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[linux-2.6/sactl.git] / drivers / lguest / page_tables.c
bloba7f64a9d67e009437b30e5007a208c25863be392
1 /*P:700 The pagetable code, on the other hand, still shows the scars of
2 * previous encounters. It's functional, and as neat as it can be in the
3 * circumstances, but be wary, for these things are subtle and break easily.
4 * The Guest provides a virtual to physical mapping, but we can neither trust
5 * it nor use it: we verify and convert it here to point the hardware to the
6 * actual Guest pages when running the Guest. :*/
8 /* Copyright (C) Rusty Russell IBM Corporation 2006.
9 * GPL v2 and any later version */
10 #include <linux/mm.h>
11 #include <linux/types.h>
12 #include <linux/spinlock.h>
13 #include <linux/random.h>
14 #include <linux/percpu.h>
15 #include <asm/tlbflush.h>
16 #include <asm/uaccess.h>
17 #include "lg.h"
19 /*M:008 We hold reference to pages, which prevents them from being swapped.
20 * It'd be nice to have a callback in the "struct mm_struct" when Linux wants
21 * to swap out. If we had this, and a shrinker callback to trim PTE pages, we
22 * could probably consider launching Guests as non-root. :*/
24 /*H:300
25 * The Page Table Code
27 * We use two-level page tables for the Guest. If you're not entirely
28 * comfortable with virtual addresses, physical addresses and page tables then
29 * I recommend you review arch/x86/lguest/boot.c's "Page Table Handling" (with
30 * diagrams!).
32 * The Guest keeps page tables, but we maintain the actual ones here: these are
33 * called "shadow" page tables. Which is a very Guest-centric name: these are
34 * the real page tables the CPU uses, although we keep them up to date to
35 * reflect the Guest's. (See what I mean about weird naming? Since when do
36 * shadows reflect anything?)
38 * Anyway, this is the most complicated part of the Host code. There are seven
39 * parts to this:
40 * (i) Looking up a page table entry when the Guest faults,
41 * (ii) Making sure the Guest stack is mapped,
42 * (iii) Setting up a page table entry when the Guest tells us one has changed,
43 * (iv) Switching page tables,
44 * (v) Flushing (throwing away) page tables,
45 * (vi) Mapping the Switcher when the Guest is about to run,
46 * (vii) Setting up the page tables initially.
47 :*/
50 /* 1024 entries in a page table page maps 1024 pages: 4MB. The Switcher is
51 * conveniently placed at the top 4MB, so it uses a separate, complete PTE
52 * page. */
53 #define SWITCHER_PGD_INDEX (PTRS_PER_PGD - 1)
55 /* We actually need a separate PTE page for each CPU. Remember that after the
56 * Switcher code itself comes two pages for each CPU, and we don't want this
57 * CPU's guest to see the pages of any other CPU. */
58 static DEFINE_PER_CPU(pte_t *, switcher_pte_pages);
59 #define switcher_pte_page(cpu) per_cpu(switcher_pte_pages, cpu)
61 /*H:320 The page table code is curly enough to need helper functions to keep it
62 * clear and clean.
64 * There are two functions which return pointers to the shadow (aka "real")
65 * page tables.
67 * spgd_addr() takes the virtual address and returns a pointer to the top-level
68 * page directory entry (PGD) for that address. Since we keep track of several
69 * page tables, the "i" argument tells us which one we're interested in (it's
70 * usually the current one). */
71 static pgd_t *spgd_addr(struct lg_cpu *cpu, u32 i, unsigned long vaddr)
73 unsigned int index = pgd_index(vaddr);
75 /* We kill any Guest trying to touch the Switcher addresses. */
76 if (index >= SWITCHER_PGD_INDEX) {
77 kill_guest(cpu, "attempt to access switcher pages");
78 index = 0;
80 /* Return a pointer index'th pgd entry for the i'th page table. */
81 return &cpu->lg->pgdirs[i].pgdir[index];
84 /* This routine then takes the page directory entry returned above, which
85 * contains the address of the page table entry (PTE) page. It then returns a
86 * pointer to the PTE entry for the given address. */
87 static pte_t *spte_addr(pgd_t spgd, unsigned long vaddr)
89 pte_t *page = __va(pgd_pfn(spgd) << PAGE_SHIFT);
90 /* You should never call this if the PGD entry wasn't valid */
91 BUG_ON(!(pgd_flags(spgd) & _PAGE_PRESENT));
92 return &page[(vaddr >> PAGE_SHIFT) % PTRS_PER_PTE];
95 /* These two functions just like the above two, except they access the Guest
96 * page tables. Hence they return a Guest address. */
97 static unsigned long gpgd_addr(struct lg_cpu *cpu, unsigned long vaddr)
99 unsigned int index = vaddr >> (PGDIR_SHIFT);
100 return cpu->lg->pgdirs[cpu->cpu_pgd].gpgdir + index * sizeof(pgd_t);
103 static unsigned long gpte_addr(pgd_t gpgd, unsigned long vaddr)
105 unsigned long gpage = pgd_pfn(gpgd) << PAGE_SHIFT;
106 BUG_ON(!(pgd_flags(gpgd) & _PAGE_PRESENT));
107 return gpage + ((vaddr>>PAGE_SHIFT) % PTRS_PER_PTE) * sizeof(pte_t);
110 /*H:350 This routine takes a page number given by the Guest and converts it to
111 * an actual, physical page number. It can fail for several reasons: the
112 * virtual address might not be mapped by the Launcher, the write flag is set
113 * and the page is read-only, or the write flag was set and the page was
114 * shared so had to be copied, but we ran out of memory.
