- 根目录:
- 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 2013.
* GPL v2 and any later version */
#include <linux/mm.h>
#include <linux/gfp.h>
#include <linux/types.h>
#include <linux/spinlock.h>
#include <linux/random.h>
#include <linux/percpu.h>
#include <asm/tlbflush.h>
#include <asm/uaccess.h>
#include "lg.h"
/*M:008
* We hold reference to pages, which prevents them from being swapped.
* It'd be nice to have a callback in the "struct mm_struct" when Linux wants
* to swap out. If we had this, and a shrinker callback to trim PTE pages, we
* could probably consider launching Guests as non-root.
:*/
/*H:300
* The Page Table Code
*
* We use two-level page tables for the Guest, or three-level with PAE. If
* you're not entirely comfortable with virtual addresses, physical addresses
* and page tables then I recommend you review arch/x86/lguest/boot.c's "Page
* Table Handling" (with diagrams!).
*
* The Guest keeps page tables, but we maintain the actual ones here: these are
* called "shadow" page tables. Which is a very Guest-centric name: these are
* the real page tables the CPU uses, although we keep them up to date to
* reflect the Guest's. (See what I mean about weird naming? Since when do
* shadows reflect anything?)
*
* Anyway, this is the most complicated part of the Host code. There are seven
* parts to this:
* (i) Looking up a page table entry when the Guest faults,
* (ii) Making sure the Guest stack is mapped,
* (iii) Setting up a page table entry when the Guest tells us one has changed,
* (iv) Switching page tables,
* (v) Flushing (throwing away) page tables,
* (vi) Mapping the Switcher when the Guest is about to run,
* (vii) Setting up the page tables initially.
:*/
/*
* The Switcher uses the complete top PTE page. That's 1024 PTE entries (4MB)
* or 512 PTE entries with PAE (2MB).
*/
#define SWITCHER_PGD_INDEX (PTRS_PER_PGD - 1)
/*
* For PAE we need the PMD index as well. We use the last 2MB, so we
* will need the last pmd entry of the last pmd page.
*/
#ifdef CONFIG_X86_PAE
#define CHECK_GPGD_MASK _PAGE_PRESENT
#else
#define CHECK_GPGD_MASK _PAGE_TABLE
#endif
/*H:320
* The page table code is curly enough to need helper functions to keep it
* clear and clean. The kernel itself provides many of them; one advantage
* of insisting that the Guest and Host use the same CONFIG_X86_PAE setting.
*
* There are two functions which return pointers to the shadow (aka "real")
* page tables.
*
* spgd_addr() takes the virtual address and returns a pointer to the top-level
* page directory entry (PGD) for that address. Since we keep track of several
* page tables, the "i" argument tells us which one we're interested in (it's
* usually the current one).
*/
static pgd_t *spgd_addr(struct lg_cpu *cpu, u32 i, unsigned long vaddr)
{
unsigned int index = pgd_index(vaddr);
/* Return a pointer index'th pgd entry for the i'th page table. */
return &cpu->lg->pgdirs[i].pgdir[index];
}
#ifdef CONFIG_X86_PAE
/*
* This routine then takes the PGD entry given above, which contains the
* address of the PMD page. It then returns a pointer to the PMD entry for the
* given address.
*/
static pmd_t *spmd_addr(struct lg_cpu *cpu, pgd_t spgd, unsigned long vaddr)
{
unsigned int index = pmd_index(vaddr);
pmd_t *page;
/* You should never call this if the PGD entry wasn't valid */
BUG_ON(!(pgd_flags(spgd) & _PAGE_PRESENT));
page = __va(pgd_pfn(spgd) << PAGE_SHIFT);
return &page[index];
}
#endif
/*
* This routine then takes the page directory entry returned above, which
* contains the address of the page table entry (PTE) page. It then returns a
* pointer to the PTE entry for the given address.
*/
static pte_t *spte_addr(struct lg_cpu *cpu, pgd_t spgd, unsigned long vaddr)
{
#ifdef CONFIG_X86_PAE
pmd_t *pmd = spmd_addr(cpu, spgd, vaddr);
pte_t *page = __va(pmd_pfn(*pmd) << PAGE_SHIFT);
/* You should never call this if the PMD entry wasn't valid */
BUG_ON(!(pmd_flags(*pmd) & _PAGE_PRESENT));
#else
pte_t *page = __va(pgd_pfn(spgd) << PAGE_SHIFT);
/* You should never call this if the PGD entry wasn't valid */
BUG_ON(!(pgd_flags(spgd) & _PAGE_PRESENT));
#endif
return &page[pte_index(vaddr)];
}
/*
* These functions are just like the above, except they access the Guest
* page tables. Hence they return a Guest address.
