lguest: documentation V: Host

[safe/jmp/linux-2.6] / drivers / lguest / page_tables.c

/*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 to point the hardware to the
 * actual 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/types.h>
#include <linux/spinlock.h>
#include <linux/random.h>
#include <linux/percpu.h>
#include <asm/tlbflush.h>
#include "lg.h"

/*H:300
 * The Page Table Code
 *
 * We use two-level page tables for the Guest.  If you're not entirely
 * comfortable with virtual addresses, physical addresses and page tables then
 * I recommend you review lguest.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) Setting up a page table entry for the Guest when it faults,
 *  (ii) Setting up the page table entry for the Guest stack,
 *  (iii) Setting up a page table entry when the Guest tells us it has changed,
 *  (iv) Switching page tables,
 *  (v) Flushing (thowing away) page tables,
 *  (vi) Mapping the Switcher when the Guest is about to run,
 *  (vii) Setting up the page tables initially.
 :*/

/* Pages a 4k long, and each page table entry is 4 bytes long, giving us 1024
 * (or 2^10) entries per page. */
#define PTES_PER_PAGE_SHIFT 10
#define PTES_PER_PAGE (1 << PTES_PER_PAGE_SHIFT)

/* 1024 entries in a page table page maps 1024 pages: 4MB.  The Switcher is
 * conveniently placed at the top 4MB, so it uses a separate, complete PTE
 * page.  */
#define SWITCHER_PGD_INDEX (PTES_PER_PAGE - 1)

/* 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(spte_t *, switcher_pte_pages);
#define switcher_pte_page(cpu) per_cpu(switcher_pte_pages, cpu)

/*H:320 With our shadow and Guest types established, we need to deal with
 * them: the page table code is curly enough to need helper functions to keep
 * it clear and clean.
 *
 * The first helper takes a virtual address, and says which entry in the top
 * level page table deals with that address.  Since each top level entry deals
 * with 4M, this effectively divides by 4M. */
static unsigned vaddr_to_pgd_index(unsigned long vaddr)
{
        return vaddr >> (PAGE_SHIFT + PTES_PER_PAGE_SHIFT);
}

/* 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 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 spgd_t *spgd_addr(struct lguest *lg, u32 i, unsigned long vaddr)
{
        unsigned int index = vaddr_to_pgd_index(vaddr);

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

/* This routine then takes the PGD entry given above, which contains the
 * address of the PTE page.  It then returns a pointer to the PTE entry for the
 * given address. */
static spte_t *spte_addr(struct lguest *lg, spgd_t spgd, unsigned long vaddr)
{
        spte_t *page = __va(spgd.pfn << PAGE_SHIFT);
        /* You should never call this if the PGD entry wasn't valid */
        BUG_ON(!(spgd.flags & _PAGE_PRESENT));
        return &page[(vaddr >> PAGE_SHIFT) % PTES_PER_PAGE];
}

/* These two functions just like the above two, except they access the Guest
 * page tables.  Hence they return a Guest address. */
static unsigned long gpgd_addr(struct lguest *lg, unsigned long vaddr)
{
        unsigned int index = vaddr >> (PAGE_SHIFT + PTES_PER_PAGE_SHIFT);
        return lg->pgdirs[lg->pgdidx].cr3 + index * sizeof(gpgd_t);
}

static unsigned long gpte_addr(struct lguest *lg,
                               gpgd_t gpgd, unsigned long vaddr)
{
        unsigned long gpage = gpgd.pfn << PAGE_SHIFT;
        BUG_ON(!(gpgd.flags & _PAGE_PRESENT));
        return gpage + ((vaddr>>PAGE_SHIFT) % PTES_PER_PAGE) * sizeof(gpte_t);
}

/*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;
        /* This value indicates failure. */
        unsigned long ret = -1UL;

        /* get_user_pages() is a complex interface: it gets the "struct
         * vm_area_struct" and "struct page" assocated with a range of pages.
         * It also needs the task's mmap_sem held, and is not very quick.
         * It returns the number of pages it got. */
        down_read(&current->mm->mmap_sem);
        if (get_user_pages(current, current->mm, virtpfn << PAGE_SHIFT,
                           1, write, 1, &page, NULL) == 1)
                ret = page_to_pfn(page);
        up_read(&current->mm->mmap_sem);
        return ret;
}

/*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 spte_t gpte_to_spte(struct lguest *lg, gpte_t gpte, int write)
{
        spte_t spte;
        unsigned long pfn;

