG_ON(!(pmd_flags(gpmd) & _PAGE_PRESENT));
	return gpage + pte_index(vaddr) * sizeof(pte_t);
}
#else
/* Follow the PGD to the PTE (no mid-level for !PAE). */
static unsigned long gpte_addr(struct lg_cpu *cpu,
				pgd_t gpgd, unsigned long vaddr)
{
	unsigned long gpage = pgd_pfn(gpgd) << PAGE_SHIFT;

	BUG_ON(!(pgd_flags(gpgd) & _PAGE_PRESENT));
	return gpage + pte_index(vaddr) * sizeof(pte_t);
}
#endif
/*:*/

/*M:014
 * get_pfn is slow: we could probably try to grab batches of pages here as
 * an optimization (ie. pre-faulting).
:*/

/*H:350
 * This routine takes a page number given by the Guest and converts it to
 * an actual, physical page number.  It can fail for several reasons: the
 * virtual address might not be mapped by the Launcher, the write flag is set
 * and the page is read-only, or the write flag was set and the page was
 * shared so had to be copied, but we ran out of memory.
 *
 * This holds a reference to the page, so release_pte() is careful to put that
 * back.
 */
static unsigned long get_pfn(unsigned long virtpfn, int write)
{
	struct page *page;

	/* gup me one page at this address please! */
	if (get_user_pages_fast(virtpfn << PAGE_SHIFT, 1, write, &page) == 1)
		return page_to_pfn(page);

	/* This value indicates failure. */
	return -1UL;
}

/*H:340
 * Converting a Guest page table entry to a shadow (ie. real) page table
 * entry can be a little tricky.  The flags are (almost) the same, but the
 * Guest PTE contains a virtual page number: the CPU needs the real page
 * number.
 */
static pte_t gpte_to_spte(struct lg_cpu *cpu, pte_t gpte, int write)
{
	unsigned long pfn, base, flags;

	/*
	 * The Guest sets the global flag, because it thinks that it is using
	 * PGE.  We only told it to use PGE so it would tell us whether it was
	 * flushing a kernel mapping or a userspace mapping.  We don't actually
	 * use the global bit, so throw it away.
	 */
	flags = (pte_flags(gpte) & ~_PAGE_GLOBAL);

	/* The Guest's pages are offset inside the Launcher. */
	base = (unsigned long)cpu->lg->mem_base / PAGE_SIZE;

	/*
	 * We need a temporary "unsigned long" variable to hold the answer from
	 * get_pfn(), because it returns 0xFFFFFFFF on failure, which wouldn't
	 * fit in spte.pfn.  get_pfn() finds the real physical number of the
	 * page, given the virtual number.
	 */
	pfn = get_pfn(base + pte_pfn(gpte), write);
	if (pfn == -1UL) {
		kill_guest(cpu, "failed to get page %lu", pte_pfn(gpte));
		/*
		 * When we destroy the Guest, we'll go through the shadow page
		 * tables and release_pte() them.  Make sure we don't think
		 * this one is valid!
		 */
		flags = 0;
	}
	/* Now we assemble our shadow PTE from the page number and flags. */
	return pfn_pte(pfn, __pgprot(flags));
}

/*H:460 And to complete the chain, release_pte() looks like this: */
static void release_pte(pte_t pte)
{
	/*
	 * Remember that get_user_pages_fast() took a reference to the page, in
	 * get_pfn()?  We have to put it back now.
	 */
	if (pte_flags(pte) & _PAGE_PRESENT)
		put_page(pte_page(pte));
}
/*:*/

static void check_gpte(struct lg_cpu *cpu, pte_t gpte)
{
	if ((pte_flags(gpte) & _PAGE_PSE) ||
	    pte_pfn(gpte) >= cpu->lg->pfn_limit)
		kill_guest(cpu, "bad page table entry");
}

static void check_gpgd(struct lg_cpu *cpu, pgd_t gpgd)
{
	if ((pgd_flags(gpgd) & ~CHECK_GPGD_MASK) ||
	   (pgd_pfn(gpgd) >= cpu->lg->pfn_limit))
		kill_guest(cpu, "bad page directory entry");
}

#ifdef CONFIG_X86_PAE
static void check_gpmd(struct lg_cpu *cpu, pmd_t gpmd)
{
	if ((pmd_flags(gpmd) & ~_PAGE_TABLE) ||
	   (pmd_pfn(gpmd) >= cpu->lg->pfn_limit))
		kill_guest(cpu, "bad page middle directory entry");
}
#endif

/*H:330
 * (i) Looking up a page table entry when the Guest faults.
 *
 * We saw this call in run_guest(): when we see a page fault in the Guest, we
 * come here.  That's because we only set up the shadow page tables lazily as
 * they're needed, so we get page faults all the time and quietly fix them up
 * and return to the Guest without it knowing.
 *
 * If we fixed up the fault (ie. we mapped the address), this routine returns
 * true.  Otherwise, it was a real fault and we need to tell the Guest.
 */
bool demand_page(struct lg_cpu *cpu, unsigned long vaddr, int errcode)
{
	pgd_t gpgd;
	pgd_t *spgd;
	unsigned long gpte_ptr;
	pte_t gpte;
	pte_t *spte;