*/
static unsigned long gpgd_addr(struct lg_cpu *cpu, unsigned long vaddr)
{
unsigned int index = vaddr >> (PGDIR_SHIFT);
return cpu->lg->pgdirs[cpu->cpu_pgd].gpgdir + index * sizeof(pgd_t);
}
#ifdef CONFIG_X86_PAE
/* Follow the PGD to the PMD. */
static unsigned long gpmd_addr(pgd_t gpgd, unsigned long vaddr)
{
unsigned long gpage = pgd_pfn(gpgd) << PAGE_SHIFT;
BUG_ON(!(pgd_flags(gpgd) & _PAGE_PRESENT));
return gpage + pmd_index(vaddr) * sizeof(pmd_t);
}
/* Follow the PMD to the PTE. */
static unsigned long gpte_addr(struct lg_cpu *cpu,
pmd_t gpmd, unsigned long vaddr)
{
unsigned long gpage = pmd_pfn(gpmd) << PAGE_SHIFT;
BUG_ON(!(pmd_flags(gpmd) & _PAGE_PRESENT));
return gpage + pte_index(vaddr) * sizeof(pte_t);
}
#else
/* Follow the PGD to the PTE (no mid-level for !PAE). */
static unsigned long gpte_addr(struct lg_cpu *cpu,
pgd_t gpgd, unsigned long vaddr)
{
unsigned long gpage = pgd_pfn(gpgd) << PAGE_SHIFT;
BUG_ON(!(pgd_flags(gpgd) & _PAGE_PRESENT));
return gpage + pte_index(vaddr) * sizeof(pte_t);
}
#endif
/*:*/
/*M:007
* get_pfn is slow: we could probably try to grab batches of pages here as
* an optimization (ie. pre-faulting).
:*/
/*H:350
* This routine takes a page number given by the Guest and converts it to
* an actual, physical page number. It can fail for several reasons: the
* virtual address might not be mapped by the Launcher, the write flag is set
* and the page is read-only, or the write flag was set and the page was
* shared so had to be copied, but we ran out of memory.
*
* This holds a reference to the page, so release_pte() is careful to put that
* back.
*/
static unsigned long get_pfn(unsigned long virtpfn, int write)
{
struct page *page;
/* gup me one page at this address please! */
if (get_user_pages_fast(virtpfn << PAGE_SHIFT, 1, write, &page) == 1)
return page_to_pfn(page);
/* This value indicates failure. */
return -1UL;
}
/*H:340
* Converting a Guest page table entry to a shadow (ie. real) page table
* entry can be a little tricky. The flags are (almost) the same, but the
* Guest PTE contains a virtual page number: the CPU needs the real page
* number.
*/
static pte_t gpte_to_spte(struct lg_cpu *cpu, pte_t gpte, int write)
{
unsigned long pfn, base, flags;
/*
* The Guest sets the global flag, because it thinks that it is using
* PGE. We only told it to use PGE so it would tell us whether it was
* flushing a kernel mapping or a userspace mapping. We don't actually
* use the global bit, so throw it away.
*/
flags = (pte_flags(gpte) & ~_PAGE_GLOBAL);
/* The Guest's pages are offset inside the Launcher. */
base = (unsigned long)cpu->lg->mem_base / PAGE_SIZE;
/*
* We need a temporary "unsigned long" variable to hold the answer from
* get_pfn(), because it returns 0xFFFFFFFF on failure, which wouldn't
* fit in spte.pfn. get_pfn() finds the real physical number of the
* page, given the virtual number.
*/
pfn = get_pfn(base + pte_pfn(gpte), write);
if (pfn == -1UL) {
kill_guest(cpu, "failed to get page %lu", pte_pfn(gpte));
/*
* When we destroy the Guest, we'll go through the shadow page
* tables and release_pte() them. Make sure we don't think
* this one is valid!
*/
flags = 0;
}
/* Now we assemble our shadow PTE from the page number and flags. */
return pfn_pte(pfn, __pgprot(flags));
}
/*H:460 And to complete the chain, release_pte() looks like this: */
static void release_pte(pte_t pte)
{
/*
* Remember that get_user_pages_fast() took a reference to the page, in
* get_pfn()? We have to put it back now.
*/
if (pte_flags(pte) & _PAGE_PRESENT)
put_page(pte_page(pte));
}
/*:*/
static bool gpte_in_iomem(struct lg_cpu *cpu, pte_t gpte)
{
/* We don't handle large pages. */
if (pte_flags(gpte) & _PAGE_PSE)
return false;
return (pte_pfn(gpte) >= cpu->lg->pfn_limit
&& pte_pfn(gpte) < cpu->lg->device_limit);
}
static bool 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");
return false;
}
return true;
}
static bool check_gpgd(struct lg_cpu *cpu, pgd_t gpgd)
{
if ((pgd_flags(gpgd) & ~CHECK_GPGD_MASK) ||
(pgd_pfn(gpgd) >= cpu->lg->pfn_limit)) {
kill_guest(cpu, "bad page directory entry");
return false;
}
return true;
}
#ifdef CONFIG_X86_PAE
static bool check_gpmd(struct lg_cpu *cpu, pmd_t gpmd)
{
if ((pmd_flags(gpmd) & ~_PAGE_TABLE) ||
(pmd_pfn(gpmd) >= cpu->lg->pfn_limit)) {
kill_guest(cpu, "bad page middle directory entry");
return false;
}
return true;
}
#endif
/*H:331
* This is the core routine to walk the shadow page tables and find the page
* table entry for a specific address.
*
* If allocate is set, then we allocate any missing levels, setting the flags
* on the new page directory and mid-level directories using the arguments
* (which are copied from the Guest's page table entries).
