drivers/lguest/page_tables.c

   1 /*P:700 The pagetable code, on the other hand, still shows the scars of
   2  * previous encounters.  It's functional, and as neat as it can be in the
   3  * circumstances, but be wary, for these things are subtle and break easily.
   4  * The Guest provides a virtual to physical mapping, but we can neither trust
   5  * it nor use it: we verify and convert it here to point the hardware to the
   6  * actual Guest pages when running the Guest. :*/
   7
   8 /* Copyright (C) Rusty Russell IBM Corporation 2006.
   9  * GPL v2 and any later version */
  10 #include <linux/mm.h>
  11 #include <linux/types.h>
  12 #include <linux/spinlock.h>
  13 #include <linux/random.h>
  14 #include <linux/percpu.h>
  15 #include <asm/tlbflush.h>
  16 #include "lg.h"
  17
  18 /*M:008 We hold reference to pages, which prevents them from being swapped.
  19  * It'd be nice to have a callback in the "struct mm_struct" when Linux wants
  20  * to swap out.  If we had this, and a shrinker callback to trim PTE pages, we
  21  * could probably consider launching Guests as non-root. :*/
  22
  23 /*H:300
  24  * The Page Table Code
  25  *
  26  * We use two-level page tables for the Guest.  If you're not entirely
  27  * comfortable with virtual addresses, physical addresses and page tables then
  28  * I recommend you review lguest.c's "Page Table Handling" (with diagrams!).
  29  *
  30  * The Guest keeps page tables, but we maintain the actual ones here: these are
  31  * called "shadow" page tables.  Which is a very Guest-centric name: these are
  32  * the real page tables the CPU uses, although we keep them up to date to
  33  * reflect the Guest's.  (See what I mean about weird naming?  Since when do
  34  * shadows reflect anything?)
  35  *
  36  * Anyway, this is the most complicated part of the Host code.  There are seven
  37  * parts to this:
  38  *  (i) Setting up a page table entry for the Guest when it faults,
  39  *  (ii) Setting up the page table entry for the Guest stack,
  40  *  (iii) Setting up a page table entry when the Guest tells us it has changed,
  41  *  (iv) Switching page tables,
  42  *  (v) Flushing (thowing away) page tables,
  43  *  (vi) Mapping the Switcher when the Guest is about to run,
  44  *  (vii) Setting up the page tables initially.
  45  :*/
  46
  47 /* Pages a 4k long, and each page table entry is 4 bytes long, giving us 1024
  48  * (or 2^10) entries per page. */
  49 #define PTES_PER_PAGE_SHIFT 10
  50 #define PTES_PER_PAGE (1 << PTES_PER_PAGE_SHIFT)
  51
  52 /* 1024 entries in a page table page maps 1024 pages: 4MB.  The Switcher is
  53  * conveniently placed at the top 4MB, so it uses a separate, complete PTE
  54  * page.  */
  55 #define SWITCHER_PGD_INDEX (PTES_PER_PAGE - 1)
  56
  57 /* We actually need a separate PTE page for each CPU.  Remember that after the
  58  * Switcher code itself comes two pages for each CPU, and we don't want this
  59  * CPU's guest to see the pages of any other CPU. */
  60 static DEFINE_PER_CPU(spte_t *, switcher_pte_pages);
  61 #define switcher_pte_page(cpu) per_cpu(switcher_pte_pages, cpu)
  62
  63 /*H:320 With our shadow and Guest types established, we need to deal with
  64  * them: the page table code is curly enough to need helper functions to keep
  65  * it clear and clean.
  66  *
  67  * The first helper takes a virtual address, and says which entry in the top
  68  * level page table deals with that address.  Since each top level entry deals
  69  * with 4M, this effectively divides by 4M. */
  70 static unsigned vaddr_to_pgd_index(unsigned long vaddr)
  71 {
  72         return vaddr >> (PAGE_SHIFT + PTES_PER_PAGE_SHIFT);
  73 }
  74
  75 /* There are two functions which return pointers to the shadow (aka "real")
  76  * page tables.