116 * This holds a reference to the page, so release_pte() is careful to
117 * put that back. */
118 static unsigned long get_pfn(unsigned long virtpfn, int write)
120 struct page *page;
121 /* This value indicates failure. */
122 unsigned long ret = -1UL;
124 /* get_user_pages() is a complex interface: it gets the "struct
125 * vm_area_struct" and "struct page" assocated with a range of pages.
126 * It also needs the task's mmap_sem held, and is not very quick.
127 * It returns the number of pages it got. */
128 down_read(&current->mm->mmap_sem);
129 if (get_user_pages(current, current->mm, virtpfn << PAGE_SHIFT,
130 1, write, 1, &page, NULL) == 1)
131 ret = page_to_pfn(page);
132 up_read(&current->mm->mmap_sem);
133 return ret;
136 /*H:340 Converting a Guest page table entry to a shadow (ie. real) page table
137 * entry can be a little tricky. The flags are (almost) the same, but the
138 * Guest PTE contains a virtual page number: the CPU needs the real page
139 * number. */
140 static pte_t gpte_to_spte(struct lg_cpu *cpu, pte_t gpte, int write)
142 unsigned long pfn, base, flags;
144 /* The Guest sets the global flag, because it thinks that it is using
145 * PGE. We only told it to use PGE so it would tell us whether it was
146 * flushing a kernel mapping or a userspace mapping. We don't actually
147 * use the global bit, so throw it away. */
148 flags = (pte_flags(gpte) & ~_PAGE_GLOBAL);
150 /* The Guest's pages are offset inside the Launcher. */
151 base = (unsigned long)cpu->lg->mem_base / PAGE_SIZE;
153 /* We need a temporary "unsigned long" variable to hold the answer from
154 * get_pfn(), because it returns 0xFFFFFFFF on failure, which wouldn't
155 * fit in spte.pfn. get_pfn() finds the real physical number of the
156 * page, given the virtual number. */
157 pfn = get_pfn(base + pte_pfn(gpte), write);
158 if (pfn == -1UL) {
159 kill_guest(cpu, "failed to get page %lu", pte_pfn(gpte));
160 /* When we destroy the Guest, we'll go through the shadow page
161 * tables and release_pte() them. Make sure we don't think
162 * this one is valid! */
163 flags = 0;
165 /* Now we assemble our shadow PTE from the page number and flags. */
166 return pfn_pte(pfn, __pgprot(flags));
169 /*H:460 And to complete the chain, release_pte() looks like this: */
170 static void release_pte(pte_t pte)
172 /* Remember that get_user_pages() took a reference to the page, in
173 * get_pfn()? We have to put it back now. */
174 if (pte_flags(pte) & _PAGE_PRESENT)
175 put_page(pfn_to_page(pte_pfn(pte)));
177 /*:*/
179 static void check_gpte(struct lg_cpu *cpu, pte_t gpte)
181 if ((pte_flags(gpte) & _PAGE_PSE) ||
182 pte_pfn(gpte) >= cpu->lg->pfn_limit)
183 kill_guest(cpu, "bad page table entry");
186 static void check_gpgd(struct lg_cpu *cpu, pgd_t gpgd)
188 if ((pgd_flags(gpgd) & ~_PAGE_TABLE) ||
189 (pgd_pfn(gpgd) >= cpu->lg->pfn_limit))
190 kill_guest(cpu, "bad page directory entry");
193 /*H:330
194 * (i) Looking up a page table entry when the Guest faults.
196 * We saw this call in run_guest(): when we see a page fault in the Guest, we
197 * come here. That's because we only set up the shadow page tables lazily as
198 * they're needed, so we get page faults all the time and quietly fix them up
199 * and return to the Guest without it knowing.