	/* Mid level for PAE. */
#ifdef CONFIG_X86_PAE
	pmd_t *spmd;
	pmd_t gpmd;
#endif

	/* First step: get the top-level Guest page table entry. */
	gpgd = lgread(cpu, gpgd_addr(cpu, vaddr), pgd_t);
	/* Toplevel not present?  We can't map it in. */
	if (!(pgd_flags(gpgd) & _PAGE_PRESENT))
		return false;

	/* Now look at the matching shadow entry. */
	spgd = spgd_addr(cpu, cpu->cpu_pgd, vaddr);
	if (!(pgd_flags(*spgd) & _PAGE_PRESENT)) {
		/* No shadow entry: allocate a new shadow PTE page. */
		unsigned long ptepage = get_zeroed_page(GFP_KERNEL);
		/*
		 * This is not really the Guest's fault, but killing it is
		 * simple for this corner case.
		 */
		if (!ptepage) {
			kill_guest(cpu, "out of memory allocating pte page");
			return false;
		}
		/* We check that the Guest pgd is OK. */
		check_gpgd(cpu, gpgd);
		/*
		 * And we copy the flags to the shadow PGD entry.  The page
		 * number in the shadow PGD is the page we just allocated.
		 */
		set_pgd(spgd, __pgd(__pa(ptepage) | pgd_flags(gpgd)));
	}

#ifdef CONFIG_X86_PAE
	gpmd = lgread(cpu, gpmd_addr(gpgd, vaddr), pmd_t);
	/* Middle level not present?  We can't map it in. */
	if (!(pmd_flags(gpmd) & _PAGE_PRESENT))
		return false;

	/* Now look at the matching shadow entry. */
	spmd = spmd_addr(cpu, *spgd, vaddr);

	if (!(pmd_flags(*spmd) & _PAGE_PRESENT)) {
		/* No shadow entry: allocate a new shadow PTE page. */
		unsigned long ptepage = get_zeroed_page(GFP_KERNEL);

		/*
		 * This is not really the Guest's fault, but killing it is
		 * simple for this corner case.
		 */
		if (!ptepage) {
			kill_guest(cpu, "out of memory allocating pte page");
			return false;
		}

		/* We check that the Guest pmd is OK. */
		check_gpmd(cpu, gpmd);

		/*
		 * And we copy the flags to the shadow PMD entry.  The page
		 * number in the shadow PMD is the page we just allocated.
		 */
		set_pmd(spmd, __pmd(__pa(ptepage) | pmd_flags(gpmd)));
	}

	/*
	 * OK, now we look at the lower level in the Guest page table: keep its
	 * address, because we might update it later.
	 */
	gpte_ptr = gpte_addr(cpu, gpmd, vaddr);
#else
	/*
	 * OK, now we look at the lower level in the Guest page table: keep its
	 * address, because we might update it later.
	 */
	gpte_ptr = gpte_addr(cpu, gpgd, vaddr);
#endif

	/* Read the actual PTE value. */
	gpte = lgread(cpu, gpte_ptr, pte_t);

	/* If this page isn't in the Guest page tables, we can't page it in. */
	if (!(pte_flags(gpte) & _PAGE_PRESENT))
		return false;

	/*
	 * Check they're not trying to write to a page the Guest wants
	 * read-only (bit 2 of errcode == write).
	 */
	if ((errcode & 2) && !(pte_flags(gpte) & _PAGE_RW))
		return false;

	/* User access to a kernel-only page? (bit 3 == user access) */
	if ((errcode & 4) && !(pte_flags(gpte) & _PAGE_USER))
		return false;

	/*
	 * Check that the Guest PTE flags are OK, and the page number is below
	 * the pfn_limit (ie. not mapping the Launcher binary).
	 */
	check_gpte(cpu, gpte);

	/* Add the _PAGE_ACCESSED and (for a write) _PAGE_DIRTY flag */
	gpte = pte_mkyoung(gpte);
	if (errcode & 2)
		gpte = pte_mkdirty(gpte);

	/* Get the pointer to the shadow PTE entry we're going to set. */
	spte = spte_addr(cpu, *spgd, vaddr);

	/*
	 * If there was a valid shadow PTE entry here before, we release it.
	 * This can happen with a write to a previously read-only entry.
	 */
	release_pte(*spte);

	/*
	 * If this is a write, we insist that the Guest page is writable (the
	 * final arg to gpte_to_spte()).
	 */
	if (pte_dirty(gpte))
		*spte = gpte_to_spte(cpu, gpte, 1);
	else
		/*
		 * If this is a read, don't set the "writable" bit in the page
		 * table entry, even if the Guest says it's writable.  That way
		 * we will come back here when a write does actually occur, so
		 * we can update the Guest's _PAGE_DIRTY flag.
		 */
		set_pte(spte, gpte_to_spte(cpu, pte_wrprotect(gpte), 0));

	/*
	 * Finally, we write the Guest PTE entry back: we've set the
	 * _PAGE_ACCESSED and maybe the _PAGE_DIRTY flags.
	 */
	lgwrite(cpu, gpte_ptr, pte_t, gpte);

