linux/drivers/lguest/page_tables.c
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   1/*P:700
   2 * The pagetable code, on the other hand, still shows the scars of
   3 * previous encounters.  It's functional, and as neat as it can be in the
   4 * circumstances, but be wary, for these things are subtle and break easily.
   5 * The Guest provides a virtual to physical mapping, but we can neither trust
   6 * it nor use it: we verify and convert it here then point the CPU to the
   7 * converted Guest pages when running the Guest.
   8:*/
   9
  10/* Copyright (C) Rusty Russell IBM Corporation 2013.
  11 * GPL v2 and any later version */
  12#include <linux/mm.h>
  13#include <linux/gfp.h>
  14#include <linux/types.h>
  15#include <linux/spinlock.h>
  16#include <linux/random.h>
  17#include <linux/percpu.h>
  18#include <asm/tlbflush.h>
  19#include <asm/uaccess.h>
  20#include "lg.h"
  21
  22/*M:008
  23 * We hold reference to pages, which prevents them from being swapped.
  24 * It'd be nice to have a callback in the "struct mm_struct" when Linux wants
  25 * to swap out.  If we had this, and a shrinker callback to trim PTE pages, we
  26 * could probably consider launching Guests as non-root.
  27:*/
  28
  29/*H:300
  30 * The Page Table Code
  31 *
  32 * We use two-level page tables for the Guest, or three-level with PAE.  If
  33 * you're not entirely comfortable with virtual addresses, physical addresses
  34 * and page tables then I recommend you review arch/x86/lguest/boot.c's "Page
  35 * Table Handling" (with diagrams!).
  36 *
  37 * The Guest keeps page tables, but we maintain the actual ones here: these are
  38 * called "shadow" page tables.  Which is a very Guest-centric name: these are
  39 * the real page tables the CPU uses, although we keep them up to date to
  40 * reflect the Guest's.  (See what I mean about weird naming?  Since when do
  41 * shadows reflect anything?)
  42 *
  43 * Anyway, this is the most complicated part of the Host code.  There are seven
  44 * parts to this:
  45 *  (i) Looking up a page table entry when the Guest faults,
  46 *  (ii) Making sure the Guest stack is mapped,
  47 *  (iii) Setting up a page table entry when the Guest tells us one has changed,
  48 *  (iv) Switching page tables,
  49 *  (v) Flushing (throwing away) page tables,
  50 *  (vi) Mapping the Switcher when the Guest is about to run,
  51 *  (vii) Setting up the page tables initially.
  52:*/
  53
  54/*
  55 * The Switcher uses the complete top PTE page.  That's 1024 PTE entries (4MB)
  56 * or 512 PTE entries with PAE (2MB).
  57 */
  58#define SWITCHER_PGD_INDEX (PTRS_PER_PGD - 1)
  59
  60/*
  61 * For PAE we need the PMD index as well. We use the last 2MB, so we
  62 * will need the last pmd entry of the last pmd page.
  63 */
  64#ifdef CONFIG_X86_PAE
  65#define CHECK_GPGD_MASK         _PAGE_PRESENT
  66#else
  67#define CHECK_GPGD_MASK         _PAGE_TABLE
  68#endif
  69
  70/*H:320
  71 * The page table code is curly enough to need helper functions to keep it
  72 * clear and clean.  The kernel itself provides many of them; one advantage
  73 * of insisting that the Guest and Host use the same CONFIG_PAE setting.
  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 (PGD) for that address.  Since we keep track of several
  80 * page tables, the "i" argument tells us which one we're interested in (it's
  81 * usually the current one).
  82 */
  83static pgd_t *spgd_addr(struct lg_cpu *cpu, u32 i, unsigned long vaddr)
  84{
  85        unsigned int index = pgd_index(vaddr);
  86
  87        /* Return a pointer index'th pgd entry for the i'th page table. */
  88        return &cpu->lg->pgdirs[i].pgdir[index];
  89}
  90
  91#ifdef CONFIG_X86_PAE
  92/*
  93 * This routine then takes the PGD entry given above, which contains the
  94 * address of the PMD page.  It then returns a pointer to the PMD entry for the
  95 * given address.
  96 */
  97static pmd_t *spmd_addr(struct lg_cpu *cpu, pgd_t spgd, unsigned long vaddr)
  98{
  99        unsigned int index = pmd_index(vaddr);
 100        pmd_t *page;
 101
 102        /* You should never call this if the PGD entry wasn't valid */
 103        BUG_ON(!(pgd_flags(spgd) & _PAGE_PRESENT));
 104        page = __va(pgd_pfn(spgd) << PAGE_SHIFT);
 105
 106        return &page[index];
 107}
 108#endif
 109
 110/*
 111 * This routine then takes the page directory entry returned above, which
 112 * contains the address of the page table entry (PTE) page.  It then returns a
 113 * pointer to the PTE entry for the given address.
 114 */
 115static pte_t *spte_addr(struct lg_cpu *cpu, pgd_t spgd, unsigned long vaddr)
 116{
 117#ifdef CONFIG_X86_PAE
 118        pmd_t *pmd = spmd_addr(cpu, spgd, vaddr);
 119        pte_t *page = __va(pmd_pfn(*pmd) << PAGE_SHIFT);
 120
 121        /* You should never call this if the PMD entry wasn't valid */
 122        BUG_ON(!(pmd_flags(*pmd) & _PAGE_PRESENT));
 123#else
 124        pte_t *page = __va(pgd_pfn(spgd) << PAGE_SHIFT);
 125        /* You should never call this if the PGD entry wasn't valid */
 126        BUG_ON(!(pgd_flags(spgd) & _PAGE_PRESENT));
 127#endif
 128
 129        return &page[pte_index(vaddr)];
 130}
 131
 132/*
 133 * These functions are just like the above, except they access the Guest
 134 * page tables.  Hence they return a Guest address.