        /* 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. */
        spte.flags = (gpte.flags & ~_PAGE_GLOBAL);

        /* 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(gpte.pfn, write);
        if (pfn == -1UL) {
                kill_guest(lg, "failed to get page %u", gpte.pfn);
                /* 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! */
                spte.flags = 0;
        }
        /* Now we assign the page number, and our shadow PTE is complete. */
        spte.pfn = pfn;
        return spte;
}

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

static void check_gpte(struct lguest *lg, gpte_t gpte)
{
        if ((gpte.flags & (_PAGE_PWT|_PAGE_PSE)) || gpte.pfn >= lg->pfn_limit)
                kill_guest(lg, "bad page table entry");
}

static void check_gpgd(struct lguest *lg, gpgd_t gpgd)
{
        if ((gpgd.flags & ~_PAGE_TABLE) || gpgd.pfn >= lg->pfn_limit)
                kill_guest(lg, "bad page directory entry");
}

/*H:330
 * (i) Setting up a page table entry for the Guest when it 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. */
int demand_page(struct lguest *lg, unsigned long vaddr, int errcode)
{
        gpgd_t gpgd;
        spgd_t *spgd;
        unsigned long gpte_ptr;
        gpte_t gpte;
        spte_t *spte;

        /* First step: get the top-level Guest page table entry. */
        gpgd = mkgpgd(lgread_u32(lg, gpgd_addr(lg, vaddr)));
        /* Toplevel not present?  We can't map it in. */
        if (!(gpgd.flags & _PAGE_PRESENT))
                return 0;

        /* Now look at the matching shadow entry. */
        spgd = spgd_addr(lg, lg->pgdidx, vaddr);
        if (!(spgd->flags & _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(lg, "out of memory allocating pte page");
                        return 0;
                }
                /* We check that the Guest pgd is OK. */
                check_gpgd(lg, gpgd);
                /* And we copy the flags to the shadow PGD entry.  The page
                 * number in the shadow PGD is the page we just allocated. */
                spgd->raw.val = (__pa(ptepage) | gpgd.flags);
        }

        /* 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(lg, gpgd, vaddr);
        gpte = mkgpte(lgread_u32(lg, gpte_ptr));

        /* If this page isn't in the Guest page tables, we can't page it in. */
        if (!(gpte.flags & _PAGE_PRESENT))
                return 0;

        /* Check they're not trying to write to a page the Guest wants
         * read-only (bit 2 of errcode == write). */
        if ((errcode & 2) && !(gpte.flags & _PAGE_RW))
                return 0;

        /* User access to a kernel page? (bit 3 == user access) */
        if ((errcode & 4) && !(gpte.flags & _PAGE_USER))
                return 0;

        /* 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(lg, gpte);
        /* Add the _PAGE_ACCESSED and (for a write) _PAGE_DIRTY flag */
        gpte.flags |= _PAGE_ACCESSED;
        if (errcode & 2)
                gpte.flags |= _PAGE_DIRTY;

        /* Get the pointer to the shadow PTE entry we're going to set. */
        spte = spte_addr(lg, *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 (gpte.flags & _PAGE_DIRTY)
                *spte = gpte_to_spte(lg, 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 come back here when a write does actually ocur, so we can
                 * update the Guest's _PAGE_DIRTY flag. */
                gpte_t ro_gpte = gpte;
                ro_gpte.flags &= ~_PAGE_RW;
                *spte = gpte_to_spte(lg, ro_gpte, 0);
        }

        /* Finally, we write the Guest PTE entry back: we've set the
         * _PAGE_ACCESSED and maybe the _PAGE_DIRTY flags. */
        lgwrite_u32(lg, gpte_ptr, gpte.raw.val);

        /* We succeeded in mapping the page! */
        return 1;
}

/*H:360 (ii) Setting up the page table entry for the Guest stack.
 *
 * Remember pin_stack_pages() which makes sure the stack is mapped?  It could
 * simply call demand_page(), but as we've seen that logic is quite long, and
 * usually the stack pages are already mapped anyway, so it's not required.
 *
 * This is a quick version which answers the question: is this virtual address
 * mapped by the shadow page tables, and is it writable? */
static int page_writable(struct lguest *lg, unsigned long vaddr)
{
        spgd_t *spgd;
        unsigned long flags;