	/*
	 * The fault is fixed, the page table is populated, the mapping
	 * manipulated, the result returned and the code complete.  A small
	 * delay and a trace of alliteration are the only indications the Guest
	 * has that a page fault occurred at all.
	 */
	return true;
}

/*H:360
 * (ii) Making sure the Guest stack is mapped.
 *
 * Remember that direct traps into the Guest need a mapped Guest kernel stack.
 * pin_stack_pages() calls us here: we could simply call demand_page(), but as
 * we've seen that logic is quite long, and usually the stack pages are already
 * mapped, so it's overkill.
 *
 * This is a quick version which answers the question: is this virtual address
 * mapped by the shadow page tables, and is it writable?
 */
static bool page_writable(struct lg_cpu *cpu, unsigned long vaddr)
{
	pgd_t *spgd;
	unsigned long flags;

#ifdef CONFIG_X86_PAE
	pmd_t *spmd;
#endif
	/* Look at the current top level entry: is it present? */
	spgd = spgd_addr(cpu, cpu->cpu_pgd, vaddr);
	if (!(pgd_flags(*spgd) & _PAGE_PRESENT))
		return false;

#ifdef CONFIG_X86_PAE
	spmd = spmd_addr(cpu, *spgd, vaddr);
	if (!(pmd_flags(*spmd) & _PAGE_PRESENT))
		return false;
#endif

	/*
	 * Check the flags on the pte entry itself: it must be present and
	 * writable.
	 */
	flags = pte_flags(*(spte_addr(cpu, *spgd, vaddr)));

	return (flags & (_PAGE_PRESENT|_PAGE_RW)) == (_PAGE_PRESENT|_PAGE_RW);
}

/*
 * So, when pin_stack_pages() asks us to pin a page, we check if it's already
 * in the page tables, and if not, we call demand_page() with error code 2
 * (meaning "write").
 */
void pin_page(struct lg_cpu *cpu, unsigned long vaddr)
{
	if (!page_writable(cpu, vaddr) && !demand_page(cpu, vaddr, 2))
		kill_guest(cpu, "bad stack page %#lx", vaddr);
}
/*:*/

#ifdef CONFIG_X86_PAE
static void release_pmd(pmd_t *spmd)
{
	/* If the entry's not present, there's nothing to release. */
	if (pmd_flags(*spmd) & _PAGE_PRESENT) {
		unsigned int i;
		pte_t *ptepage = __va(pmd_pfn(*spmd) << PAGE_SHIFT);
		/* For each entry in the page, we might need to release it. */
		for (i = 0; i < PTRS_PER_PTE; i++)
			release_pte(ptepage[i]);
		/* Now we can free the page of PTEs */
		free_page((long)ptepage);
		/* And zero out the PMD entry so we never release it twice. */
		set_pmd(spmd, __pmd(0));
	}
}

static void release_pgd(pgd_t *spgd)
{
	/* If the entry's not present, there's nothing to release. */
	if (pgd_flags(*spgd) & _PAGE_PRESENT) {
		unsigned int i;
		pmd_t *pmdpage = __va(pgd_pfn(*spgd) << PAGE_SHIFT);

		for (i = 0; i < PTRS_PER_PMD; i++)
			release_pmd(&pmdpage[i]);

		/* Now we can free the page of PMDs */
		free_page((long)pmdpage);
		/* And zero out the PGD entry so we never release it twice. */
		set_pgd(spgd, __pgd(0));
	}
}

#else /* !CONFIG_X86_PAE */
/*H:450
 * If we chase down the release_pgd() code, the non-PAE version looks like
 * this.  The PAE version is almost identical, but instead of calling
 * release_pte it calls release_pmd(), which looks much like this.
 */
static void release_pgd(pgd_t *spgd)
{
	/* If the entry's not present, there's nothing to release. */
	if (pgd_flags(*spgd) & _PAGE_PRESENT) {
		unsigned int i;
		/*
		 * Converting the pfn to find the actual PTE page is easy: turn
		 * the page number into a physical address, then convert to a
		 * virtual address (easy for kernel pages like this one).
		 */
		pte_t *ptepage = __va(pgd_pfn(*spgd) << PAGE_SHIFT);
		/* For each entry in the page, we might need to release it. */
		for (i = 0; i < PTRS_PER_PTE; i++)
			release_pte(ptepage[i]);
		/* Now we can free the page of PTEs */
		free_page((long)ptepage);
		/* And zero out the PGD entry so we never release it twice. */
		*spgd = __pgd(0);
	}
}
#endif

/*H:445
 * We saw flush_user_mappings() twice: once from the flush_user_mappings()
 * hypercall and once in new_pgdir() when we re-used a top-level pgdir page.
 * It simply releases every PTE page from 0 up to the Guest's kernel address.
 */
static void flush_user_mappings(struct lguest *lg, int idx)
{
	unsigned int i;
	/* Release every pgd entry up to the kernel's address. */
	for (i = 0; i < pgd_index(lg->kernel_address); i++)
		release_pgd(lg->pgdirs[idx].pgdir + i);
}

/*H:440
 * (v) Flushing (throwing away) page tables,
 *
 * The Guest has a hypercall to throw away the page tables: it's used when a
 * large number of mappings have been changed.
 */
void guest_pagetable_flush_user(struct lg_cpu *cpu)
{
	/* Drop the userspace part of the current page table. */
	flush_user_mappings(cpu->lg, cpu->cpu_pgd);
}
/*:*/