*/
static pte_t *find_spte(struct lg_cpu *cpu, unsigned long vaddr, bool allocate,
int pgd_flags, int pmd_flags)
{
pgd_t *spgd;
/* Mid level for PAE. */
#ifdef CONFIG_X86_PAE
pmd_t *spmd;
#endif
/* Get top level entry. */
spgd = spgd_addr(cpu, cpu->cpu_pgd, vaddr);
if (!(pgd_flags(*spgd) & _PAGE_PRESENT)) {
/* No shadow entry: allocate a new shadow PTE page. */
unsigned long ptepage;
/* If they didn't want us to allocate anything, stop. */
if (!allocate)
return NULL;
ptepage = get_zeroed_page(GFP_KERNEL);
/*
* This is not really the Guest's fault, but killing it is
* simple for this corner case.
*/
if (!ptepage) {
kill_guest(cpu, "out of memory allocating pte page");
return NULL;
}
/*
* And we copy the flags to the shadow PGD entry. The page
* number in the shadow PGD is the page we just allocated.
*/
set_pgd(spgd, __pgd(__pa(ptepage) | pgd_flags));
}
/*
* Intel's Physical Address Extension actually uses three levels of
* page tables, so we need to look in the mid-level.
*/
#ifdef CONFIG_X86_PAE
/* Now look at the mid-level shadow entry. */
spmd = spmd_addr(cpu, *spgd, vaddr);
if (!(pmd_flags(*spmd) & _PAGE_PRESENT)) {
/* No shadow entry: allocate a new shadow PTE page. */
unsigned long ptepage;
/* If they didn't want us to allocate anything, stop. */
if (!allocate)
return NULL;
ptepage = get_zeroed_page(GFP_KERNEL);
/*
* This is not really the Guest's fault, but killing it is
* simple for this corner case.
*/
if (!ptepage) {
kill_guest(cpu, "out of memory allocating pmd page");
return NULL;
}
/*
* And we copy the flags to the shadow PMD entry. The page
* number in the shadow PMD is the page we just allocated.
*/
set_pmd(spmd, __pmd(__pa(ptepage) | pmd_flags));
}
#endif
/* Get the pointer to the shadow PTE entry we're going to set. */
return spte_addr(cpu, *spgd, vaddr);
}
/*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.
*
* There's a corner case: they're trying to access memory between
* pfn_limit and device_limit, which is I/O memory. In this case, we
* return false and set @iomem to the physical address, so the the
* Launcher can handle the instruction manually.
*/
bool demand_page(struct lg_cpu *cpu, unsigned long vaddr, int errcode,
unsigned long *iomem)
{
unsigned long gpte_ptr;
pte_t gpte;
pte_t *spte;
pmd_t gpmd;
pgd_t gpgd;
*iomem = 0;
/* We never demand page the Switcher, so trying is a mistake. */
if (vaddr >= switcher_addr)
return false;
/* First step: get the top-level Guest page table entry. */
if (unlikely(cpu->linear_pages)) {
/* Faking up a linear mapping. */
gpgd = __pgd(CHECK_GPGD_MASK);
} else {
gpgd = lgread(cpu, gpgd_addr(cpu, vaddr), pgd_t);
/* Toplevel not present? We can't map it in. */
if (!(pgd_flags(gpgd) & _PAGE_PRESENT))
return false;
/*
* This kills the Guest if it has weird flags or tries to
* refer to a "physical" address outside the bounds.
*/
if (!check_gpgd(cpu, gpgd))
return false;
}
/* This "mid-level" entry is only used for non-linear, PAE mode. */
gpmd = __pmd(_PAGE_TABLE);
#ifdef CONFIG_X86_PAE
if (likely(!cpu->linear_pages)) {
gpmd = lgread(cpu, gpmd_addr(gpgd, vaddr), pmd_t);
/* Middle level not present? We can't map it in. */
if (!(pmd_flags(gpmd) & _PAGE_PRESENT))
return false;
/*
* This kills the Guest if it has weird flags or tries to
* refer to a "physical" address outside the bounds.
*/
if (!check_gpmd(cpu, gpmd))
return false;
}
/*
* OK, now we look at the lower level in the Guest page table: keep its
* address, because we might update it later.
*/
gpte_ptr = gpte_addr(cpu, gpmd, vaddr);
#else
/*
* OK, now we look at the lower level in the Guest page table: keep its
* address, because we might update it later.
*/
gpte_ptr = gpte_addr(cpu, gpgd, vaddr);
#endif
if (unlikely(cpu->linear_pages)) {
/* Linear? Make up a PTE which points to same page. */
gpte = __pte((vaddr & PAGE_MASK) | _PAGE_RW | _PAGE_PRESENT);
} else {
/* Read the actual PTE value. */
gpte = lgread(cpu, gpte_ptr, pte_t);
}
/* If this page isn't in the Guest page tables, we can't page it in. */
if (!(pte_flags(gpte) & _PAGE_PRESENT))
return false;
/*
* Check they're not trying to write to a page the Guest wants
* read-only (bit 2 of errcode == write).
*/
if ((errcode & 2) && !(pte_flags(gpte) & _PAGE_RW))
return false;
/* User access to a kernel-only page? (bit 3 == user access) */
if ((errcode & 4) && !(pte_flags(gpte) & _PAGE_USER))
return false;
/* If they're accessing io memory, we expect a fault. */
if (gpte_in_iomem(cpu, gpte)) {
*iomem = (pte_pfn(gpte) << PAGE_SHIFT) | (vaddr & ~PAGE_MASK);
return false;
}
/*
* Check that the Guest PTE flags are OK, and the page number is below
* the pfn_limit (ie. not mapping the Launcher binary).