        /* Look at the top level entry: is it present? */
        spgd = spgd_addr(lg, lg->pgdidx, vaddr);
        if (!(spgd->flags & _PAGE_PRESENT))
                return 0;

        /* Check the flags on the pte entry itself: it must be present and
         * writable. */
        flags = spte_addr(lg, *spgd, vaddr)->flags;
        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 lguest *lg, unsigned long vaddr)
{
        if (!page_writable(lg, vaddr) && !demand_page(lg, vaddr, 2))
                kill_guest(lg, "bad stack page %#lx", vaddr);
}

/*H:450 If we chase down the release_pgd() code, it looks like this: */
static void release_pgd(struct lguest *lg, spgd_t *spgd)
{
        /* If the entry's not present, there's nothing to release. */
        if (spgd->flags & _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). */
                spte_t *ptepage = __va(spgd->pfn << PAGE_SHIFT);
                /* For each entry in the page, we might need to release it. */
                for (i = 0; i < PTES_PER_PAGE; i++)
                        release_pte(ptepage[i]);
                /* Now we can free the page of PTEs */
                free_page((long)ptepage);
                /* And zero out the PGD entry we we never release it twice. */
                spgd->raw.val = 0;
        }
}

/*H:440 (v) Flushing (thowing away) page tables,
 *
 * We saw flush_user_mappings() called when we re-used a top-level pgdir page.
 * It simply releases every PTE page from 0 up to the 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 < vaddr_to_pgd_index(lg->page_offset); i++)
                release_pgd(lg, lg->pgdirs[idx].pgdir + i);
}

/* The Guest also has a hypercall to do this manually: it's used when a large
 * number of mappings have been changed. */
void guest_pagetable_flush_user(struct lguest *lg)
{
        /* Drop the userspace part of the current page table. */
        flush_user_mappings(lg, lg->pgdidx);
}
/*:*/

/* 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].cr3 == 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 lguest *lg,
                              unsigned long cr3,
                              int *blank_pgdir)
{
        unsigned int next;

        /* 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(lg->pgdirs);
        /* If it's never been allocated at all before, try now. */
        if (!lg->pgdirs[next].pgdir) {
                lg->pgdirs[next].pgdir = (spgd_t *)get_zeroed_page(GFP_KERNEL);
                /* If the allocation fails, just keep using the one we have */
                if (!lg->pgdirs[next].pgdir)
                        next = lg->pgdidx;
                else
                        /* This is a blank page, so there are no kernel
                         * mappings: caller must map the stack! */
                        *blank_pgdir = 1;
        }
        /* Record which Guest toplevel this shadows. */
        lg->pgdirs[next].cr3 = cr3;
        /* Release all the non-kernel mappings. */
        flush_user_mappings(lg, next);

        return next;
}

/*H:430 (iv) Switching page tables
 *
 * This is what happens when the Guest changes page tables (ie. changes the
 * top-level pgdir).  This happens on almost every context switch. */
void guest_new_pagetable(struct lguest *lg, unsigned long pgtable)
{
        int newpgdir, repin = 0;

        /* Look to see if we have this one already. */
        newpgdir = find_pgdir(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(lg->pgdirs))
                newpgdir = new_pgdir(lg, pgtable, &repin);
        /* Change the current pgd index to the new one. */
        lg->pgdidx = newpgdir;
        /* If it was completely blank, we map in the Guest kernel stack */
        if (repin)
                pin_stack_pages(lg);
}

/*H:470 Finally, a routine which throws away everything: all PGD entries in all
 * the shadow page tables.  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)
                        /* Every PGD entry except the Switcher at the top */
                        for (j = 0; j < SWITCHER_PGD_INDEX; j++)
                                release_pgd(lg, 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 is amazingly slow, but thankfully rare. */
void guest_pagetable_clear_all(struct lguest *lg)
{
        release_all_pagetables(lg);
        /* We need the Guest kernel stack mapped again. */
        pin_stack_pages(lg);
}

/*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 lguest *lg, int idx,
                       unsigned long vaddr, gpte_t gpte)
{
        /* Look up the matching shadow page directot entry. */
        spgd_t *spgd = spgd_addr(lg, idx, vaddr);