 135 */
 136static unsigned long gpgd_addr(struct lg_cpu *cpu, unsigned long vaddr)
 137{
 138        unsigned int index = vaddr >> (PGDIR_SHIFT);
 139        return cpu->lg->pgdirs[cpu->cpu_pgd].gpgdir + index * sizeof(pgd_t);
 140}
 141
 142#ifdef CONFIG_X86_PAE
 143/* Follow the PGD to the PMD. */
 144static unsigned long gpmd_addr(pgd_t gpgd, unsigned long vaddr)
 145{
 146        unsigned long gpage = pgd_pfn(gpgd) << PAGE_SHIFT;
 147        BUG_ON(!(pgd_flags(gpgd) & _PAGE_PRESENT));
 148        return gpage + pmd_index(vaddr) * sizeof(pmd_t);
 149}
 150
 151/* Follow the PMD to the PTE. */
 152static unsigned long gpte_addr(struct lg_cpu *cpu,
 153                               pmd_t gpmd, unsigned long vaddr)
 154{
 155        unsigned long gpage = pmd_pfn(gpmd) << PAGE_SHIFT;
 156
 157        BUG_ON(!(pmd_flags(gpmd) & _PAGE_PRESENT));
 158        return gpage + pte_index(vaddr) * sizeof(pte_t);
 159}
 160#else
 161/* Follow the PGD to the PTE (no mid-level for !PAE). */
 162static unsigned long gpte_addr(struct lg_cpu *cpu,
 163                                pgd_t gpgd, unsigned long vaddr)
 164{
 165        unsigned long gpage = pgd_pfn(gpgd) << PAGE_SHIFT;
 166
 167        BUG_ON(!(pgd_flags(gpgd) & _PAGE_PRESENT));
 168        return gpage + pte_index(vaddr) * sizeof(pte_t);
 169}
 170#endif
 171/*:*/
 172
 173/*M:007
 174 * get_pfn is slow: we could probably try to grab batches of pages here as
 175 * an optimization (ie. pre-faulting).
 176:*/
 177
 178/*H:350
 179 * This routine takes a page number given by the Guest and converts it to
 180 * an actual, physical page number.  It can fail for several reasons: the
 181 * virtual address might not be mapped by the Launcher, the write flag is set
 182 * and the page is read-only, or the write flag was set and the page was
 183 * shared so had to be copied, but we ran out of memory.
 184 *
 185 * This holds a reference to the page, so release_pte() is careful to put that
 186 * back.
 187 */
 188static unsigned long get_pfn(unsigned long virtpfn, int write)
 189{
 190        struct page *page;
 191
 192        /* gup me one page at this address please! */
 193        if (get_user_pages_fast(virtpfn << PAGE_SHIFT, 1, write, &page) == 1)
 194                return page_to_pfn(page);
 195
 196        /* This value indicates failure. */
 197        return -1UL;
 198}
 199
 200/*H:340
 201 * Converting a Guest page table entry to a shadow (ie. real) page table
 202 * entry can be a little tricky.  The flags are (almost) the same, but the
 203 * Guest PTE contains a virtual page number: the CPU needs the real page
 204 * number.
 205 */
 206static pte_t gpte_to_spte(struct lg_cpu *cpu, pte_t gpte, int write)
 207{
 208        unsigned long pfn, base, flags;
 209
 210        /*
 211         * The Guest sets the global flag, because it thinks that it is using
 212         * PGE.  We only told it to use PGE so it would tell us whether it was
 213         * flushing a kernel mapping or a userspace mapping.  We don't actually
 214         * use the global bit, so throw it away.
 215         */
 216        flags = (pte_flags(gpte) & ~_PAGE_GLOBAL);
 217
 218        /* The Guest's pages are offset inside the Launcher. */
 219        base = (unsigned long)cpu->lg->mem_base / PAGE_SIZE;
 220
 221        /*
 222         * We need a temporary "unsigned long" variable to hold the answer from
 223         * get_pfn(), because it returns 0xFFFFFFFF on failure, which wouldn't
 224         * fit in spte.pfn.  get_pfn() finds the real physical number of the
 225         * page, given the virtual number.
 226         */
 227        pfn = get_pfn(base + pte_pfn(gpte), write);
 228        if (pfn == -1UL) {
 229                kill_guest(cpu, "failed to get page %lu", pte_pfn(gpte));
 230                /*
 231                 * When we destroy the Guest, we'll go through the shadow page
 232                 * tables and release_pte() them.  Make sure we don't think
 233                 * this one is valid!
 234                 */
 235                flags = 0;
 236        }
 237        /* Now we assemble our shadow PTE from the page number and flags. */
 238        return pfn_pte(pfn, __pgprot(flags));
 239}
 240
 241/*H:460 And to complete the chain, release_pte() looks like this: */
 242static void release_pte(pte_t pte)
 243{
 244        /*
 245         * Remember that get_user_pages_fast() took a reference to the page, in
 246         * get_pfn()?  We have to put it back now.
 247         */
 248        if (pte_flags(pte) & _PAGE_PRESENT)
 249                put_page(pte_page(pte));
 250}
 251/*:*/
 252
 253static bool check_gpte(struct lg_cpu *cpu, pte_t gpte)
 254{
 255        if ((pte_flags(gpte) & _PAGE_PSE) ||
 256            pte_pfn(gpte) >= cpu->lg->pfn_limit) {
 257                kill_guest(cpu, "bad page table entry");
 258                return false;
 259        }
 260        return true;
 261}
 262
 263static bool check_gpgd(struct lg_cpu *cpu, pgd_t gpgd)
 264{
 265        if ((pgd_flags(gpgd) & ~CHECK_GPGD_MASK) ||
 266            (pgd_pfn(gpgd) >= cpu->lg->pfn_limit)) {
 267                kill_guest(cpu, "bad page directory entry");
 268                return false;
 269        }
 270        return true;
 271}
 272
 273#ifdef CONFIG_X86_PAE
 274static bool check_gpmd(struct lg_cpu *cpu, pmd_t gpmd)
 275{
 276        if ((pmd_flags(gpmd) & ~_PAGE_TABLE) ||
 277            (pmd_pfn(gpmd) >= cpu->lg->pfn_limit)) {
 278                kill_guest(cpu, "bad page middle directory entry");
 279                return false;
 280        }
 281        return true;
 282}
 283#endif
 284
 285/*H:331
 286 * This is the core routine to walk the shadow page tables and find the page
 287 * table entry for a specific address.
 288 *
 289 * If allocate is set, then we allocate any missing levels, setting the flags
 290 * on the new page directory and mid-level directories using the arguments
 291 * (which are copied from the Guest's page table entries).