*/
if (!check_gpte(cpu, gpte))
return false;
/* Add the _PAGE_ACCESSED and (for a write) _PAGE_DIRTY flag */
gpte = pte_mkyoung(gpte);
if (errcode & 2)
gpte = pte_mkdirty(gpte);
/* Get the pointer to the shadow PTE entry we're going to set. */
spte = find_spte(cpu, vaddr, true, pgd_flags(gpgd), pmd_flags(gpmd));
if (!spte)
return false;
/*
* If there was a valid shadow PTE entry here before, we release it.
* This can happen with a write to a previously read-only entry.
*/
release_pte(*spte);
/*
* If this is a write, we insist that the Guest page is writable (the
* final arg to gpte_to_spte()).
*/
if (pte_dirty(gpte))
*spte = gpte_to_spte(cpu, gpte, 1);
else
/*
* If this is a read, don't set the "writable" bit in the page
* table entry, even if the Guest says it's writable. That way
* we will come back here when a write does actually occur, so
* we can update the Guest's _PAGE_DIRTY flag.
*/
set_pte(spte, gpte_to_spte(cpu, pte_wrprotect(gpte), 0));
/*
* Finally, we write the Guest PTE entry back: we've set the
* _PAGE_ACCESSED and maybe the _PAGE_DIRTY flags.
*/
if (likely(!cpu->linear_pages))
lgwrite(cpu, gpte_ptr, pte_t, gpte);
/*
* The fault is fixed, the page table is populated, the mapping
* manipulated, the result returned and the code complete. A small
* delay and a trace of alliteration are the only indications the Guest
* has that a page fault occurred at all.
*/
return true;
}
/*H:360
* (ii) Making sure the Guest stack is mapped.
*
* Remember that direct traps into the Guest need a mapped Guest kernel stack.
* pin_stack_pages() calls us here: we could simply call demand_page(), but as
* we've seen that logic is quite long, and usually the stack pages are already
* mapped, so it's overkill.
*
* This is a quick version which answers the question: is this virtual address
* mapped by the shadow page tables, and is it writable?
*/
static bool page_writable(struct lg_cpu *cpu, unsigned long vaddr)
{
pte_t *spte;
unsigned long flags;
/* You can't put your stack in the Switcher! */
if (vaddr >= switcher_addr)
return false;
/* If there's no shadow PTE, it's not writable. */
spte = find_spte(cpu, vaddr, false, 0, 0);
if (!spte)
return false;
/*
* Check the flags on the pte entry itself: it must be present and
* writable.
*/
flags = pte_flags(*spte);
return (flags & (_PAGE_PRESENT|_PAGE_RW)) == (_PAGE_PRESENT|_PAGE_RW);
}
/*
* So, when pin_stack_pages() asks us to pin a page, we check if it's already
* in the page tables, and if not, we call demand_page() with error code 2
* (meaning "write").
*/
void pin_page(struct lg_cpu *cpu, unsigned long vaddr)
{
unsigned long iomem;
if (!page_writable(cpu, vaddr) && !demand_page(cpu, vaddr, 2, &iomem))
kill_guest(cpu, "bad stack page %#lx", vaddr);
}
/*:*/
#ifdef CONFIG_X86_PAE
static void release_pmd(pmd_t *spmd)
{
/* If the entry's not present, there's nothing to release. */
if (pmd_flags(*spmd) & _PAGE_PRESENT) {
unsigned int i;
pte_t *ptepage = __va(pmd_pfn(*spmd) << PAGE_SHIFT);
/* For each entry in the page, we might need to release it. */
for (i = 0; i < PTRS_PER_PTE; i++)
release_pte(ptepage[i]);
/* Now we can free the page of PTEs */
free_page((long)ptepage);
/* And zero out the PMD entry so we never release it twice. */
set_pmd(spmd, __pmd(0));
}
}
static void release_pgd(pgd_t *spgd)
{
/* If the entry's not present, there's nothing to release. */
if (pgd_flags(*spgd) & _PAGE_PRESENT) {
unsigned int i;
pmd_t *pmdpage = __va(pgd_pfn(*spgd) << PAGE_SHIFT);
for (i = 0; i < PTRS_PER_PMD; i++)
release_pmd(&pmdpage[i]);
/* Now we can free the page of PMDs */
free_page((long)pmdpage);
/* And zero out the PGD entry so we never release it twice. */
set_pgd(spgd, __pgd(0));
}
}
#else /* !CONFIG_X86_PAE */
/*H:450
* If we chase down the release_pgd() code, the non-PAE version looks like
* this. The PAE version is almost identical, but instead of calling
* release_pte it calls release_pmd(), which looks much like this.
*/
static void release_pgd(pgd_t *spgd)
{
/* If the entry's not present, there's nothing to release. */
if (pgd_flags(*spgd) & _PAGE_PRESENT) {
unsigned int i;
/*
* Converting the pfn to find the actual PTE page is easy: turn
* the page number into a physical address, then convert to a
* virtual address (easy for kernel pages like this one).