/* We walk down the guest page tables to get a guest-physical address */
unsigned long guest_pa(struct lg_cpu *cpu, unsigned long vaddr)
{
	pgd_t gpgd;
	pte_t gpte;
#ifdef CONFIG_X86_PAE
	pmd_t gpmd;
#endif
	/* First step: get the top-level Guest page table entry. */
	gpgd = lgread(cpu, gpgd_addr(cpu, vaddr), pgd_t);
	/* Toplevel not present?  We can't map it in. */
	if (!(pgd_flags(gpgd) & _PAGE_PRESENT)) {
		kill_guest(cpu, "Bad address %#lx", vaddr);
		return -1UL;
	}

#ifdef CONFIG_X86_PAE
	gpmd = lgread(cpu, gpmd_addr(gpgd, vaddr), pmd_t);
	if (!(pmd_flags(gpmd) & _PAGE_PRESENT))
		kill_guest(cpu, "Bad address %#lx", vaddr);
	gpte = lgread(cpu, gpte_addr(cpu, gpmd, vaddr), pte_t);
#else
	gpte = lgread(cpu, gpte_addr(cpu, gpgd, vaddr), pte_t);
#endif
	if (!(pte_flags(gpte) & _PAGE_PRESENT))
		kill_guest(cpu, "Bad address %#lx", vaddr);

	return pte_pfn(gpte) * PAGE_SIZE | (vaddr & ~PAGE_MASK);
}

/*
 * We keep several page tables.  This is a simple routine to find the page
 * table (if any) corresponding to this top-level address the Guest has given
 * us.
 */
static unsigned int find_pgdir(struct lguest *lg, unsigned long pgtable)
{
	unsigned int i;
	for (i = 0; i < ARRAY_SIZE(lg->pgdirs); i++)
		if (lg->pgdirs[i].pgdir && lg->pgdirs[i].gpgdir == pgtable)
			break;
	return i;
}

/*H:435
 * And this is us, creating the new page directory.  If we really do
 * allocate a new one (and so the kernel parts are not there), we set
 * blank_pgdir.
 */
static unsigned int new_pgdir(struct lg_cpu *cpu,
			      unsigned long gpgdir,
			      int *blank_pgdir)
{
	unsigned int next;
#ifdef CONFIG_X86_PAE
	pmd_t *pmd_table;
#endif

	/*
	 * We pick one entry at random to throw out.  Choosing the Least
	 * Recently Used might be better, but this is easy.
	 */
	next = random32() % ARRAY_SIZE(cpu->lg->pgdirs);
	/* If it's never been allocated at all before, try now. */
	if (!cpu->lg->pgdirs[next].pgdir) {
		cpu->lg->pgdirs[next].pgdir =
					(pgd_t *)get_zeroed_page(GFP_KERNEL);
		/* If the allocation fails, just keep using the one we have */
		if (!cpu->lg->pgdirs[next].pgdir)
			next = cpu->cpu_pgd;
		else {
#ifdef CONFIG_X86_PAE
			/*
			 * In PAE mode, allocate a pmd page and populate the
			 * last pgd entry.
			 */
			pmd_table = (pmd_t *)get_zeroed_page(GFP_KERNEL);
			if (!pmd_table) {
				free_page((long)cpu->lg->pgdirs[next].pgdir);
				set_pgd(cpu->lg->pgdirs[next].pgdir, __pgd(0));
				next = cpu->cpu_pgd;
			} else {
				set_pgd(cpu->lg->pgdirs[next].pgdir +
					SWITCHER_PGD_INDEX,
					__pgd(__pa(pmd_table) | _PAGE_PRESENT));
				/*
				 * This is a blank page, so there are no kernel
				 * mappings: caller must map the stack!
				 */
				*blank_pgdir = 1;
			}
#else
			*blank_pgdir = 1;
#endif
		}
	}
	/* Record which Guest toplevel this shadows. */
	cpu->lg->pgdirs[next].gpgdir = gpgdir;
	/* Release all the non-kernel mappings. */
	flush_user_mappings(cpu->lg, next);

	return next;
}

/*H:430
 * (iv) Switching page tables
 *
 * Now we've seen all the page table setting and manipulation, let's see
 * what happens when the Guest changes page tables (ie. changes the top-level
 * pgdir).  This occurs on almost every context switch.
 */
void guest_new_pagetable(struct lg_cpu *cpu, unsigned long pgtable)
{
	int newpgdir, repin = 0;

	/* Look to see if we have this one already. */
	newpgdir = find_pgdir(cpu->lg, pgtable);
	/*
	 * If not, we allocate or mug an existing one: if it's a fresh one,
	 * repin gets set to 1.
	 */
	if (newpgdir == ARRAY_SIZE(cpu->lg->pgdirs))
		newpgdir = new_pgdir(cpu, pgtable, &repin);
	/* Change the current pgd index to the new one. */
	cpu->cpu_pgd = newpgdir;
	/* If it was completely blank, we map in the Guest kernel stack */
	if (repin)
		pin_stack_pages(cpu);
}