 292 */
 293static pte_t *find_spte(struct lg_cpu *cpu, unsigned long vaddr, bool allocate,
 294                        int pgd_flags, int pmd_flags)
 295{
 296        pgd_t *spgd;
 297        /* Mid level for PAE. */
 298#ifdef CONFIG_X86_PAE
 299        pmd_t *spmd;
 300#endif
 301
 302        /* Get top level entry. */
 303        spgd = spgd_addr(cpu, cpu->cpu_pgd, vaddr);
 304        if (!(pgd_flags(*spgd) & _PAGE_PRESENT)) {
 305                /* No shadow entry: allocate a new shadow PTE page. */
 306                unsigned long ptepage;
 307
 308                /* If they didn't want us to allocate anything, stop. */
 309                if (!allocate)
 310                        return NULL;
 311
 312                ptepage = get_zeroed_page(GFP_KERNEL);
 313                /*
 314                 * This is not really the Guest's fault, but killing it is
 315                 * simple for this corner case.
 316                 */
 317                if (!ptepage) {
 318                        kill_guest(cpu, "out of memory allocating pte page");
 319                        return NULL;
 320                }
 321                /*
 322                 * And we copy the flags to the shadow PGD entry.  The page
 323                 * number in the shadow PGD is the page we just allocated.
 324                 */
 325                set_pgd(spgd, __pgd(__pa(ptepage) | pgd_flags));
 326        }
 327
 328        /*
 329         * Intel's Physical Address Extension actually uses three levels of
 330         * page tables, so we need to look in the mid-level.
 331         */
 332#ifdef CONFIG_X86_PAE
 333        /* Now look at the mid-level shadow entry. */
 334        spmd = spmd_addr(cpu, *spgd, vaddr);
 335
 336        if (!(pmd_flags(*spmd) & _PAGE_PRESENT)) {
 337                /* No shadow entry: allocate a new shadow PTE page. */
 338                unsigned long ptepage;
 339
 340                /* If they didn't want us to allocate anything, stop. */
 341                if (!allocate)
 342                        return NULL;
 343
 344                ptepage = get_zeroed_page(GFP_KERNEL);
 345
 346                /*
 347                 * This is not really the Guest's fault, but killing it is
 348                 * simple for this corner case.
 349                 */
 350                if (!ptepage) {
 351                        kill_guest(cpu, "out of memory allocating pmd page");
 352                        return NULL;
 353                }
 354
 355                /*
 356                 * And we copy the flags to the shadow PMD entry.  The page
 357                 * number in the shadow PMD is the page we just allocated.
 358                 */
 359                set_pmd(spmd, __pmd(__pa(ptepage) | pmd_flags));
 360        }
 361#endif
 362
 363        /* Get the pointer to the shadow PTE entry we're going to set. */
 364        return spte_addr(cpu, *spgd, vaddr);
 365}
 366
 367/*H:330
 368 * (i) Looking up a page table entry when the Guest faults.
 369 *
 370 * We saw this call in run_guest(): when we see a page fault in the Guest, we
 371 * come here.  That's because we only set up the shadow page tables lazily as
 372 * they're needed, so we get page faults all the time and quietly fix them up
 373 * and return to the Guest without it knowing.
 374 *
 375 * If we fixed up the fault (ie. we mapped the address), this routine returns
 376 * true.  Otherwise, it was a real fault and we need to tell the Guest.
 377 */
 378bool demand_page(struct lg_cpu *cpu, unsigned long vaddr, int errcode)
 379{
 380        unsigned long gpte_ptr;
 381        pte_t gpte;
 382        pte_t *spte;
 383        pmd_t gpmd;
 384        pgd_t gpgd;
 385
 386        /* We never demand page the Switcher, so trying is a mistake. */
 387        if (vaddr >= switcher_addr)
 388                return false;
 389
 390        /* First step: get the top-level Guest page table entry. */
 391        if (unlikely(cpu->linear_pages)) {
 392                /* Faking up a linear mapping. */
 393                gpgd = __pgd(CHECK_GPGD_MASK);
 394        } else {
 395                gpgd = lgread(cpu, gpgd_addr(cpu, vaddr), pgd_t);
 396                /* Toplevel not present?  We can't map it in. */
 397                if (!(pgd_flags(gpgd) & _PAGE_PRESENT))
 398                        return false;
 399
 400                /* 
 401                 * This kills the Guest if it has weird flags or tries to
 402                 * refer to a "physical" address outside the bounds.
 403                 */
 404                if (!check_gpgd(cpu, gpgd))
 405                        return false;
 406        }
 407
 408        /* This "mid-level" entry is only used for non-linear, PAE mode. */
 409        gpmd = __pmd(_PAGE_TABLE);
 410
 411#ifdef CONFIG_X86_PAE
 412        if (likely(!cpu->linear_pages)) {
 413                gpmd = lgread(cpu, gpmd_addr(gpgd, vaddr), pmd_t);
 414                /* Middle level not present?  We can't map it in. */
 415                if (!(pmd_flags(gpmd) & _PAGE_PRESENT))
 416                        return false;
 417
 418                /* 
 419                 * This kills the Guest if it has weird flags or tries to
 420                 * refer to a "physical" address outside the bounds.
 421                 */
 422                if (!check_gpmd(cpu, gpmd))
 423                        return false;
 424        }
 425
 426        /*
 427         * OK, now we look at the lower level in the Guest page table: keep its
 428         * address, because we might update it later.
 429         */
 430        gpte_ptr = gpte_addr(cpu, gpmd, vaddr);
 431#else
 432        /*
 433         * OK, now we look at the lower level in the Guest page table: keep its
 434         * address, because we might update it later.
 435         */
 436        gpte_ptr = gpte_addr(cpu, gpgd, vaddr);
 437#endif
 438
 439        if (unlikely(cpu->linear_pages)) {
 440                /* Linear?  Make up a PTE which points to same page. */
 441                gpte = __pte((vaddr & PAGE_MASK) | _PAGE_RW | _PAGE_PRESENT);
 442        } else {
 443                /* Read the actual PTE value. */
 444                gpte = lgread(cpu, gpte_ptr, pte_t);
 445        }
 446
 447        /* If this page isn't in the Guest page tables, we can't page it in. */
 448        if (!(pte_flags(gpte) & _PAGE_PRESENT))
 449                return false;
 450
 451        /*
 452         * Check they're not trying to write to a page the Guest wants
 453         * read-only (bit 2 of errcode == write).