  77  *
  78  * spgd_addr() takes the virtual address and returns a pointer to the top-level
  79  * page directory entry for that address.  Since we keep track of several page
  80  * tables, the "i" argument tells us which one we're interested in (it's
  81  * usually the current one). */
  82 static spgd_t *spgd_addr(struct lguest *lg, u32 i, unsigned long vaddr)
  83 {
  84         unsigned int index = vaddr_to_pgd_index(vaddr);
  85
  86         /* We kill any Guest trying to touch the Switcher addresses. */
  87         if (index >= SWITCHER_PGD_INDEX) {
  88                 kill_guest(lg, "attempt to access switcher pages");
  89                 index = 0;
  90         }
  91         /* Return a pointer index'th pgd entry for the i'th page table. */
  92         return &lg->pgdirs[i].pgdir[index];
  93 }
  94
  95 /* This routine then takes the PGD entry given above, which contains the
  96  * address of the PTE page.  It then returns a pointer to the PTE entry for the
  97  * given address. */
  98 static spte_t *spte_addr(struct lguest *lg, spgd_t spgd, unsigned long vaddr)
  99 {
 100         spte_t *page = __va(spgd.pfn << PAGE_SHIFT);
 101         /* You should never call this if the PGD entry wasn't valid */
 102         BUG_ON(!(spgd.flags & _PAGE_PRESENT));
 103         return &page[(vaddr >> PAGE_SHIFT) % PTES_PER_PAGE];
 104 }
 105
 106 /* These two functions just like the above two, except they access the Guest
 107  * page tables.  Hence they return a Guest address. */
 108 static unsigned long gpgd_addr(struct lguest *lg, unsigned long vaddr)
 109 {
 110         unsigned int index = vaddr >> (PAGE_SHIFT + PTES_PER_PAGE_SHIFT);
 111         return lg->pgdirs[lg->pgdidx].cr3 + index * sizeof(gpgd_t);
 112 }
 113
 114 static unsigned long gpte_addr(struct lguest *lg,
 115                                gpgd_t gpgd, unsigned long vaddr)
 116 {
 117         unsigned long gpage = gpgd.pfn << PAGE_SHIFT;
 118         BUG_ON(!(gpgd.flags & _PAGE_PRESENT));
 119         return gpage + ((vaddr>>PAGE_SHIFT) % PTES_PER_PAGE) * sizeof(gpte_t);
 120 }
 121
 122 /*H:350 This routine takes a page number given by the Guest and converts it to
 123  * an actual, physical page number.  It can fail for several reasons: the
 124  * virtual address might not be mapped by the Launcher, the write flag is set
 125  * and the page is read-only, or the write flag was set and the page was
 126  * shared so had to be copied, but we ran out of memory.
 127  *
 128  * This holds a reference to the page, so release_pte() is careful to
 129  * put that back. */
 130 static unsigned long get_pfn(unsigned long virtpfn, int write)
 131 {
 132         struct page *page;
 133         /* This value indicates failure. */
 134         unsigned long ret = -1UL;
 135
 136         /* get_user_pages() is a complex interface: it gets the "struct
 137          * vm_area_struct" and "struct page" assocated with a range of pages.
 138          * It also needs the task's mmap_sem held, and is not very quick.