/*H:470
 * Finally, a routine which throws away everything: all PGD entries in all
 * the shadow page tables, including the Guest's kernel mappings.  This is used
 * when we destroy the Guest.
 */
static void release_all_pagetables(struct lguest *lg)
{
	unsigned int i, j;

	/* Every shadow pagetable this Guest has */
	for (i = 0; i < ARRAY_SIZE(lg->pgdirs); i++)
		if (lg->pgdirs[i].pgdir) {
#ifdef CONFIG_X86_PAE
			pgd_t *spgd;
			pmd_t *pmdpage;
			unsigned int k;

			/* Get the last pmd page. */
			spgd = lg->pgdirs[i].pgdir + SWITCHER_PGD_INDEX;
			pmdpage = __va(pgd_pfn(*spgd) << PAGE_SHIFT);

			/*
			 * And release the pmd entries of that pmd page,
			 * except for the switcher pmd.
			 */
			for (k = 0; k < SWITCHER_PMD_INDEX; k++)
				release_pmd(&pmdpage[k]);
#endif
			/* Every PGD entry except the Switcher at the top */
			for (j = 0; j < SWITCHER_PGD_INDEX; j++)
				release_pgd(lg->pgdirs[i].pgdir + j);
		}
}

/*
 * We also throw away everything when a Guest tells us it's changed a kernel
 * mapping.  Since kernel mappings are in every page table, it's easiest to
 * throw them all away.  This traps the Guest in amber for a while as
 * everything faults back in, but it's rare.
 */
void guest_pagetable_clear_all(struct lg_cpu *cpu)
{
	release_all_pagetables(cpu->lg);
	/* We need the Guest kernel stack mapped again. */
	pin_stack_pages(cpu);
}
/*:*/

/*M:009
 * Since we throw away all mappings when a kernel mapping changes, our
 * performance sucks for guests using highmem.  In fact, a guest with
 * PAGE_OFFSET 0xc0000000 (the default) and more than about 700MB of RAM is
 * usually slower than a Guest with less memory.
 *
 * This, of course, cannot be fixed.  It would take some kind of... well, I
 * don't know, but the term "puissant code-fu" comes to mind.
:*/

/*H:420
 * This is the routine which actually sets the page table entry for then
 * "idx"'th shadow page table.
 *
 * Normally, we can just throw out the old entry and replace it with 0: if they
 * use it demand_page() will put the new entry in.  We need to do this anyway:
 * The Guest expects _PAGE_ACCESSED to be set on its PTE the first time a page
 * is read from, and _PAGE_DIRTY when it's written to.
 *
 * But Avi Kivity pointed out that most Operating Systems (Linux included) set
 * these bits on PTEs immediately anyway.  This is done to save the CPU from
 * having to update them, but it helps us the same way: if they set
 * _PAGE_ACCESSED then we can put a read-only PTE entry in immediately, and if
 * they set _PAGE_DIRTY then we can put a writable PTE entry in immediately.
 */
static void do_set_pte(struct lg_cpu *cpu, int idx,
		       unsigned long vaddr, pte_t gpte)
{
	/* Look up the matching shadow page directory entry. */
	pgd_t *spgd = spgd_addr(cpu, idx, vaddr);
#ifdef CONFIG_X86_PAE
	pmd_t *spmd;
#endif

	/* If the top level isn't present, there's no entry to update. */
	if (pgd_flags(*spgd) & _PAGE_PRESENT) {
#ifdef CONFIG_X86_PAE
		spmd = spmd_addr(cpu, *spgd, vaddr);
		if (pmd_flags(*spmd) & _PAGE_PRESENT) {
#endif
			/* Otherwise, start by releasing the existing entry. */
			pte_t *spte = spte_addr(cpu, *spgd, vaddr);
			release_pte(*spte);

			/*
			 * If they're setting this entry as dirty or accessed,
			 * we might as well put that entry they've given us in
			 * now.  This shaves 10% off a copy-on-write
			 * micro-benchmark.
			 */
			if (pte_flags(gpte) & (_PAGE_DIRTY | _PAGE_ACCESSED)) {
				check_gpte(cpu, gpte);
				set_pte(spte,
					gpte_to_spte(cpu, gpte,
						pte_flags(gpte) & _PAGE_DIRTY));
			} else {
				/*
				 * Otherwise kill it and we can demand_page()
				 * it in later.
				 */
				set_pte(spte, __pte(0));
			}
#ifdef CONFIG_X86_PAE
		}
#endif
	}
}

/*H:410
 * Updating a PTE entry is a little trickier.
 *
 * We keep track of several different page tables (the Guest uses one for each
 * process, so it makes sense to cache at least a few).  Each of these have
 * identical kernel parts: ie. every mapping above PAGE_OFFSET is the same for
 * all processes.  So when the page table above that address changes, we update
 * all the page tables, not just the current one.  This is rare.
 *
 * The benefit is that when we have to track a new page table, we can keep all
 * the kernel mappings.  This speeds up context switch immensely.
 */
void guest_set_pte(struct lg_cpu *cpu,
		   unsigned long gpgdir, unsigned long vaddr, pte_t gpte)
{
	/*
	 * Kernel mappings must be changed on all top levels.  Slow, but doesn't
	 * happen often.
	 */
	if (vaddr >= cpu->lg->kernel_address) {
		unsigned int i;
		for (i = 0; i < ARRAY_SIZE(cpu->lg->pgdirs); i++)
			if (cpu->lg->pgdirs[i].pgdir)
				do_set_pte(cpu, i, vaddr, gpte);
	} else {
		/* Is this page table one we have a shadow for? */
		int pgdir = find_pgdir(cpu->lg, gpgdir);
		if (pgdir != ARRAY_SIZE(cpu->lg->pgdirs))
			/* If so, do the update. */
			do_set_pte(cpu, pgdir, vaddr, gpte);
	}
}