 454         */
 455        if ((errcode & 2) && !(pte_flags(gpte) & _PAGE_RW))
 456                return false;
 457
 458        /* User access to a kernel-only page? (bit 3 == user access) */
 459        if ((errcode & 4) && !(pte_flags(gpte) & _PAGE_USER))
 460                return false;
 461
 462        /*
 463         * Check that the Guest PTE flags are OK, and the page number is below
 464         * the pfn_limit (ie. not mapping the Launcher binary).
 465         */
 466        if (!check_gpte(cpu, gpte))
 467                return false;
 468
 469        /* Add the _PAGE_ACCESSED and (for a write) _PAGE_DIRTY flag */
 470        gpte = pte_mkyoung(gpte);
 471        if (errcode & 2)
 472                gpte = pte_mkdirty(gpte);
 473
 474        /* Get the pointer to the shadow PTE entry we're going to set. */
 475        spte = find_spte(cpu, vaddr, true, pgd_flags(gpgd), pmd_flags(gpmd));
 476        if (!spte)
 477                return false;
 478
 479        /*
 480         * If there was a valid shadow PTE entry here before, we release it.
 481         * This can happen with a write to a previously read-only entry.
 482         */
 483        release_pte(*spte);
 484
 485        /*
 486         * If this is a write, we insist that the Guest page is writable (the
 487         * final arg to gpte_to_spte()).
 488         */
 489        if (pte_dirty(gpte))
 490                *spte = gpte_to_spte(cpu, gpte, 1);
 491        else
 492                /*
 493                 * If this is a read, don't set the "writable" bit in the page
 494                 * table entry, even if the Guest says it's writable.  That way
 495                 * we will come back here when a write does actually occur, so
 496                 * we can update the Guest's _PAGE_DIRTY flag.
 497                 */
 498                set_pte(spte, gpte_to_spte(cpu, pte_wrprotect(gpte), 0));
 499
 500        /*
 501         * Finally, we write the Guest PTE entry back: we've set the
 502         * _PAGE_ACCESSED and maybe the _PAGE_DIRTY flags.
 503         */
 504        if (likely(!cpu->linear_pages))
 505                lgwrite(cpu, gpte_ptr, pte_t, gpte);
 506
 507        /*
 508         * The fault is fixed, the page table is populated, the mapping
 509         * manipulated, the result returned and the code complete.  A small
 510         * delay and a trace of alliteration are the only indications the Guest
 511         * has that a page fault occurred at all.
 512         */
 513        return true;
 514}
 515
 516/*H:360
 517 * (ii) Making sure the Guest stack is mapped.
 518 *
 519 * Remember that direct traps into the Guest need a mapped Guest kernel stack.
 520 * pin_stack_pages() calls us here: we could simply call demand_page(), but as
 521 * we've seen that logic is quite long, and usually the stack pages are already
 522 * mapped, so it's overkill.
 523 *
 524 * This is a quick version which answers the question: is this virtual address
 525 * mapped by the shadow page tables, and is it writable?
 526 */
 527static bool page_writable(struct lg_cpu *cpu, unsigned long vaddr)
 528{
 529        pte_t *spte;
 530        unsigned long flags;
 531
 532        /* You can't put your stack in the Switcher! */
 533        if (vaddr >= switcher_addr)
 534                return false;
 535
 536        /* If there's no shadow PTE, it's not writable. */
 537        spte = find_spte(cpu, vaddr, false, 0, 0);
 538        if (!spte)
 539                return false;
 540
 541        /*
 542         * Check the flags on the pte entry itself: it must be present and
 543         * writable.
 544         */
 545        flags = pte_flags(*spte);
 546        return (flags & (_PAGE_PRESENT|_PAGE_RW)) == (_PAGE_PRESENT|_PAGE_RW);
 547}
 548
 549/*
 550 * So, when pin_stack_pages() asks us to pin a page, we check if it's already
 551 * in the page tables, and if not, we call demand_page() with error code 2
 552 * (meaning "write").
 553 */
 554void pin_page(struct lg_cpu *cpu, unsigned long vaddr)
 555{
 556        if (!page_writable(cpu, vaddr) && !demand_page(cpu, vaddr, 2))
 557                kill_guest(cpu, "bad stack page %#lx", vaddr);
 558}
 559/*:*/
 560
 561#ifdef CONFIG_X86_PAE
 562static void release_pmd(pmd_t *spmd)
 563{
 564        /* If the entry's not present, there's nothing to release. */
 565        if (pmd_flags(*spmd) & _PAGE_PRESENT) {
 566                unsigned int i;
 567                pte_t *ptepage = __va(pmd_pfn(*spmd) << PAGE_SHIFT);
 568                /* For each entry in the page, we might need to release it. */
 569                for (i = 0; i < PTRS_PER_PTE; i++)
 570                        release_pte(ptepage[i]);
 571                /* Now we can free the page of PTEs */
 572                free_page((long)ptepage);
 573                /* And zero out the PMD entry so we never release it twice. */
 574                set_pmd(spmd, __pmd(0));
 575        }
 576}
 577
 578static void release_pgd(pgd_t *spgd)
 579{
 580        /* If the entry's not present, there's nothing to release. */
 581        if (pgd_flags(*spgd) & _PAGE_PRESENT) {
 582                unsigned int i;
 583                pmd_t *pmdpage = __va(pgd_pfn(*spgd) << PAGE_SHIFT);
 584
 585                for (i = 0; i < PTRS_PER_PMD; i++)
 586                        release_pmd(&pmdpage[i]);
 587
 588                /* Now we can free the page of PMDs */
 589                free_page((long)pmdpage);
 590                /* And zero out the PGD entry so we never release it twice. */
 591                set_pgd(spgd, __pgd(0));
 592        }
 593}
 594
 595#else /* !CONFIG_X86_PAE */
 596/*H:450
 597 * If we chase down the release_pgd() code, the non-PAE version looks like
 598 * this.  The PAE version is almost identical, but instead of calling
 599 * release_pte it calls release_pmd(), which looks much like this.
 600 */
 601static void release_pgd(pgd_t *spgd)
 602{
 603        /* If the entry's not present, there's nothing to release. */
 604        if (pgd_flags(*spgd) & _PAGE_PRESENT) {
 605                unsigned int i;
 606                /*
 607                 * Converting the pfn to find the actual PTE page is easy: turn
 608                 * the page number into a physical address, then convert to a
 609                 * virtual address (easy for kernel pages like this one).