 139          * It returns the number of pages it got. */
 140         down_read(&current->mm->mmap_sem);
 141         if (get_user_pages(current, current->mm, virtpfn << PAGE_SHIFT,
 142                            1, write, 1, &page, NULL) == 1)
 143                 ret = page_to_pfn(page);
 144         up_read(&current->mm->mmap_sem);
 145         return ret;
 146 }
 147
 148 /*H:340 Converting a Guest page table entry to a shadow (ie. real) page table
 149  * entry can be a little tricky.  The flags are (almost) the same, but the
 150  * Guest PTE contains a virtual page number: the CPU needs the real page
 151  * number. */
 152 static spte_t gpte_to_spte(struct lguest *lg, gpte_t gpte, int write)
 153 {
 154         spte_t spte;
 155         unsigned long pfn;
 156
 157         /* The Guest sets the global flag, because it thinks that it is using
 158          * PGE.  We only told it to use PGE so it would tell us whether it was
 159          * flushing a kernel mapping or a userspace mapping.  We don't actually
 160          * use the global bit, so throw it away. */
 161         spte.flags = (gpte.flags & ~_PAGE_GLOBAL);
 162
 163         /* We need a temporary "unsigned long" variable to hold the answer from
 164          * get_pfn(), because it returns 0xFFFFFFFF on failure, which wouldn't
 165          * fit in spte.pfn.  get_pfn() finds the real physical number of the
 166          * page, given the virtual number. */
 167         pfn = get_pfn(gpte.pfn, write);
 168         if (pfn == -1UL) {
 169                 kill_guest(lg, "failed to get page %u", gpte.pfn);
 170                 /* When we destroy the Guest, we'll go through the shadow page
 171                  * tables and release_pte() them.  Make sure we don't think
 172                  * this one is valid! */
 173                 spte.flags = 0;
 174         }
 175         /* Now we assign the page number, and our shadow PTE is complete. */
 176         spte.pfn = pfn;
 177         return spte;
 178 }
 179
 180 /*H:460 And to complete the chain, release_pte() looks like this: */
 181 static void release_pte(spte_t pte)
 182 {
 183         /* Remember that get_user_pages() took a reference to the page, in
 184          * get_pfn()?  We have to put it back now. */
 185         if (pte.flags & _PAGE_PRESENT)
 186                 put_page(pfn_to_page(pte.pfn));
 187 }
 188 /*:*/
 189
 190 static void check_gpte(struct lguest *lg, gpte_t gpte)
 191 {
 192         if ((gpte.flags & (_PAGE_PWT|_PAGE_PSE)) || gpte.pfn >= lg->pfn_limit)
 193                 kill_guest(lg, "bad page table entry");
 194 }
 195
 196 static void check_gpgd(struct lguest *lg, gpgd_t gpgd)
 197 {
 198         if ((gpgd.flags & ~_PAGE_TABLE) || gpgd.pfn >= lg->pfn_limit)
 199                 kill_guest(lg, "bad page directory entry");
 200 }
 201
 202 /*H:330
 203  * (i) Setting up a page table entry for the Guest when it faults
 204  *
 205  * We saw this call in run_guest(): when we see a page fault in the Guest, we
 206  * come here.  That's because we only set up the shadow page tables lazily as
 207  * they're needed, so we get page faults all the time and quietly fix them up
 208  * and return to the Guest without it knowing.
 209  *
 210  * If we fixed up the fault (ie. we mapped the address), this routine returns
 211  * true. */
 212 int demand_page(struct lguest *lg, unsigned long vaddr, int errcode)
 213 {
 214         gpgd_t gpgd;
 215         spgd_t *spgd;
 216         unsigned long gpte_ptr;
 217         gpte_t gpte;
 218         spte_t *spte;
 219
 220         /* First step: get the top-level Guest page table entry. */
 221         gpgd = mkgpgd(lgread_u32(lg, gpgd_addr(lg, vaddr)));
 222         /* Toplevel not present?  We can't map it in. */
 223         if (!(gpgd.flags & _PAGE_PRESENT))
 224                 return 0;
 225
 226         /* Now look at the matching shadow entry. */
 227         spgd = spgd_addr(lg, lg->pgdidx, vaddr);
 228         if (!(spgd->flags & _PAGE_PRESENT)) {
 229                 /* No shadow entry: allocate a new shadow PTE page. */
 230                 unsigned long ptepage = get_zeroed_page(GFP_KERNEL);
 231                 /* This is not really the Guest's fault, but killing it is
 232                  * simple for this corner case. */
 233                 if (!ptepage) {
 234                         kill_guest(lg, "out of memory allocating pte page");
 235                         return 0;
 236                 }
 237                 /* We check that the Guest pgd is OK. */
 238                 check_gpgd(lg, gpgd);
 239                 /* And we copy the flags to the shadow PGD entry.  The page
 240                  * number in the shadow PGD is the page we just allocated. */
 241                 spgd->raw.val = (__pa(ptepage) | gpgd.flags);
 242         }
 243
 244         /* OK, now we look at the lower level in the Guest page table: keep its
 245          * address, because we might update it later. */
 246         gpte_ptr = gpte_addr(lg, gpgd, vaddr);
 247         gpte = mkgpte(lgread_u32(lg, gpte_ptr));
 248
 249         /* If this page isn't in the Guest page tables, we can't page it in. */
 250         if (!(gpte.flags & _PAGE_PRESENT))
 251                 return 0;
 252
 253         /* Check they're not trying to write to a page the Guest wants
 254          * read-only (bit 2 of errcode == write). */
 255         if ((errcode & 2) && !(gpte.flags & _PAGE_RW))
 256                 return 0;
 257
 258         /* User access to a kernel page? (bit 3 == user access) */
 259         if ((errcode & 4) && !(gpte.flags & _PAGE_USER))
 260                 return 0;
 261
 262         /* Check that the Guest PTE flags are OK, and the page number is below
 263          * the pfn_limit (ie. not mapping the Launcher binary). */
 264         check_gpte(lg, gpte);
 265         /* Add the _PAGE_ACCESSED and (for a write) _PAGE_DIRTY flag */
 266         gpte.flags |= _PAGE_ACCESSED;
 267         if (errcode & 2)
 268                 gpte.flags |= _PAGE_DIRTY;
 269
 270         /* Get the pointer to the shadow PTE entry we're going to set. */
 271         spte = spte_addr(lg, *spgd, vaddr);
 272         /* If there was a valid shadow PTE entry here before, we release it.