*/
pte_t *ptepage = __va(pgd_pfn(*spgd) << PAGE_SHIFT);
/* For each entry in the page, we might need to release it. */
for (i = 0; i < PTRS_PER_PTE; i++)
release_pte(ptepage[i]);
/* Now we can free the page of PTEs */
free_page((long)ptepage);
/* And zero out the PGD entry so we never release it twice. */
*spgd = __pgd(0);
}
}
#endif
/*H:445
* We saw flush_user_mappings() twice: once from the flush_user_mappings()
* hypercall and once in new_pgdir() when we re-used a top-level pgdir page.
* It simply releases every PTE page from 0 up to the Guest's kernel address.
*/
static void flush_user_mappings(struct lguest *lg, int idx)
{
unsigned int i;
/* Release every pgd entry up to the kernel's address. */
for (i = 0; i < pgd_index(lg->kernel_address); i++)
release_pgd(lg->pgdirs[idx].pgdir + i);
}
/*H:440
* (v) Flushing (throwing away) page tables,
*
* The Guest has a hypercall to throw away the page tables: it's used when a
* large number of mappings have been changed.
*/
void guest_pagetable_flush_user(struct lg_cpu *cpu)
{
/* Drop the userspace part of the current page table. */
flush_user_mappings(cpu->lg, cpu->cpu_pgd);
}
/*:*/
/* We walk down the guest page tables to get a guest-physical address */
bool __guest_pa(struct lg_cpu *cpu, unsigned long vaddr, unsigned long *paddr)
{
pgd_t gpgd;
pte_t gpte;
#ifdef CONFIG_X86_PAE
pmd_t gpmd;
#endif
/* Still not set up? Just map 1:1. */
if (unlikely(cpu->linear_pages)) {
*paddr = vaddr;
return true;
}
/* First step: get the top-level Guest page table entry. */
gpgd = lgread(cpu, gpgd_addr(cpu, vaddr), pgd_t);
/* Toplevel not present? We can't map it in. */
if (!(pgd_flags(gpgd) & _PAGE_PRESENT))
goto fail;
#ifdef CONFIG_X86_PAE
gpmd = lgread(cpu, gpmd_addr(gpgd, vaddr), pmd_t);
if (!(pmd_flags(gpmd) & _PAGE_PRESENT))
goto fail;
gpte = lgread(cpu, gpte_addr(cpu, gpmd, vaddr), pte_t);
#else
gpte = lgread(cpu, gpte_addr(cpu, gpgd, vaddr), pte_t);
#endif
if (!(pte_flags(gpte) & _PAGE_PRESENT))
goto fail;
*paddr = pte_pfn(gpte) * PAGE_SIZE | (vaddr & ~PAGE_MASK);
return true;
fail:
*paddr = -1UL;
return false;
}
/*
* This is the version we normally use: kills the Guest if it uses a
* bad address
*/
unsigned long guest_pa(struct lg_cpu *cpu, unsigned long vaddr)
{
unsigned long paddr;
if (!__guest_pa(cpu, vaddr, &paddr))
kill_guest(cpu, "Bad address %#lx", vaddr);
return paddr;
}
/*
* We keep several page tables. This is a simple routine to find the page
* table (if any) corresponding to this top-level address the Guest has given
* us.
*/
static unsigned int find_pgdir(struct lguest *lg, unsigned long pgtable)
{
unsigned int i;
for (i = 0; i < ARRAY_SIZE(lg->pgdirs); i++)
if (lg->pgdirs[i].pgdir && lg->pgdirs[i].gpgdir == pgtable)
break;
return i;
}
/*H:435
* And this is us, creating the new page directory. If we really do
* allocate a new one (and so the kernel parts are not there), we set
* blank_pgdir.
*/
static unsigned int new_pgdir(struct lg_cpu *cpu,
unsigned long gpgdir,
int *blank_pgdir)
{
unsigned int next;
/*
* We pick one entry at random to throw out. Choosing the Least
* Recently Used might be better, but this is easy.
*/
next = prandom_u32() % ARRAY_SIZE(cpu->lg->pgdirs);
/* If it's never been allocated at all before, try now. */
if (!cpu->lg->pgdirs[next].pgdir) {
cpu->lg->pgdirs[next].pgdir =
(pgd_t *)get_zeroed_page(GFP_KERNEL);
/* If the allocation fails, just keep using the one we have */
if (!cpu->lg->pgdirs[next].pgdir)
next = cpu->cpu_pgd;
else {
/*
* 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. */
cpu->lg->pgdirs[next].gpgdir = gpgdir;
/* Release all the non-kernel mappings. */
flush_user_mappings(cpu->lg, next);
/* This hasn't run on any CPU at all. */
cpu->lg->pgdirs[next].last_host_cpu = -1;
return next;
}
/*H:501
* We do need the Switcher code mapped at all times, so we allocate that
* part of the Guest page table here. We map the Switcher code immediately,
* but defer mapping of the guest register page and IDT/LDT etc page until
* just before we run the guest in map_switcher_in_guest().
*
* We *could* do this setup in map_switcher_in_guest(), but at that point
* we've interrupts disabled, and allocating pages like that is fraught: we
* can't sleep if we need to free up some memory.