 610                 */
 611                pte_t *ptepage = __va(pgd_pfn(*spgd) << PAGE_SHIFT);
 612                /* For each entry in the page, we might need to release it. */
 613                for (i = 0; i < PTRS_PER_PTE; i++)
 614                        release_pte(ptepage[i]);
 615                /* Now we can free the page of PTEs */
 616                free_page((long)ptepage);
 617                /* And zero out the PGD entry so we never release it twice. */
 618                *spgd = __pgd(0);
 619        }
 620}
 621#endif
 622
 623/*H:445
 624 * We saw flush_user_mappings() twice: once from the flush_user_mappings()
 625 * hypercall and once in new_pgdir() when we re-used a top-level pgdir page.
 626 * It simply releases every PTE page from 0 up to the Guest's kernel address.
 627 */
 628static void flush_user_mappings(struct lguest *lg, int idx)
 629{
 630        unsigned int i;
 631        /* Release every pgd entry up to the kernel's address. */
 632        for (i = 0; i < pgd_index(lg->kernel_address); i++)
 633                release_pgd(lg->pgdirs[idx].pgdir + i);
 634}
 635
 636/*H:440
 637 * (v) Flushing (throwing away) page tables,
 638 *
 639 * The Guest has a hypercall to throw away the page tables: it's used when a
 640 * large number of mappings have been changed.
 641 */
 642void guest_pagetable_flush_user(struct lg_cpu *cpu)
 643{
 644        /* Drop the userspace part of the current page table. */
 645        flush_user_mappings(cpu->lg, cpu->cpu_pgd);
 646}
 647/*:*/
 648
 649/* We walk down the guest page tables to get a guest-physical address */
 650unsigned long guest_pa(struct lg_cpu *cpu, unsigned long vaddr)
 651{
 652        pgd_t gpgd;
 653        pte_t gpte;
 654#ifdef CONFIG_X86_PAE
 655        pmd_t gpmd;
 656#endif
 657
 658        /* Still not set up?  Just map 1:1. */
 659        if (unlikely(cpu->linear_pages))
 660                return vaddr;
 661
 662        /* First step: get the top-level Guest page table entry. */
 663        gpgd = lgread(cpu, gpgd_addr(cpu, vaddr), pgd_t);
 664        /* Toplevel not present?  We can't map it in. */
 665        if (!(pgd_flags(gpgd) & _PAGE_PRESENT)) {
 666                kill_guest(cpu, "Bad address %#lx", vaddr);
 667                return -1UL;
 668        }
 669
 670#ifdef CONFIG_X86_PAE
 671        gpmd = lgread(cpu, gpmd_addr(gpgd, vaddr), pmd_t);
 672        if (!(pmd_flags(gpmd) & _PAGE_PRESENT))
 673                kill_guest(cpu, "Bad address %#lx", vaddr);
 674        gpte = lgread(cpu, gpte_addr(cpu, gpmd, vaddr), pte_t);
 675#else
 676        gpte = lgread(cpu, gpte_addr(cpu, gpgd, vaddr), pte_t);
 677#endif
 678        if (!(pte_flags(gpte) & _PAGE_PRESENT))
 679                kill_guest(cpu, "Bad address %#lx", vaddr);
 680
 681        return pte_pfn(gpte) * PAGE_SIZE | (vaddr & ~PAGE_MASK);
 682}
 683
 684/*
 685 * We keep several page tables.  This is a simple routine to find the page
 686 * table (if any) corresponding to this top-level address the Guest has given
 687 * us.
 688 */
 689static unsigned int find_pgdir(struct lguest *lg, unsigned long pgtable)
 690{
 691        unsigned int i;
 692        for (i = 0; i < ARRAY_SIZE(lg->pgdirs); i++)
 693                if (lg->pgdirs[i].pgdir && lg->pgdirs[i].gpgdir == pgtable)
 694                        break;
 695        return i;
 696}
 697
 698/*H:435
 699 * And this is us, creating the new page directory.  If we really do
 700 * allocate a new one (and so the kernel parts are not there), we set
 701 * blank_pgdir.
 702 */
 703static unsigned int new_pgdir(struct lg_cpu *cpu,
 704                              unsigned long gpgdir,
 705                              int *blank_pgdir)
 706{
 707        unsigned int next;
 708
 709        /*
 710         * We pick one entry at random to throw out.  Choosing the Least
 711         * Recently Used might be better, but this is easy.
 712         */
 713        next = prandom_u32() % ARRAY_SIZE(cpu->lg->pgdirs);
 714        /* If it's never been allocated at all before, try now. */
 715        if (!cpu->lg->pgdirs[next].pgdir) {
 716                cpu->lg->pgdirs[next].pgdir =
 717                                        (pgd_t *)get_zeroed_page(GFP_KERNEL);
 718                /* If the allocation fails, just keep using the one we have */
 719                if (!cpu->lg->pgdirs[next].pgdir)
 720                        next = cpu->cpu_pgd;
 721                else {
 722                        /*
 723                         * This is a blank page, so there are no kernel
 724                         * mappings: caller must map the stack!
 725                         */
 726                        *blank_pgdir = 1;
 727                }
 728        }
 729        /* Record which Guest toplevel this shadows. */
 730        cpu->lg->pgdirs[next].gpgdir = gpgdir;
 731        /* Release all the non-kernel mappings. */
 732        flush_user_mappings(cpu->lg, next);
 733
 734        /* This hasn't run on any CPU at all. */
 735        cpu->lg->pgdirs[next].last_host_cpu = -1;
 736
 737        return next;
 738}
 739
 740/*H:501
 741 * We do need the Switcher code mapped at all times, so we allocate that
 742 * part of the Guest page table here.  We map the Switcher code immediately,
 743 * but defer mapping of the guest register page and IDT/LDT etc page until
 744 * just before we run the guest in map_switcher_in_guest().
 745 *
 746 * We *could* do this setup in map_switcher_in_guest(), but at that point
 747 * we've interrupts disabled, and allocating pages like that is fraught: we
 748 * can't sleep if we need to free up some memory.