 273          * This can happen with a write to a previously read-only entry. */
 274         release_pte(*spte);
 275
 276         /* If this is a write, we insist that the Guest page is writable (the
 277          * final arg to gpte_to_spte()). */
 278         if (gpte.flags & _PAGE_DIRTY)
 279                 *spte = gpte_to_spte(lg, gpte, 1);
 280         else {
 281                 /* If this is a read, don't set the "writable" bit in the page
 282                  * table entry, even if the Guest says it's writable.  That way
 283                  * we come back here when a write does actually ocur, so we can
 284                  * update the Guest's _PAGE_DIRTY flag. */
 285                 gpte_t ro_gpte = gpte;
 286                 ro_gpte.flags &= ~_PAGE_RW;
 287                 *spte = gpte_to_spte(lg, ro_gpte, 0);
 288         }
 289
 290         /* Finally, we write the Guest PTE entry back: we've set the
 291          * _PAGE_ACCESSED and maybe the _PAGE_DIRTY flags. */
 292         lgwrite_u32(lg, gpte_ptr, gpte.raw.val);
 293
 294         /* We succeeded in mapping the page! */
 295         return 1;
 296 }
 297
 298 /*H:360 (ii) Setting up the page table entry for the Guest stack.
 299  *
 300  * Remember pin_stack_pages() which makes sure the stack is mapped?  It could
 301  * simply call demand_page(), but as we've seen that logic is quite long, and
 302  * usually the stack pages are already mapped anyway, so it's not required.
 303  *
 304  * This is a quick version which answers the question: is this virtual address
 305  * mapped by the shadow page tables, and is it writable? */
 306 static int page_writable(struct lguest *lg, unsigned long vaddr)
 307 {
 308         spgd_t *spgd;
 309         unsigned long flags;
 310
 311         /* Look at the top level entry: is it present? */
 312         spgd = spgd_addr(lg, lg->pgdidx, vaddr);
 313         if (!(spgd->flags & _PAGE_PRESENT))
 314                 return 0;
 315
 316         /* Check the flags on the pte entry itself: it must be present and
 317          * writable. */
 318         flags = spte_addr(lg, *spgd, vaddr)->flags;
 319         return (flags & (_PAGE_PRESENT|_PAGE_RW)) == (_PAGE_PRESENT|_PAGE_RW);
 320 }
 321
 322 /* So, when pin_stack_pages() asks us to pin a page, we check if it's already
 323  * in the page tables, and if not, we call demand_page() with error code 2
 324  * (meaning "write"). */
 325 void pin_page(struct lguest *lg, unsigned long vaddr)
 326 {
 327         if (!page_writable(lg, vaddr) && !demand_page(lg, vaddr, 2))
 328                 kill_guest(lg, "bad stack page %#lx", vaddr);
 329 }
 330
 331 /*H:450 If we chase down the release_pgd() code, it looks like this: */
 332 static void release_pgd(struct lguest *lg, spgd_t *spgd)
 333 {
 334         /* If the entry's not present, there's nothing to release. */
 335         if (spgd->flags & _PAGE_PRESENT) {
 336                 unsigned int i;
 337                 /* Converting the pfn to find the actual PTE page is easy: turn
 338                  * the page number into a physical address, then convert to a
 339                  * virtual address (easy for kernel pages like this one). */
 340                 spte_t *ptepage = __va(spgd->pfn << PAGE_SHIFT);
 341                 /* For each entry in the page, we might need to release it. */
 342                 for (i = 0; i < PTES_PER_PAGE; i++)
 343                         release_pte(ptepage[i]);
 344                 /* Now we can free the page of PTEs */
 345                 free_page((long)ptepage);
 346                 /* And zero out the PGD entry we we never release it twice. */
 347                 spgd->raw.val = 0;
 348         }
 349 }
 350
 351 /*H:440 (v) Flushing (thowing away) page tables,
 352  *
 353  * We saw flush_user_mappings() called when we re-used a top-level pgdir page.