*/
static bool allocate_switcher_mapping(struct lg_cpu *cpu)
{
int i;
for (i = 0; i < TOTAL_SWITCHER_PAGES; i++) {
pte_t *pte = find_spte(cpu, switcher_addr + i * PAGE_SIZE, true,
CHECK_GPGD_MASK, _PAGE_TABLE);
if (!pte)
return false;
/*
* Map the switcher page if not already there. It might
* already be there because we call allocate_switcher_mapping()
* in guest_set_pgd() just in case it did discard our Switcher
* mapping, but it probably didn't.
*/
if (i == 0 && !(pte_flags(*pte) & _PAGE_PRESENT)) {
/* Get a reference to the Switcher page. */
get_page(lg_switcher_pages[0]);
/* Create a read-only, exectuable, kernel-style PTE */
set_pte(pte,
mk_pte(lg_switcher_pages[0], PAGE_KERNEL_RX));
}
}
cpu->lg->pgdirs[cpu->cpu_pgd].switcher_mapped = true;
return true;
}
/*H:470
* Finally, a routine which throws away everything: all PGD entries in all
* the shadow page tables, including the Guest's kernel mappings. This is used
* when we destroy the Guest.
*/
static void release_all_pagetables(struct lguest *lg)
{
unsigned int i, j;
/* Every shadow pagetable this Guest has */
for (i = 0; i < ARRAY_SIZE(lg->pgdirs); i++) {
if (!lg->pgdirs[i].pgdir)
continue;
/* Every PGD entry. */
for (j = 0; j < PTRS_PER_PGD; j++)
release_pgd(lg->pgdirs[i].pgdir + j);
lg->pgdirs[i].switcher_mapped = false;
lg->pgdirs[i].last_host_cpu = -1;
}
}
/*
* We also throw away everything when a Guest tells us it's changed a kernel
* mapping. Since kernel mappings are in every page table, it's easiest to
* throw them all away. This traps the Guest in amber for a while as
* everything faults back in, but it's rare.
*/
void guest_pagetable_clear_all(struct lg_cpu *cpu)
{
release_all_pagetables(cpu->lg);
/* We need the Guest kernel stack mapped again. */
pin_stack_pages(cpu);
/* And we need Switcher allocated. */
if (!allocate_switcher_mapping(cpu))
kill_guest(cpu, "Cannot populate switcher mapping");
}
/*H:430
* (iv) Switching page tables
*
* Now we've seen all the page table setting and manipulation, let's see
* what happens when the Guest changes page tables (ie. changes the top-level
* pgdir). This occurs on almost every context switch.
*/
void guest_new_pagetable(struct lg_cpu *cpu, unsigned long pgtable)
{
int newpgdir, repin = 0;
/*
* The very first time they call this, we're actually running without
* any page tables; we've been making it up. Throw them away now.
*/
if (unlikely(cpu->linear_pages)) {
release_all_pagetables(cpu->lg);
cpu->linear_pages = false;
/* Force allocation of a new pgdir. */
newpgdir = ARRAY_SIZE(cpu->lg->pgdirs);
} else {
/* Look to see if we have this one already. */
newpgdir = find_pgdir(cpu->lg, pgtable);
}
/*
* If not, we allocate or mug an existing one: if it's a fresh one,
* repin gets set to 1.
*/
if (newpgdir == ARRAY_SIZE(cpu->lg->pgdirs))
newpgdir = new_pgdir(cpu, pgtable, &repin);
/* Change the current pgd index to the new one. */
cpu->cpu_pgd = newpgdir;
/*
* If it was completely blank, we map in the Guest kernel stack and
* the Switcher.
*/
if (repin)
pin_stack_pages(cpu);
if (!cpu->lg->pgdirs[cpu->cpu_pgd].switcher_mapped) {
if (!allocate_switcher_mapping(cpu))
kill_guest(cpu, "Cannot populate switcher mapping");
}
}
/*:*/
/*M:009
* Since we throw away all mappings when a kernel mapping changes, our
* performance sucks for guests using highmem. In fact, a guest with
* PAGE_OFFSET 0xc0000000 (the default) and more than about 700MB of RAM is
* usually slower than a Guest with less memory.
*
* This, of course, cannot be fixed. It would take some kind of... well, I
* don't know, but the term "puissant code-fu" comes to mind.
:*/
/*H:420
* This is the routine which actually sets the page table entry for then
* "idx"'th shadow page table.
*
* Normally, we can just throw out the old entry and replace it with 0: if they
* use it demand_page() will put the new entry in. We need to do this anyway:
* The Guest expects _PAGE_ACCESSED to be set on its PTE the first time a page
* is read from, and _PAGE_DIRTY when it's written to.
*
* But Avi Kivity pointed out that most Operating Systems (Linux included) set
* these bits on PTEs immediately anyway. This is done to save the CPU from
* having to update them, but it helps us the same way: if they set
* _PAGE_ACCESSED then we can put a read-only PTE entry in immediately, and if
* they set _PAGE_DIRTY then we can put a writable PTE entry in immediately.