 749 */
 750static bool allocate_switcher_mapping(struct lg_cpu *cpu)
 751{
 752        int i;
 753
 754        for (i = 0; i < TOTAL_SWITCHER_PAGES; i++) {
 755                pte_t *pte = find_spte(cpu, switcher_addr + i * PAGE_SIZE, true,
 756                                       CHECK_GPGD_MASK, _PAGE_TABLE);
 757                if (!pte)
 758                        return false;
 759
 760                /*
 761                 * Map the switcher page if not already there.  It might
 762                 * already be there because we call allocate_switcher_mapping()
 763                 * in guest_set_pgd() just in case it did discard our Switcher
 764                 * mapping, but it probably didn't.
 765                 */
 766                if (i == 0 && !(pte_flags(*pte) & _PAGE_PRESENT)) {
 767                        /* Get a reference to the Switcher page. */
 768                        get_page(lg_switcher_pages[0]);
 769                        /* Create a read-only, exectuable, kernel-style PTE */
 770                        set_pte(pte,
 771                                mk_pte(lg_switcher_pages[0], PAGE_KERNEL_RX));
 772                }
 773        }
 774        cpu->lg->pgdirs[cpu->cpu_pgd].switcher_mapped = true;
 775        return true;
 776}
 777
 778/*H:470
 779 * Finally, a routine which throws away everything: all PGD entries in all
 780 * the shadow page tables, including the Guest's kernel mappings.  This is used
 781 * when we destroy the Guest.
 782 */
 783static void release_all_pagetables(struct lguest *lg)
 784{
 785        unsigned int i, j;
 786
 787        /* Every shadow pagetable this Guest has */
 788        for (i = 0; i < ARRAY_SIZE(lg->pgdirs); i++) {
 789                if (!lg->pgdirs[i].pgdir)
 790                        continue;
 791
 792                /* Every PGD entry. */
 793                for (j = 0; j < PTRS_PER_PGD; j++)
 794                        release_pgd(lg->pgdirs[i].pgdir + j);
 795                lg->pgdirs[i].switcher_mapped = false;
 796                lg->pgdirs[i].last_host_cpu = -1;
 797        }
 798}
 799
 800/*
 801 * We also throw away everything when a Guest tells us it's changed a kernel
 802 * mapping.  Since kernel mappings are in every page table, it's easiest to
 803 * throw them all away.  This traps the Guest in amber for a while as
 804 * everything faults back in, but it's rare.
 805 */
 806void guest_pagetable_clear_all(struct lg_cpu *cpu)
 807{
 808        release_all_pagetables(cpu->lg);
 809        /* We need the Guest kernel stack mapped again. */
 810        pin_stack_pages(cpu);
 811        /* And we need Switcher allocated. */
 812        if (!allocate_switcher_mapping(cpu))
 813                kill_guest(cpu, "Cannot populate switcher mapping");
 814}
 815
 816/*H:430
 817 * (iv) Switching page tables
 818 *
 819 * Now we've seen all the page table setting and manipulation, let's see
 820 * what happens when the Guest changes page tables (ie. changes the top-level
 821 * pgdir).  This occurs on almost every context switch.
 822 */
 823void guest_new_pagetable(struct lg_cpu *cpu, unsigned long pgtable)
 824{
 825        int newpgdir, repin = 0;
 826
 827        /*
 828         * The very first time they call this, we're actually running without
 829         * any page tables; we've been making it up.  Throw them away now.
 830         */
 831        if (unlikely(cpu->linear_pages)) {
 832                release_all_pagetables(cpu->lg);
 833                cpu->linear_pages = false;
 834                /* Force allocation of a new pgdir. */
 835                newpgdir = ARRAY_SIZE(cpu->lg->pgdirs);
 836        } else {
 837                /* Look to see if we have this one already. */
 838                newpgdir = find_pgdir(cpu->lg, pgtable);
 839        }
 840
 841        /*
 842         * If not, we allocate or mug an existing one: if it's a fresh one,
 843         * repin gets set to 1.
 844         */
 845        if (newpgdir == ARRAY_SIZE(cpu->lg->pgdirs))
 846                newpgdir = new_pgdir(cpu, pgtable, &repin);
 847        /* Change the current pgd index to the new one. */
 848        cpu->cpu_pgd = newpgdir;
 849        /*
 850         * If it was completely blank, we map in the Guest kernel stack and
 851         * the Switcher.
 852         */
 853        if (repin)
 854                pin_stack_pages(cpu);
 855
 856        if (!cpu->lg->pgdirs[cpu->cpu_pgd].switcher_mapped) {
 857                if (!allocate_switcher_mapping(cpu))
 858                        kill_guest(cpu, "Cannot populate switcher mapping");
 859        }
 860}
 861/*:*/
 862
 863/*M:009
 864 * Since we throw away all mappings when a kernel mapping changes, our
 865 * performance sucks for guests using highmem.  In fact, a guest with
 866 * PAGE_OFFSET 0xc0000000 (the default) and more than about 700MB of RAM is
 867 * usually slower than a Guest with less memory.
 868 *
 869 * This, of course, cannot be fixed.  It would take some kind of... well, I
 870 * don't know, but the term "puissant code-fu" comes to mind.
 871:*/
 872
 873/*H:420
 874 * This is the routine which actually sets the page table entry for then
 875 * "idx"'th shadow page table.
 876 *
 877 * Normally, we can just throw out the old entry and replace it with 0: if they
 878 * use it demand_page() will put the new entry in.  We need to do this anyway:
 879 * The Guest expects _PAGE_ACCESSED to be set on its PTE the first time a page
 880 * is read from, and _PAGE_DIRTY when it's written to.
 881 *
 882 * But Avi Kivity pointed out that most Operating Systems (Linux included) set
 883 * these bits on PTEs immediately anyway.  This is done to save the CPU from
 884 * having to update them, but it helps us the same way: if they set
 885 * _PAGE_ACCESSED then we can put a read-only PTE entry in immediately, and if
 886 * they set _PAGE_DIRTY then we can put a writable PTE entry in immediately.
 887 */
 888static void do_set_pte(struct lg_cpu *cpu, int idx,
 889                       unsigned long vaddr, pte_t gpte)
 890{
 891        /* Look up the matching shadow page directory entry. */
 892        pgd_t *spgd = spgd_addr(cpu, idx, vaddr);
 893#ifdef CONFIG_X86_PAE
 894        pmd_t *spmd;
 895#endif
 896
 897        /* If the top level isn't present, there's no entry to update. */
 898        if (pgd_flags(*spgd) & _PAGE_PRESENT) {
 899#ifdef CONFIG_X86_PAE
 900                spmd = spmd_addr(cpu, *spgd, vaddr);
 901                if (pmd_flags(*spmd) & _PAGE_PRESENT) {
 902#endif
 903                        /* Otherwise, start by releasing the existing entry. */
 904                        pte_t *spte = spte_addr(cpu, *spgd, vaddr);
 905                        release_pte(*spte);
 906
 907                        /*
 908                         * If they're setting this entry as dirty or accessed,
 909                         * we might as well put that entry they've given us in
 910                         * now.  This shaves 10% off a copy-on-write
 911                         * micro-benchmark.