 354  * It simply releases every PTE page from 0 up to the kernel address. */
 355 static void flush_user_mappings(struct lguest *lg, int idx)
 356 {
 357         unsigned int i;
 358         /* Release every pgd entry up to the kernel's address. */
 359         for (i = 0; i < vaddr_to_pgd_index(lg->page_offset); i++)
 360                 release_pgd(lg, lg->pgdirs[idx].pgdir + i);
 361 }
 362
 363 /* The Guest also has a hypercall to do this manually: it's used when a large
 364  * number of mappings have been changed. */
 365 void guest_pagetable_flush_user(struct lguest *lg)
 366 {
 367         /* Drop the userspace part of the current page table. */
 368         flush_user_mappings(lg, lg->pgdidx);
 369 }
 370 /*:*/
 371
 372 /* We keep several page tables.  This is a simple routine to find the page
 373  * table (if any) corresponding to this top-level address the Guest has given
 374  * us. */
 375 static unsigned int find_pgdir(struct lguest *lg, unsigned long pgtable)
 376 {
 377         unsigned int i;
 378         for (i = 0; i < ARRAY_SIZE(lg->pgdirs); i++)
 379                 if (lg->pgdirs[i].cr3 == pgtable)
 380                         break;
 381         return i;
 382 }
 383
 384 /*H:435 And this is us, creating the new page directory.  If we really do
 385  * allocate a new one (and so the kernel parts are not there), we set
 386  * blank_pgdir. */
 387 static unsigned int new_pgdir(struct lguest *lg,
 388                               unsigned long cr3,
 389                               int *blank_pgdir)
 390 {
 391         unsigned int next;
 392
 393         /* We pick one entry at random to throw out.  Choosing the Least
 394          * Recently Used might be better, but this is easy. */
 395         next = random32() % ARRAY_SIZE(lg->pgdirs);
 396         /* If it's never been allocated at all before, try now. */
 397         if (!lg->pgdirs[next].pgdir) {
 398                 lg->pgdirs[next].pgdir = (spgd_t *)get_zeroed_page(GFP_KERNEL);
 399                 /* If the allocation fails, just keep using the one we have */
 400                 if (!lg->pgdirs[next].pgdir)
 401                         next = lg->pgdidx;
 402                 else
 403                         /* This is a blank page, so there are no kernel
 404                          * mappings: caller must map the stack! */
 405                         *blank_pgdir = 1;
 406         }
 407         /* Record which Guest toplevel this shadows. */
 408         lg->pgdirs[next].cr3 = cr3;
 409         /* Release all the non-kernel mappings. */
 410         flush_user_mappings(lg, next);
 411
 412         return next;
 413 }
 414
 415 /*H:430 (iv) Switching page tables
 416  *
 417  * This is what happens when the Guest changes page tables (ie. changes the
 418  * top-level pgdir).  This happens on almost every context switch. */
 419 void guest_new_pagetable(struct lguest *lg, unsigned long pgtable)
 420 {
 421         int newpgdir, repin = 0;
 422
 423         /* Look to see if we have this one already. */
 424         newpgdir = find_pgdir(lg, pgtable);
 425         /* If not, we allocate or mug an existing one: if it's a fresh one,
 426          * repin gets set to 1. */
 427         if (newpgdir == ARRAY_SIZE(lg->pgdirs))
 428                 newpgdir = new_pgdir(lg, pgtable, &repin);
 429         /* Change the current pgd index to the new one. */
 430         lg->pgdidx = newpgdir;
 431         /* If it was completely blank, we map in the Guest kernel stack */
 432         if (repin)
 433                 pin_stack_pages(lg);
 434 }
 435
