path: root/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 then point the CPU to the
 * converted Guest pages when running the Guest. :*/

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

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

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


/* 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 (PTRS_PER_PGD - 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(pte_t *, switcher_pte_pages);
#define switcher_pte_page(cpu) per_cpu(switcher_pte_pages, cpu)

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

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

/* This routine then takes the page directory entry returned above, which
 * contains the address of the page table entry (PTE) page.  It then returns a
 * pointer to the PTE entry for the given address. */
static pte_t *spte_addr(pgd_t spgd, unsigned long vaddr)
{
	pte_t *page = __va(pgd_pfn(spgd) << PAGE_SHIFT);
	/* You should never call this if the PGD entry wasn't valid */
	BUG_ON(!(pgd_flags(spgd) & _PAGE_PRESENT));
	return &page[(vaddr >> PAGE_SHIFT) % PTRS_PER_PTE];
}

/* 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 lg_cpu *cpu, unsigned long vaddr)
{
	unsigned int index = vaddr >> (PGDIR_SHIFT);
	return cpu->lg->pgdirs[cpu->cpu_pgd].gpgdir + index * sizeof(pgd_t);
}

static unsigned long gpte_addr(pgd_t gpgd, unsigned long vaddr)
{
	unsigned long gpage = pgd_pfn(gpgd) << PAGE_SHIFT;
	BUG_ON(!(pgd_flags(gpgd) & _PAGE_PRESENT));
	return gpage + ((vaddr>>PAGE_SHIFT) % PTRS_PER_PTE) * sizeof(pte_t);
}
/*:*/

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

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

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

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

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

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

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

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

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

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

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

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

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