/*H:400
 * (iii) Setting up a page table entry when the Guest tells us one has changed.
 *
 * Just like we did in interrupts_and_traps.c, it makes sense for us to deal
 * with the other side of page tables while we're here: what happens when the
 * Guest asks for a page table to be updated?
 *
 * We already saw that demand_page() will fill in the shadow page tables when
 * needed, so we can simply remove shadow page table entries whenever the Guest
 * tells us they've changed.  When the Guest tries to use the new entry it will
 * fault and demand_page() will fix it up.
 *
 * So with that in mind here's our code to update a (top-level) PGD entry:
 */
void guest_set_pgd(struct lguest *lg, unsigned long gpgdir, u32 idx)
{
	int pgdir;

	if (idx >= SWITCHER_PGD_INDEX)
		return;

	/* If they're talking about a page table we have a shadow for... */
	pgdir = find_pgdir(lg, gpgdir);
	if (pgdir < ARRAY_SIZE(lg->pgdirs))
		/* ... throw it away. */
		release_pgd(lg->pgdirs[pgdir].pgdir + idx);
}

#ifdef CONFIG_X86_PAE
/* For setting a mid-level, we just throw everything away.  It's easy. */
void guest_set_pmd(struct lguest *lg, unsigned long pmdp, u32 idx)
{
	guest_pagetable_clear_all(&lg->cpus[0]);
}
#endif

/*H:505
 * To get through boot, we construct simple identity page mappings (which
 * set virtual == physical) and linear mappings which will get the Guest far
 * enough into the boot to create its own.  The linear mapping means we
 * simplify the Guest boot, but it makes assumptions about their PAGE_OFFSET,
 * as you'll see.
 *
 * We lay them out of the way, just below the initrd (which is why we need to
 * know its size here).
 */
static unsigned long setup_pagetables(struct lguest *lg,
				      unsigned long mem,
				      unsigned long initrd_size)
{
	pgd_t __user *pgdir;
	pte_t __user *linear;
	unsigned long mem_base = (unsigned long)lg->mem_base;
	unsigned int mapped_pages, i, linear_pages;
#ifdef CONFIG_X86_PAE
	pmd_t __user *pmds;
	unsigned int j;
	pgd_t pgd;
	pmd_t pmd;
#else
	unsigned int phys_linear;
#endif

	/*
	 * We have mapped_pages frames to map, so we need linear_pages page
	 * tables to map them.
	 */
	mapped_pages = mem / PAGE_SIZE;
	linear_pages = (mapped_pages + PTRS_PER_PTE - 1) / PTRS_PER_PTE;

	/* We put the toplevel page directory page at the top of memory. */
	pgdir = (pgd_t *)(mem + mem_base - initrd_size - PAGE_SIZE);

	/* Now we use the next linear_pages pages as pte pages */
	linear = (void *)pgdir - linear_pages * PAGE_SIZE;

#ifdef CONFIG_X86_PAE
	/*
	 * And the single mid page goes below that.  We only use one, but
	 * that's enough to map 1G, which definitely gets us through boot.
	 */
	pmds = (void *)linear - PAGE_SIZE;
#endif
	/*
	 * Linear mapping is easy: put every page's address into the
	 * mapping in order.
	 */
	for (i = 0; i < mapped_pages; i++) {
		pte_t pte;
		pte = pfn_pte(i, __pgprot(_PAGE_PRESENT|_PAGE_RW|_PAGE_USER));
		if (copy_to_user(&linear[i], &pte, sizeof(pte)) != 0)
			return -EFAULT;
	}

#ifdef CONFIG_X86_PAE
	/*
	 * Make the Guest PMD entries point to the corresponding place in the
	 * linear mapping (up to one page worth of PMD).
	 */
	for (i = j = 0; i < mapped_pages && j < PTRS_PER_PMD;
	     i += PTRS_PER_PTE, j++) {
		pmd = pfn_pmd(((unsigned long)&linear[i] - mem_base)/PAGE_SIZE,
			      __pgprot(_PAGE_PRESENT | _PAGE_RW | _PAGE_USER));