 436 /*H:470 Finally, a routine which throws away everything: all PGD entries in all
 437  * the shadow page tables.  This is used when we destroy the Guest. */
 438 static void release_all_pagetables(struct lguest *lg)
 439 {
 440         unsigned int i, j;
 441
 442         /* Every shadow pagetable this Guest has */
 443         for (i = 0; i < ARRAY_SIZE(lg->pgdirs); i++)
 444                 if (lg->pgdirs[i].pgdir)
 445                         /* Every PGD entry except the Switcher at the top */
 446                         for (j = 0; j < SWITCHER_PGD_INDEX; j++)
 447                                 release_pgd(lg, lg->pgdirs[i].pgdir + j);
 448 }
 449
 450 /* We also throw away everything when a Guest tells us it's changed a kernel
 451  * mapping.  Since kernel mappings are in every page table, it's easiest to
 452  * throw them all away.  This is amazingly slow, but thankfully rare. */
 453 void guest_pagetable_clear_all(struct lguest *lg)
 454 {
 455         release_all_pagetables(lg);
 456         /* We need the Guest kernel stack mapped again. */
 457         pin_stack_pages(lg);
 458 }
 459
 460 /*H:420 This is the routine which actually sets the page table entry for then
 461  * "idx"'th shadow page table.
 462  *
 463  * Normally, we can just throw out the old entry and replace it with 0: if they
 464  * use it demand_page() will put the new entry in.  We need to do this anyway:
 465  * The Guest expects _PAGE_ACCESSED to be set on its PTE the first time a page
 466  * is read from, and _PAGE_DIRTY when it's written to.
 467  *
 468  * But Avi Kivity pointed out that most Operating Systems (Linux included) set
 469  * these bits on PTEs immediately anyway.  This is done to save the CPU from
 470  * having to update them, but it helps us the same way: if they set
 471  * _PAGE_ACCESSED then we can put a read-only PTE entry in immediately, and if
 472  * they set _PAGE_DIRTY then we can put a writable PTE entry in immediately.
 473  */
 474 static void do_set_pte(struct lguest *lg, int idx,
 475                        unsigned long vaddr, gpte_t gpte)
 476 {
 477         /* Look up the matching shadow page directot entry. */
 478         spgd_t *spgd = spgd_addr(lg, idx, vaddr);
 479
 480         /* If the top level isn't present, there's no entry to update. */
 481         if (spgd->flags & _PAGE_PRESENT) {
 482                 /* Otherwise, we start by releasing the existing entry. */
 483                 spte_t *spte = spte_addr(lg, *spgd, vaddr);
 484                 release_pte(*spte);
 485
 486                 /* If they're setting this entry as dirty or accessed, we might
 487                  * as well put that entry they've given us in now.  This shaves
 488                  * 10% off a copy-on-write micro-benchmark. */
 489                 if (gpte.flags & (_PAGE_DIRTY | _PAGE_ACCESSED)) {
 490                         check_gpte(lg, gpte);
 491                         *spte = gpte_to_spte(lg, gpte, gpte.flags&_PAGE_DIRTY);
 492                 } else
 493                         /* Otherwise we can demand_page() it in later. */
 494                         spte->raw.val = 0;
 495         }
 496 }
 497
 498 /*H:410 Updating a PTE entry is a little trickier.
 499  *
 500  * We keep track of several different page tables (the Guest uses one for each
 501  * process, so it makes sense to cache at least a few).  Each of these have
 502  * identical kernel parts: ie. every mapping above PAGE_OFFSET is the same for
 503  * all processes.  So when the page table above that address changes, we update
 504  * all the page tables, not just the current one.  This is rare.