*/
static void __guest_set_pte(struct lg_cpu *cpu, int idx,
unsigned long vaddr, pte_t gpte)
{
/* Look up the matching shadow page directory entry. */
pgd_t *spgd = spgd_addr(cpu, idx, vaddr);
#ifdef CONFIG_X86_PAE
pmd_t *spmd;
#endif
/* If the top level isn't present, there's no entry to update. */
if (pgd_flags(*spgd) & _PAGE_PRESENT) {
#ifdef CONFIG_X86_PAE
spmd = spmd_addr(cpu, *spgd, vaddr);
if (pmd_flags(*spmd) & _PAGE_PRESENT) {
#endif
/* Otherwise, start by releasing the existing entry. */
pte_t *spte = spte_addr(cpu, *spgd, vaddr);
release_pte(*spte);
/*
* If they're setting this entry as dirty or accessed,
* we might as well put that entry they've given us in
* now. This shaves 10% off a copy-on-write
* micro-benchmark.
*/
if ((pte_flags(gpte) & (_PAGE_DIRTY | _PAGE_ACCESSED))
&& !gpte_in_iomem(cpu, gpte)) {
if (!check_gpte(cpu, gpte))
return;
set_pte(spte,
gpte_to_spte(cpu, gpte,
pte_flags(gpte) & _PAGE_DIRTY));
} else {
/*
* Otherwise kill it and we can demand_page()
* it in later.
*/
set_pte(spte, __pte(0));
}
#ifdef CONFIG_X86_PAE
}
#endif
}
}
/*H:410
* Updating a PTE entry is a little trickier.
*
* We keep track of several different page tables (the Guest uses one for each
* process, so it makes sense to cache at least a few). Each of these have
* identical kernel parts: ie. every mapping above PAGE_OFFSET is the same for
* all processes. So when the page table above that address changes, we update
* all the page tables, not just the current one. This is rare.
*
* The benefit is that when we have to track a new page table, we can keep all
* the kernel mappings. This speeds up context switch immensely.
*/
void guest_set_pte(struct lg_cpu *cpu,
unsigned long gpgdir, unsigned long vaddr, pte_t gpte)
{
/* We don't let you remap the Switcher; we need it to get back! */
if (vaddr >= switcher_addr) {
kill_guest(cpu, "attempt to set pte into Switcher pages");
return;
}
/*
* Kernel mappings must be changed on all top levels. Slow, but doesn't
* happen often.
*/
if (vaddr >= cpu->lg->kernel_address) {
unsigned int i;
for (i = 0; i < ARRAY_SIZE(cpu->lg->pgdirs); i++)
if (cpu->lg->pgdirs[i].pgdir)
__guest_set_pte(cpu, i, vaddr, gpte);
} else {
/* Is this page table one we have a shadow for? */
int pgdir = find_pgdir(cpu->lg, gpgdir);
if (pgdir != ARRAY_SIZE(cpu->lg->pgdirs))
/* If so, do the update. */
__guest_set_pte(cpu, pgdir, vaddr, gpte);
}
}
/*H:400
* (iii) Setting up a page table entry when the Guest tells us one has changed.
*
* Just like we did in interrupts_and_traps.c, it makes sense for us to deal
* with the other side of page tables while we're here: what happens when the
* Guest asks for a page table to be updated?
*
* We already saw that demand_page() will fill in the shadow page tables when
* needed, so we can simply remove shadow page table entries whenever the Guest
* tells us they've changed. When the Guest tries to use the new entry it will
* fault and demand_page() will fix it up.
*
* So with that in mind here's our code to update a (top-level) PGD entry:
*/
void guest_set_pgd(struct lguest *lg, unsigned long gpgdir, u32 idx)
{
int pgdir;
if (idx > PTRS_PER_PGD) {
kill_guest(&lg->cpus[0], "Attempt to set pgd %u/%u",
idx, PTRS_PER_PGD);
return;
}
/* If they're talking about a page table we have a shadow for... */
pgdir = find_pgdir(lg, gpgdir);
if (pgdir < ARRAY_SIZE(lg->pgdirs)) {
/* ... throw it away. */
release_pgd(lg->pgdirs[pgdir].pgdir + idx);
/* That might have been the Switcher mapping, remap it. */
if (!allocate_switcher_mapping(&lg->cpus[0])) {
kill_guest(&lg->cpus[0],
"Cannot populate switcher mapping");
}
lg->pgdirs[pgdir].last_host_cpu = -1;
}
}
#ifdef CONFIG_X86_PAE
/* For setting a mid-level, we just throw everything away. It's easy. */
void guest_set_pmd(struct lguest *lg, unsigned long pmdp, u32 idx)
{
guest_pagetable_clear_all(&lg->cpus[0]);
}
#endif
/*H:500
* (vii) Setting up the page tables initially.
*
* When a Guest is first created, set initialize a shadow page table which
* we will populate on future faults. The Guest doesn't have any actual
* pagetables yet, so we set linear_pages to tell demand_page() to fake it
* for the moment.
*
* We do need the Switcher to be mapped at all times, so we allocate that
* part of the Guest page table here.
*/
int init_guest_pagetable(struct lguest *lg)
{
struct lg_cpu *cpu = &lg->cpus[0];
int allocated = 0;
/* lg (and lg->cpus[]) starts zeroed: this allocates a new pgdir */
cpu->cpu_pgd = new_pgdir(cpu, 0, &allocated);
if (!allocated)
return -ENOMEM;
/* We start with a linear mapping until the initialize. */
cpu->linear_pages = true;
/* Allocate the page tables for the Switcher. */
if (!allocate_switcher_mapping(cpu)) {
release_all_pagetables(lg);
return -ENOMEM;
}
return 0;
}
/*H:508 When the Guest calls LHCALL_LGUEST_INIT we do more setup. */
void page_table_guest_data_init(struct lg_cpu *cpu)
{
/*
* We tell the Guest that it can't use the virtual addresses
* used by the Switcher. This trick is equivalent to 4GB -
* switcher_addr.