		if (copy_to_user(&pmds[j], &pmd, sizeof(pmd)) != 0)
			return -EFAULT;
	}

	/* One PGD entry, pointing to that PMD page. */
	pgd = __pgd(((unsigned long)pmds - mem_base) | _PAGE_PRESENT);
	/* Copy it in as the first PGD entry (ie. addresses 0-1G). */
	if (copy_to_user(&pgdir[0], &pgd, sizeof(pgd)) != 0)
		return -EFAULT;
	/*
	 * And the other PGD entry to make the linear mapping at PAGE_OFFSET
	 */
	if (copy_to_user(&pgdir[KERNEL_PGD_BOUNDARY], &pgd, sizeof(pgd)))
		return -EFAULT;
#else
	/*
	 * The top level points to the linear page table pages above.
	 * We setup the identity and linear mappings here.
	 */
	phys_linear = (unsigned long)linear - mem_base;
	for (i = 0; i < mapped_pages; i += PTRS_PER_PTE) {
		pgd_t pgd;
		/*
		 * Create a PGD entry which points to the right part of the
		 * linear PTE pages.
		 */
		pgd = __pgd((phys_linear + i * sizeof(pte_t)) |
			    (_PAGE_PRESENT | _PAGE_RW | _PAGE_USER));

		/*
		 * Copy it into the PGD page at 0 and PAGE_OFFSET.
		 */
		if (copy_to_user(&pgdir[i / PTRS_PER_PTE], &pgd, sizeof(pgd))
		    || copy_to_user(&pgdir[pgd_index(PAGE_OFFSET)
					   + i / PTRS_PER_PTE],
				    &pgd, sizeof(pgd)))
			return -EFAULT;
	}
#endif

	/*
	 * We return the top level (guest-physical) address: we remember where
	 * this is to write it into lguest_data when the Guest initializes.
	 */
	return (unsigned long)pgdir - mem_base;
}

/*H:500
 * (vii) Setting up the page tables initially.
 *
 * When a Guest is first created, the Launcher tells us where the toplevel of
 * its first page table is.  We set some things up here:
 */
int init_guest_pagetable(struct lguest *lg)
{
	u64 mem;
	u32 initrd_size;
	struct boot_params __user *boot = (struct boot_params *)lg->mem_base;
#ifdef CONFIG_X86_PAE
	pgd_t *pgd;
	pmd_t *pmd_table;
#endif
	/*
	 * Get the Guest memory size and the ramdisk size from the boot header
	 * located at lg->mem_base (Guest address 0).
	 */
	if (copy_from_user(&mem, &boot->e820_map[0].size, sizeof(mem))
	    || get_user(initrd_size, &boot->hdr.ramdisk_size))
		return -EFAULT;

	/*
	 * We start on the first shadow page table, and give it a blank PGD
	 * page.
	 */
	lg->pgdirs[0].gpgdir = setup_pagetables(lg, mem, initrd_size);
	if (IS_ERR_VALUE(lg->pgdirs[0].gpgdir))
		return lg->pgdirs[0].gpgdir;
	lg->pgdirs[0].pgdir = (pgd_t *)get_zeroed_page(GFP_KERNEL);
	if (!lg->pgdirs[0].pgdir)
		return -ENOMEM;

#ifdef CONFIG_X86_PAE
	/* For PAE, we also create the initial mid-level. */
	pgd = lg->pgdirs[0].pgdir;
	pmd_table = (pmd_t *) get_zeroed_page(GFP_KERNEL);
	if (!pmd_table)
		return -ENOMEM;

	set_pgd(pgd + SWITCHER_PGD_INDEX,
		__pgd(__pa(pmd_table) | _PAGE_PRESENT));
#endif

	/* This is the current page table. */
	lg->cpus[0].cpu_pgd = 0;
	return 0;
}

/*H:508 When the Guest calls LHCALL_LGUEST_INIT we do more setup. */
void page_table_guest_data_init(struct lg_cpu *cpu)
{
	/* We get the kernel address: above this is all kernel memory. */
	if (get_user(cpu->lg->kernel_address,
		&cpu->lg->lguest_data->kernel_address)
		/*
		 * We tell the Guest that it can't use the top 2 or 4 MB
		 * of virtual addresses used by the Switcher.
		 */
		|| put_user(RESERVE_MEM * 1024 * 1024,
			&cpu->lg->lguest_data->reserve_mem)
		|| put_user(cpu->lg->pgdirs[0].gpgdir,
			&cpu->lg->lguest_data->pgdir))
		kill_guest(cpu, "bad guest page %p", cpu->lg->lguest_data);

	/*
	 * In flush_user_mappings() we loop from 0 to
	 * "pgd_index(lg->kernel_address)".  This assumes it won't hit the
	 * Switcher mappings, so check that now.
	 */
#ifdef CONFIG_X86_PAE
	if (pgd_index(cpu->lg->kernel_address) == SWITCHER_PGD_INDEX &&
		pmd_index(cpu->lg->kernel_address) == SWITCHER_PMD_INDEX)
#else
	if (pgd_index(cpu->lg->kernel_address) >= SWITCHER_PGD_INDEX)
#endif
		kill_guest(cpu, "bad kernel address %#lx",
				 cpu->lg->kernel_address);
}

/* When a Guest dies, our cleanup is fairly simple. */
void free_guest_pagetable(struct lguest *lg)
{
	unsigned int i;

	/* Throw away all page table pages. */
	release_all_pagetables(lg);
	/* Now free the top levels: free_page() can handle 0 just fine. */
	for (i = 0; i < ARRAY_SIZE(lg->pgdirs); i++)
		free_page((long)lg->pgdirs[i].pgdir);
}