 505  *
 506  * The benefit is that when we have to track a new page table, we can copy keep
 507  * all the kernel mappings.  This speeds up context switch immensely. */
 508 void guest_set_pte(struct lguest *lg,
 509                    unsigned long cr3, unsigned long vaddr, gpte_t gpte)
 510 {
 511         /* Kernel mappings must be changed on all top levels.  Slow, but
 512          * doesn't happen often. */
 513         if (vaddr >= lg->page_offset) {
 514                 unsigned int i;
 515                 for (i = 0; i < ARRAY_SIZE(lg->pgdirs); i++)
 516                         if (lg->pgdirs[i].pgdir)
 517                                 do_set_pte(lg, i, vaddr, gpte);
 518         } else {
 519                 /* Is this page table one we have a shadow for? */
 520                 int pgdir = find_pgdir(lg, cr3);
 521                 if (pgdir != ARRAY_SIZE(lg->pgdirs))
 522                         /* If so, do the update. */
 523                         do_set_pte(lg, pgdir, vaddr, gpte);
 524         }
 525 }
 526
 527 /*H:400
 528  * (iii) Setting up a page table entry when the Guest tells us it has changed.
 529  *
 530  * Just like we did in interrupts_and_traps.c, it makes sense for us to deal
 531  * with the other side of page tables while we're here: what happens when the
 532  * Guest asks for a page table to be updated?
 533  *
 534  * We already saw that demand_page() will fill in the shadow page tables when
 535  * needed, so we can simply remove shadow page table entries whenever the Guest
 536  * tells us they've changed.  When the Guest tries to use the new entry it will
 537  * fault and demand_page() will fix it up.
 538  *
 539  * So with that in mind here's our code to to update a (top-level) PGD entry:
 540  */
 541 void guest_set_pmd(struct lguest *lg, unsigned long cr3, u32 idx)
 542 {
 543         int pgdir;
 544
 545         /* The kernel seems to try to initialize this early on: we ignore its
 546          * attempts to map over the Switcher. */
 547         if (idx >= SWITCHER_PGD_INDEX)
 548                 return;
 549
 550         /* If they're talking about a page table we have a shadow for... */
 551         pgdir = find_pgdir(lg, cr3);
 552         if (pgdir < ARRAY_SIZE(lg->pgdirs))
 553                 /* ... throw it away. */
 554                 release_pgd(lg, lg->pgdirs[pgdir].pgdir + idx);
 555 }
 556
 557 /*H:500 (vii) Setting up the page tables initially.
 558  *
 559  * When a Guest is first created, the Launcher tells us where the toplevel of
 560  * its first page table is.  We set some things up here: */
 561 int init_guest_pagetable(struct lguest *lg, unsigned long pgtable)
 562 {
 563         /* In flush_user_mappings() we loop from 0 to
 564          * "vaddr_to_pgd_index(lg->page_offset)".  This assumes it won't hit
 565          * the Switcher mappings, so check that now. */
 566         if (vaddr_to_pgd_index(lg->page_offset) >= SWITCHER_PGD_INDEX)
 567                 return -EINVAL;
 568         /* We start on the first shadow page table, and give it a blank PGD
 569          * page. */
 570         lg->pgdidx = 0;
 571         lg->pgdirs[lg->pgdidx].cr3 = pgtable;
 572         lg->pgdirs[lg->pgdidx].pgdir = (spgd_t*)get_zeroed_page(GFP_KERNEL);
 573         if (!lg->pgdirs[lg->pgdidx].pgdir)
 574                 return -ENOMEM;
 575         return 0;
 576 }
 577
 578 /* When a Guest dies, our cleanup is fairly simple. */
 579 void free_guest_pagetable(struct lguest *lg)
 580 {
 581         unsigned int i;
 582
 583         /* Throw away all page table pages. */
 584         release_all_pagetables(lg);
 585         /* Now free the top levels: free_page() can handle 0 just fine. */
 586         for (i = 0; i < ARRAY_SIZE(lg->pgdirs); i++)
 587                 free_page((long)lg->pgdirs[i].pgdir);
 588 }
 589
 590 /*H:480 (vi) Mapping the Switcher when the Guest is about to run.