 912                         */
 913                        if (pte_flags(gpte) & (_PAGE_DIRTY | _PAGE_ACCESSED)) {
 914                                if (!check_gpte(cpu, gpte))
 915                                        return;
 916                                set_pte(spte,
 917                                        gpte_to_spte(cpu, gpte,
 918                                                pte_flags(gpte) & _PAGE_DIRTY));
 919                        } else {
 920                                /*
 921                                 * Otherwise kill it and we can demand_page()
 922                                 * it in later.
 923                                 */
 924                                set_pte(spte, __pte(0));
 925                        }
 926#ifdef CONFIG_X86_PAE
 927                }
 928#endif
 929        }
 930}
 931
 932/*H:410
 933 * Updating a PTE entry is a little trickier.
 934 *
 935 * We keep track of several different page tables (the Guest uses one for each
 936 * process, so it makes sense to cache at least a few).  Each of these have
 937 * identical kernel parts: ie. every mapping above PAGE_OFFSET is the same for
 938 * all processes.  So when the page table above that address changes, we update
 939 * all the page tables, not just the current one.  This is rare.
 940 *
 941 * The benefit is that when we have to track a new page table, we can keep all
 942 * the kernel mappings.  This speeds up context switch immensely.
 943 */
 944void guest_set_pte(struct lg_cpu *cpu,
 945                   unsigned long gpgdir, unsigned long vaddr, pte_t gpte)
 946{
 947        /* We don't let you remap the Switcher; we need it to get back! */
 948        if (vaddr >= switcher_addr) {
 949                kill_guest(cpu, "attempt to set pte into Switcher pages");
 950                return;
 951        }
 952
 953        /*
 954         * Kernel mappings must be changed on all top levels.  Slow, but doesn't
 955         * happen often.
 956         */
 957        if (vaddr >= cpu->lg->kernel_address) {
 958                unsigned int i;
 959                for (i = 0; i < ARRAY_SIZE(cpu->lg->pgdirs); i++)
 960                        if (cpu->lg->pgdirs[i].pgdir)
 961                                do_set_pte(cpu, i, vaddr, gpte);
 962        } else {
 963                /* Is this page table one we have a shadow for? */
 964                int pgdir = find_pgdir(cpu->lg, gpgdir);
 965                if (pgdir != ARRAY_SIZE(cpu->lg->pgdirs))
 966                        /* If so, do the update. */
 967                        do_set_pte(cpu, pgdir, vaddr, gpte);
 968        }
 969}
 970
 971/*H:400
 972 * (iii) Setting up a page table entry when the Guest tells us one has changed.
 973 *
 974 * Just like we did in interrupts_and_traps.c, it makes sense for us to deal
 975 * with the other side of page tables while we're here: what happens when the
 976 * Guest asks for a page table to be updated?
 977 *
 978 * We already saw that demand_page() will fill in the shadow page tables when
 979 * needed, so we can simply remove shadow page table entries whenever the Guest
 980 * tells us they've changed.  When the Guest tries to use the new entry it will
 981 * fault and demand_page() will fix it up.
 982 *
 983 * So with that in mind here's our code to update a (top-level) PGD entry:
 984 */
 985void guest_set_pgd(struct lguest *lg, unsigned long gpgdir, u32 idx)
 986{
 987        int pgdir;
 988
 989        if (idx > PTRS_PER_PGD) {
 990                kill_guest(&lg->cpus[0], "Attempt to set pgd %u/%u",
 991                           idx, PTRS_PER_PGD);
 992                return;
 993        }
 994
 995        /* If they're talking about a page table we have a shadow for... */
 996        pgdir = find_pgdir(lg, gpgdir);
 997        if (pgdir < ARRAY_SIZE(lg->pgdirs)) {
 998                /* ... throw it away. */
 999                release_pgd(lg->pgdirs[pgdir].pgdir + idx);
1000                /* That might have been the Switcher mapping, remap it. */
1001                if (!allocate_switcher_mapping(&lg->cpus[0])) {
1002                        kill_guest(&lg->cpus[0],
1003                                   "Cannot populate switcher mapping");
1004                }
1005                lg->pgdirs[pgdir].last_host_cpu = -1;
1006        }
1007}
1008
1009#ifdef CONFIG_X86_PAE
1010/* For setting a mid-level, we just throw everything away.  It's easy. */
1011void guest_set_pmd(struct lguest *lg, unsigned long pmdp, u32 idx)
1012{
1013        guest_pagetable_clear_all(&lg->cpus[0]);
1014}
1015#endif
1016
1017/*H:500
1018 * (vii) Setting up the page tables initially.
1019 *
1020 * When a Guest is first created, set initialize a shadow page table which
1021 * we will populate on future faults.  The Guest doesn't have any actual
1022 * pagetables yet, so we set linear_pages to tell demand_page() to fake it
1023 * for the moment.
1024 *
1025 * We do need the Switcher to be mapped at all times, so we allocate that
1026 * part of the Guest page table here.
1027 */
1028int init_guest_pagetable(struct lguest *lg)
1029{
1030        struct lg_cpu *cpu = &lg->cpus[0];
1031        int allocated = 0;
1032
1033        /* lg (and lg->cpus[]) starts zeroed: this allocates a new pgdir */
1034        cpu->cpu_pgd = new_pgdir(cpu, 0, &allocated);
1035        if (!allocated)
1036                return -ENOMEM;
1037
1038        /* We start with a linear mapping until the initialize. */
1039        cpu->linear_pages = true;
1040
1041        /* Allocate the page tables for the Switcher. */
1042        if (!allocate_switcher_mapping(cpu)) {
1043                release_all_pagetables(lg);
1044                return -ENOMEM;
1045        }
1046
1047        return 0;
1048}
1049
1050/*H:508 When the Guest calls LHCALL_LGUEST_INIT we do more setup. */
1051void page_table_guest_data_init(struct lg_cpu *cpu)
1052{
1053        /*
1054         * We tell the Guest that it can't use the virtual addresses
1055         * used by the Switcher.  This trick is equivalent to 4GB -
1056         * switcher_addr.