*/
u32 top = ~switcher_addr + 1;
/* We get the kernel address: above this is all kernel memory. */
if (get_user(cpu->lg->kernel_address,
&cpu->lg->lguest_data->kernel_address)
/*
* We tell the Guest that it can't use the top virtual
* addresses (used by the Switcher).
*/
|| put_user(top, &cpu->lg->lguest_data->reserve_mem)) {
kill_guest(cpu, "bad guest page %p", cpu->lg->lguest_data);
return;
}
/*
* In flush_user_mappings() we loop from 0 to
* "pgd_index(lg->kernel_address)". This assumes it won't hit the
* Switcher mappings, so check that now.
*/
if (cpu->lg->kernel_address >= switcher_addr)
kill_guest(cpu, "bad kernel address %#lx",
cpu->lg->kernel_address);
}
/* When a Guest dies, our cleanup is fairly simple. */
void free_guest_pagetable(struct lguest *lg)
{
unsigned int i;
/* Throw away all page table pages. */
release_all_pagetables(lg);
/* Now free the top levels: free_page() can handle 0 just fine. */
for (i = 0; i < ARRAY_SIZE(lg->pgdirs); i++)
free_page((long)lg->pgdirs[i].pgdir);
}
/*H:481
* This clears the Switcher mappings for cpu #i.
*/
static void remove_switcher_percpu_map(struct lg_cpu *cpu, unsigned int i)
{
unsigned long base = switcher_addr + PAGE_SIZE + i * PAGE_SIZE*2;
pte_t *pte;
/* Clear the mappings for both pages. */
pte = find_spte(cpu, base, false, 0, 0);
release_pte(*pte);
set_pte(pte, __pte(0));
pte = find_spte(cpu, base + PAGE_SIZE, false, 0, 0);
release_pte(*pte);
set_pte(pte, __pte(0));
}
/*H:480
* (vi) Mapping the Switcher when the Guest is about to run.
*
* The Switcher and the two pages for this CPU need to be visible in the Guest
* (and not the pages for other CPUs).
*
* The pages for the pagetables have all been allocated before: we just need
* to make sure the actual PTEs are up-to-date for the CPU we're about to run
* on.
*/
void map_switcher_in_guest(struct lg_cpu *cpu, struct lguest_pages *pages)
{
unsigned long base;
struct page *percpu_switcher_page, *regs_page;
pte_t *pte;
struct pgdir *pgdir = &cpu->lg->pgdirs[cpu->cpu_pgd];
/* Switcher page should always be mapped by now! */
BUG_ON(!pgdir->switcher_mapped);
/*
* Remember that we have two pages for each Host CPU, so we can run a
* Guest on each CPU without them interfering. We need to make sure
* those pages are mapped correctly in the Guest, but since we usually
* run on the same CPU, we cache that, and only update the mappings
* when we move.
*/
if (pgdir->last_host_cpu == raw_smp_processor_id())
return;
/* -1 means unknown so we remove everything. */
if (pgdir->last_host_cpu == -1) {
unsigned int i;
for_each_possible_cpu(i)
remove_switcher_percpu_map(cpu, i);
} else {
/* We know exactly what CPU mapping to remove. */
remove_switcher_percpu_map(cpu, pgdir->last_host_cpu);
}
/*
* 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.
*/
/* Find the shadow PTE for this regs page. */
base = switcher_addr + PAGE_SIZE
+ raw_smp_processor_id() * sizeof(struct lguest_pages);
pte = find_spte(cpu, base, false, 0, 0);
regs_page = pfn_to_page(__pa(cpu->regs_page) >> PAGE_SHIFT);
get_page(regs_page);
set_pte(pte, mk_pte(regs_page, __pgprot(__PAGE_KERNEL & ~_PAGE_GLOBAL)));
/*
* We map the second page of the struct lguest_pages read-only in
* the Guest: the IDT, GDT and other things it's not supposed to
* change.
*/
pte = find_spte(cpu, base + PAGE_SIZE, false, 0, 0);
percpu_switcher_page
= lg_switcher_pages[1 + raw_smp_processor_id()*2 + 1];
get_page(percpu_switcher_page);
set_pte(pte, mk_pte(percpu_switcher_page,
__pgprot(__PAGE_KERNEL_RO & ~_PAGE_GLOBAL)));
pgdir->last_host_cpu = raw_smp_processor_id();
}
/*H:490
* We've made it through the page table code. Perhaps our tired brains are
* still processing the details, or perhaps we're simply glad it's over.
*
* If nothing else, note that all this complexity in juggling shadow page tables
* in sync with the Guest's page tables is for one reason: for most Guests this
* page table dance determines how bad performance will be. This is why Xen
* uses exotic direct Guest pagetable manipulation, and why both Intel and AMD
* have implemented shadow page table support directly into hardware.
*
* There is just one file remaining in the Host.
*/