 591  *
 592  * The Switcher and the two pages for this CPU need to be available to the
 593  * Guest (and not the pages for other CPUs).  We have the appropriate PTE pages
 594  * for each CPU already set up, we just need to hook them in. */
 595 void map_switcher_in_guest(struct lguest *lg, struct lguest_pages *pages)
 596 {
 597         spte_t *switcher_pte_page = __get_cpu_var(switcher_pte_pages);
 598         spgd_t switcher_pgd;
 599         spte_t regs_pte;
 600
 601         /* Make the last PGD entry for this Guest point to the Switcher's PTE
 602          * page for this CPU (with appropriate flags). */
 603         switcher_pgd.pfn = __pa(switcher_pte_page) >> PAGE_SHIFT;
 604         switcher_pgd.flags = _PAGE_KERNEL;
 605         lg->pgdirs[lg->pgdidx].pgdir[SWITCHER_PGD_INDEX] = switcher_pgd;
 606
 607         /* We also change the Switcher PTE page.  When we're running the Guest,
 608          * we want the Guest's "regs" page to appear where the first Switcher
 609          * page for this CPU is.  This is an optimization: when the Switcher
 610          * saves the Guest registers, it saves them into the first page of this
 611          * CPU's "struct lguest_pages": if we make sure the Guest's register
 612          * page is already mapped there, we don't have to copy them out
 613          * again. */
 614         regs_pte.pfn = __pa(lg->regs_page) >> PAGE_SHIFT;
 615         regs_pte.flags = _PAGE_KERNEL;
 616         switcher_pte_page[(unsigned long)pages/PAGE_SIZE%PTES_PER_PAGE]
 617                 = regs_pte;
 618 }
 619 /*:*/
 620
 621 static void free_switcher_pte_pages(void)
 622 {
 623         unsigned int i;
 624
 625         for_each_possible_cpu(i)
 626                 free_page((long)switcher_pte_page(i));
 627 }
 628
 629 /*H:520 Setting up the Switcher PTE page for given CPU is fairly easy, given
 630  * the CPU number and the "struct page"s for the Switcher code itself.
 631  *
 632  * Currently the Switcher is less than a page long, so "pages" is always 1. */
 633 static __init void populate_switcher_pte_page(unsigned int cpu,
 634                                               struct page *switcher_page[],
 635                                               unsigned int pages)
 636 {
 637         unsigned int i;
 638         spte_t *pte = switcher_pte_page(cpu);
 639
 640         /* The first entries are easy: they map the Switcher code. */
 641         for (i = 0; i < pages; i++) {
 642                 pte[i].pfn = page_to_pfn(switcher_page[i]);
 643                 pte[i].flags = _PAGE_PRESENT|_PAGE_ACCESSED;
 644         }
 645
 646         /* The only other thing we map is this CPU's pair of pages. */
 647         i = pages + cpu*2;
 648
 649         /* First page (Guest registers) is writable from the Guest */
 650         pte[i].pfn = page_to_pfn(switcher_page[i]);
 651         pte[i].flags = _PAGE_PRESENT|_PAGE_ACCESSED|_PAGE_RW;
 652         /* The second page contains the "struct lguest_ro_state", and is
 653          * read-only. */
 654         pte[i+1].pfn = page_to_pfn(switcher_page[i+1]);
 655         pte[i+1].flags = _PAGE_PRESENT|_PAGE_ACCESSED;
 656 }
 657
 658 /*H:510 At boot or module load time, init_pagetables() allocates and populates
 659  * the Switcher PTE page for each CPU. */
 660 __init int init_pagetables(struct page **switcher_page, unsigned int pages)
 661 {
 662         unsigned int i;
 663
 664         for_each_possible_cpu(i) {
 665                 switcher_pte_page(i) = (spte_t *)get_zeroed_page(GFP_KERNEL);
 666                 if (!switcher_pte_page(i)) {
 667                         free_switcher_pte_pages();
 668                         return -ENOMEM;
 669                 }
 670                 populate_switcher_pte_page(i, switcher_page, pages);
 671         }
 672         return 0;
 673 }
 674 /*:*/
 675
 676 /* Cleaning up simply involves freeing the PTE page for each CPU. */
 677 void free_pagetables(void)
 678 {
 679         free_switcher_pte_pages();
 680 }