        /* If the top level isn't present, there's no entry to update. */
        if (spgd->flags & _PAGE_PRESENT) {
                /* Otherwise, we start by releasing the existing entry. */
                spte_t *spte = spte_addr(lg, *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 (gpte.flags & (_PAGE_DIRTY | _PAGE_ACCESSED)) {
                        check_gpte(lg, gpte);
                        *spte = gpte_to_spte(lg, gpte, gpte.flags&_PAGE_DIRTY);
                } else
                        /* Otherwise we can demand_page() it in later. */
                        spte->raw.val = 0;
        }
}

/*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 copy keep
 * all the kernel mappings.  This speeds up context switch immensely. */
void guest_set_pte(struct lguest *lg,
                   unsigned long cr3, unsigned long vaddr, gpte_t gpte)
{
        /* Kernel mappings must be changed on all top levels.  Slow, but
         * doesn't happen often. */
        if (vaddr >= lg->page_offset) {
                unsigned int i;
                for (i = 0; i < ARRAY_SIZE(lg->pgdirs); i++)
                        if (lg->pgdirs[i].pgdir)
                                do_set_pte(lg, i, vaddr, gpte);
        } else {
                /* Is this page table one we have a shadow for? */
                int pgdir = find_pgdir(lg, cr3);
                if (pgdir != ARRAY_SIZE(lg->pgdirs))
                        /* If so, do the update. */
                        do_set_pte(lg, pgdir, vaddr, gpte);
        }
}

/*H:400
 * (iii) Setting up a page table entry when the Guest tells us it 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 to update a (top-level) PGD entry:
 */
void guest_set_pmd(struct lguest *lg, unsigned long cr3, u32 idx)
{
        int pgdir;

        /* The kernel seems to try to initialize this early on: we ignore its
         * attempts to map over the Switcher. */
        if (idx >= SWITCHER_PGD_INDEX)
                return;

        /* If they're talking about a page table we have a shadow for... */
        pgdir = find_pgdir(lg, cr3);
        if (pgdir < ARRAY_SIZE(lg->pgdirs))
                /* ... throw it away. */
                release_pgd(lg, lg->pgdirs[pgdir].pgdir + idx);
}

/*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, unsigned long pgtable)
{
        /* In flush_user_mappings() we loop from 0 to
         * "vaddr_to_pgd_index(lg->page_offset)".  This assumes it won't hit
         * the Switcher mappings, so check that now. */
        if (vaddr_to_pgd_index(lg->page_offset) >= SWITCHER_PGD_INDEX)
                return -EINVAL;
        /* We start on the first shadow page table, and give it a blank PGD
         * page. */
        lg->pgdidx = 0;
        lg->pgdirs[lg->pgdidx].cr3 = pgtable;
        lg->pgdirs[lg->pgdidx].pgdir = (spgd_t*)get_zeroed_page(GFP_KERNEL);
        if (!lg->pgdirs[lg->pgdidx].pgdir)
                return -ENOMEM;
        return 0;
}

/* 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 available to 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. */
void map_switcher_in_guest(struct lguest *lg, struct lguest_pages *pages)
{
        spte_t *switcher_pte_page = __get_cpu_var(switcher_pte_pages);
        spgd_t switcher_pgd;
        spte_t regs_pte;

        /* Make the last PGD entry for this Guest point to the Switcher's PTE
         * page for this CPU (with appropriate flags). */
        switcher_pgd.pfn = __pa(switcher_pte_page) >> PAGE_SHIFT;
        switcher_pgd.flags = _PAGE_KERNEL;
        lg->pgdirs[lg->pgdidx].pgdir[SWITCHER_PGD_INDEX] = switcher_pgd;

        /* 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 = __pa(lg->regs_page) >> PAGE_SHIFT;
        regs_pte.flags = _PAGE_KERNEL;
        switcher_pte_page[(unsigned long)pages/PAGE_SIZE%PTES_PER_PAGE]
                = 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;
        spte_t *pte = switcher_pte_page(cpu);

        /* The first entries are easy: they map the Switcher code. */
        for (i = 0; i < pages; i++) {
                pte[i].pfn = page_to_pfn(switcher_page[i]);
                pte[i].flags = _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 */
        pte[i].pfn = page_to_pfn(switcher_page[i]);
        pte[i].flags = _PAGE_PRESENT|_PAGE_ACCESSED|_PAGE_RW;
        /* The second page contains the "struct lguest_ro_state", and is
         * read-only. */
        pte[i+1].pfn = page_to_pfn(switcher_page[i+1]);
        pte[i+1].flags = _PAGE_PRESENT|_PAGE_ACCESSED;
}

/*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) = (spte_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();
}