/*H:480
 * (vi) Mapping the Switcher when the Guest is about to run.
 *
 * The Switcher and the two pages for this CPU need to be visible in the
 * Guest (and not the pages for other CPUs).  We have the appropriate PTE pages
 * for each CPU already set up, we just need to hook them in now we know which
 * Guest is about to run on this CPU.
 */
void map_switcher_in_guest(struct lg_cpu *cpu, struct lguest_pages *pages)
{
	pte_t *switcher_pte_page = __get_cpu_var(switcher_pte_pages);
	pte_t regs_pte;

#ifdef CONFIG_X86_PAE
	pmd_t switcher_pmd;
	pmd_t *pmd_table;

	switcher_pmd = pfn_pmd(__pa(switcher_pte_page) >> PAGE_SHIFT,
			       PAGE_KERNEL_EXEC);

	/* Figure out where the pmd page is, by reading the PGD, and converting
	 * it to a virtual address. */
	pmd_table = __va(pgd_pfn(cpu->lg->
			pgdirs[cpu->cpu_pgd].pgdir[SWITCHER_PGD_INDEX])
								<< PAGE_SHIFT);
	/* Now write it into the shadow page table. */
	set_pmd(&pmd_table[SWITCHER_PMD_INDEX], switcher_pmd);
#else
	pgd_t switcher_pgd;

	/*
	 * Make the last PGD entry for this Guest point to the Switcher's PTE
	 * page for this CPU (with appropriate flags).
	 */
	switcher_pgd = __pgd(__pa(switcher_pte_page) | __PAGE_KERNEL_EXEC);

	cpu->lg->pgdirs[cpu->cpu_pgd].pgdir[SWITCHER_PGD_INDEX] = switcher_pgd;

#endif
	/*
	 * We also change the Switcher PTE page.  When we're running the Guest,
	 * we want the Guest's "regs" page to appear where the first Switcher
	 * page for this CPU is.  This is an optimization: when the Switcher
	 * saves the Guest registers, it saves them into the first page of this
	 * CPU's "struct lguest_pages": if we make sure the Guest's register
	 * page is already mapped there, we don't have to copy them out
	 * again.
	 */
	regs_pte = pfn_pte(__pa(cpu->regs_page) >> PAGE_SHIFT, PAGE_KERNEL);
	set_pte(&switcher_pte_page[pte_index((unsigned long)pages)], regs_pte);
}
/*:*/

static void free_switcher_pte_pages(void)
{
	unsigned int i;

	for_each_possible_cpu(i)
		free_page((long)switcher_pte_page(i));
}

/*H:520
 * Setting up the Switcher PTE page for given CPU is fairly easy, given
 * the CPU number and the "struct page"s for the Switcher code itself.
 *
 * Currently the Switcher is less than a page long, so "pages" is always 1.
 */
static __init void populate_switcher_pte_page(unsigned int cpu,
					      struct page *switcher_page[],
					      unsigned int pages)
{
	unsigned int i;
	pte_t *pte = switcher_pte_page(cpu);

	/* The first entries are easy: they map the Switcher code. */
	for (i = 0; i < pages; i++) {
		set_pte(&pte[i], mk_pte(switcher_page[i],
				__pgprot(_PAGE_PRESENT|_PAGE_ACCESSED)));
	}

	/* The only other thing we map is this CPU's pair of pages. */
	i = pages + cpu*2;

	/* First page (Guest registers) is writable from the Guest */
	set_pte(&pte[i], pfn_pte(page_to_pfn(switcher_page[i]),
			 __pgprot(_PAGE_PRESENT|_PAGE_ACCESSED|_PAGE_RW)));

	/*
	 * The second page contains the "struct lguest_ro_state", and is
	 * read-only.
	 */
	set_pte(&pte[i+1], pfn_pte(page_to_pfn(switcher_page[i+1]),
			   __pgprot(_PAGE_PRESENT|_PAGE_ACCESSED)));
}

/*
 * We've made it through the page table code.  Perhaps our tired brains are
 * still processing the details, or perhaps we're simply glad it's over.
 *
 * If nothing else, note that all this complexity in juggling shadow page tables
 * in sync with the Guest's page tables is for one reason: for most Guests this
 * page table dance determines how bad performance will be.  This is why Xen
 * uses exotic direct Guest pagetable manipulation, and why both Intel and AMD
 * have implemented shadow page table support directly into hardware.
 *
 * There is just one file remaining in the Host.
 */

/*H:510
 * At boot or module load time, init_pagetables() allocates and populates
 * the Switcher PTE page for each CPU.
 */
__init int init_pagetables(struct page **switcher_page, unsigned int pages)
{
	unsigned int i;

	for_each_possible_cpu(i) {
		switcher_pte_page(i) = (pte_t *)get_zeroed_page(GFP_KERNEL);
		if (!switcher_pte_page(i)) {
			free_switcher_pte_pages();
			return -ENOMEM;
		}
		populate_switcher_pte_page(i, switcher_page, pages);
	}
	return 0;
}
/*:*/

/* Cleaning up simply involves freeing the PTE page for each CPU. */
void free_pagetables(void)
{
	free_switcher_pte_pages();
}