1057         */
1058        u32 top = ~switcher_addr + 1;
1059
1060        /* We get the kernel address: above this is all kernel memory. */
1061        if (get_user(cpu->lg->kernel_address,
1062                     &cpu->lg->lguest_data->kernel_address)
1063                /*
1064                 * We tell the Guest that it can't use the top virtual
1065                 * addresses (used by the Switcher).
1066                 */
1067            || put_user(top, &cpu->lg->lguest_data->reserve_mem)) {
1068                kill_guest(cpu, "bad guest page %p", cpu->lg->lguest_data);
1069                return;
1070        }
1071
1072        /*
1073         * In flush_user_mappings() we loop from 0 to
1074         * "pgd_index(lg->kernel_address)".  This assumes it won't hit the
1075         * Switcher mappings, so check that now.
1076         */
1077        if (cpu->lg->kernel_address >= switcher_addr)
1078                kill_guest(cpu, "bad kernel address %#lx",
1079                                 cpu->lg->kernel_address);
1080}
1081
1082/* When a Guest dies, our cleanup is fairly simple. */
1083void free_guest_pagetable(struct lguest *lg)
1084{
1085        unsigned int i;
1086
1087        /* Throw away all page table pages. */
1088        release_all_pagetables(lg);
1089        /* Now free the top levels: free_page() can handle 0 just fine. */
1090        for (i = 0; i < ARRAY_SIZE(lg->pgdirs); i++)
1091                free_page((long)lg->pgdirs[i].pgdir);
1092}
1093
1094/*H:481
1095 * This clears the Switcher mappings for cpu #i.
1096 */
1097static void remove_switcher_percpu_map(struct lg_cpu *cpu, unsigned int i)
1098{
1099        unsigned long base = switcher_addr + PAGE_SIZE + i * PAGE_SIZE*2;
1100        pte_t *pte;
1101
1102        /* Clear the mappings for both pages. */
1103        pte = find_spte(cpu, base, false, 0, 0);
1104        release_pte(*pte);
1105        set_pte(pte, __pte(0));
1106
1107        pte = find_spte(cpu, base + PAGE_SIZE, false, 0, 0);
1108        release_pte(*pte);
1109        set_pte(pte, __pte(0));
1110}
1111
1112/*H:480
1113 * (vi) Mapping the Switcher when the Guest is about to run.
1114 *
1115 * The Switcher and the two pages for this CPU need to be visible in the Guest
1116 * (and not the pages for other CPUs).
1117 *
1118 * The pages for the pagetables have all been allocated before: we just need
1119 * to make sure the actual PTEs are up-to-date for the CPU we're about to run
1120 * on.
1121 */
1122void map_switcher_in_guest(struct lg_cpu *cpu, struct lguest_pages *pages)
1123{
1124        unsigned long base;
1125        struct page *percpu_switcher_page, *regs_page;
1126        pte_t *pte;
1127        struct pgdir *pgdir = &cpu->lg->pgdirs[cpu->cpu_pgd];
1128
1129        /* Switcher page should always be mapped by now! */
1130        BUG_ON(!pgdir->switcher_mapped);
1131
1132        /* 
1133         * Remember that we have two pages for each Host CPU, so we can run a
1134         * Guest on each CPU without them interfering.  We need to make sure
1135         * those pages are mapped correctly in the Guest, but since we usually
1136         * run on the same CPU, we cache that, and only update the mappings
1137         * when we move.
1138         */
1139        if (pgdir->last_host_cpu == raw_smp_processor_id())
1140                return;
1141
1142        /* -1 means unknown so we remove everything. */
1143        if (pgdir->last_host_cpu == -1) {
1144                unsigned int i;
1145                for_each_possible_cpu(i)
1146                        remove_switcher_percpu_map(cpu, i);
1147        } else {
1148                /* We know exactly what CPU mapping to remove. */
1149                remove_switcher_percpu_map(cpu, pgdir->last_host_cpu);
1150        }
1151
1152        /*
1153         * When we're running the Guest, we want the Guest's "regs" page to
1154         * appear where the first Switcher page for this CPU is.  This is an
1155         * optimization: when the Switcher saves the Guest registers, it saves
1156         * them into the first page of this CPU's "struct lguest_pages": if we
1157         * make sure the Guest's register page is already mapped there, we
1158         * don't have to copy them out again.
1159         */
1160        /* Find the shadow PTE for this regs page. */
1161        base = switcher_addr + PAGE_SIZE
1162                + raw_smp_processor_id() * sizeof(struct lguest_pages);
1163        pte = find_spte(cpu, base, false, 0, 0);
1164        regs_page = pfn_to_page(__pa(cpu->regs_page) >> PAGE_SHIFT);
1165        get_page(regs_page);
1166        set_pte(pte, mk_pte(regs_page, __pgprot(__PAGE_KERNEL & ~_PAGE_GLOBAL)));
1167
1168        /*
1169         * We map the second page of the struct lguest_pages read-only in
1170         * the Guest: the IDT, GDT and other things it's not supposed to
1171         * change.
1172         */
1173        pte = find_spte(cpu, base + PAGE_SIZE, false, 0, 0);
1174        percpu_switcher_page
1175                = lg_switcher_pages[1 + raw_smp_processor_id()*2 + 1];
1176        get_page(percpu_switcher_page);
1177        set_pte(pte, mk_pte(percpu_switcher_page,
1178                            __pgprot(__PAGE_KERNEL_RO & ~_PAGE_GLOBAL)));
1179
1180        pgdir->last_host_cpu = raw_smp_processor_id();
1181}
1182
1183/*H:490
1184 * We've made it through the page table code.  Perhaps our tired brains are
1185 * still processing the details, or perhaps we're simply glad it's over.
1186 *
1187 * If nothing else, note that all this complexity in juggling shadow page tables
1188 * in sync with the Guest's page tables is for one reason: for most Guests this
1189 * page table dance determines how bad performance will be.  This is why Xen
1190 * uses exotic direct Guest pagetable manipulation, and why both Intel and AMD
1191 * have implemented shadow page table support directly into hardware.
1192 *
1193 * There is just one file remaining in the Host.
1194 */
1195
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