201 * If we fixed up the fault (ie. we mapped the address), this routine returns
202 * true. Otherwise, it was a real fault and we need to tell the Guest. */
203 int demand_page(struct lg_cpu *cpu, unsigned long vaddr, int errcode)
205 pgd_t gpgd;
206 pgd_t *spgd;
207 unsigned long gpte_ptr;
208 pte_t gpte;
209 pte_t *spte;
211 /* First step: get the top-level Guest page table entry. */
212 gpgd = lgread(cpu, gpgd_addr(cpu, vaddr), pgd_t);
213 /* Toplevel not present? We can't map it in. */
214 if (!(pgd_flags(gpgd) & _PAGE_PRESENT))
215 return 0;
217 /* Now look at the matching shadow entry. */
218 spgd = spgd_addr(cpu, cpu->cpu_pgd, vaddr);
219 if (!(pgd_flags(*spgd) & _PAGE_PRESENT)) {
220 /* No shadow entry: allocate a new shadow PTE page. */
221 unsigned long ptepage = get_zeroed_page(GFP_KERNEL);
222 /* This is not really the Guest's fault, but killing it is
223 * simple for this corner case. */
224 if (!ptepage) {
225 kill_guest(cpu, "out of memory allocating pte page");
226 return 0;
228 /* We check that the Guest pgd is OK. */
229 check_gpgd(cpu, gpgd);
230 /* And we copy the flags to the shadow PGD entry. The page
231 * number in the shadow PGD is the page we just allocated. */
232 *spgd = __pgd(__pa(ptepage) | pgd_flags(gpgd));
235 /* OK, now we look at the lower level in the Guest page table: keep its
236 * address, because we might update it later. */
237 gpte_ptr = gpte_addr(gpgd, vaddr);
238 gpte = lgread(cpu, gpte_ptr, pte_t);
240 /* If this page isn't in the Guest page tables, we can't page it in. */
241 if (!(pte_flags(gpte) & _PAGE_PRESENT))
242 return 0;
244 /* Check they're not trying to write to a page the Guest wants
245 * read-only (bit 2 of errcode == write). */
246 if ((errcode & 2) && !(pte_flags(gpte) & _PAGE_RW))
247 return 0;
249 /* User access to a kernel-only page? (bit 3 == user access) */
250 if ((errcode & 4) && !(pte_flags(gpte) & _PAGE_USER))
251 return 0;
253 /* Check that the Guest PTE flags are OK, and the page number is below
254 * the pfn_limit (ie. not mapping the Launcher binary). */
255 check_gpte(cpu, gpte);
257 /* Add the _PAGE_ACCESSED and (for a write) _PAGE_DIRTY flag */
258 gpte = pte_mkyoung(gpte);
259 if (errcode & 2)
260 gpte = pte_mkdirty(gpte);
262 /* Get the pointer to the shadow PTE entry we're going to set. */
263 spte = spte_addr(*spgd, vaddr);
264 /* If there was a valid shadow PTE entry here before, we release it.
265 * This can happen with a write to a previously read-only entry. */
266 release_pte(*spte);
268 /* If this is a write, we insist that the Guest page is writable (the
269 * final arg to gpte_to_spte()). */
270 if (pte_dirty(gpte))
271 *spte = gpte_to_spte(cpu, gpte, 1);
272 else
273 /* If this is a read, don't set the "writable" bit in the page
274 * table entry, even if the Guest says it's writable. That way
275 * we will come back here when a write does actually occur, so
276 * we can update the Guest's _PAGE_DIRTY flag. */
277 *spte = gpte_to_spte(cpu, pte_wrprotect(gpte), 0);
279 /* Finally, we write the Guest PTE entry back: we've set the
280 * _PAGE_ACCESSED and maybe the _PAGE_DIRTY flags. */
281 lgwrite(cpu, gpte_ptr, pte_t, gpte);
283 /* The fault is fixed, the page table is populated, the mapping
284 * manipulated, the result returned and the code complete. A small
285 * delay and a trace of alliteration are the only indications the Guest
286 * has that a page fault occurred at all. */
287 return 1;
290 /*H:360
291 * (ii) Making sure the Guest stack is mapped.
293 * Remember that direct traps into the Guest need a mapped Guest kernel stack.
294 * pin_stack_pages() calls us here: we could simply call demand_page(), but as
295 * we've seen that logic is quite long, and usually the stack pages are already
296 * mapped, so it's overkill.
298 * This is a quick version which answers the question: is this virtual address
299 * mapped by the shadow page tables, and is it writable? */
300 static int page_writable(struct lg_cpu *cpu, unsigned long vaddr)
302 pgd_t *spgd;
303 unsigned long flags;
305 /* Look at the current top level entry: is it present? */
306 spgd = spgd_addr(cpu, cpu->cpu_pgd, vaddr);
307 if (!(pgd_flags(*spgd) & _PAGE_PRESENT))
308 return 0;
310 /* Check the flags on the pte entry itself: it must be present and
311 * writable. */
312 flags = pte_flags(*(spte_addr(*spgd, vaddr)));
314 return (flags & (_PAGE_PRESENT|_PAGE_RW)) == (_PAGE_PRESENT|_PAGE_RW);
317 /* So, when pin_stack_pages() asks us to pin a page, we check if it's already
318 * in the page tables, and if not, we call demand_page() with error code 2
319 * (meaning "write"). */
320 void pin_page(struct lg_cpu *cpu, unsigned long vaddr)
322 if (!page_writable(cpu, vaddr) && !demand_page(cpu, vaddr, 2))
323 kill_guest(cpu, "bad stack page %#lx", vaddr);
326 /*H:450 If we chase down the release_pgd() code, it looks like this: */
327 static void release_pgd(struct lguest *lg, pgd_t *spgd)
329 /* If the entry's not present, there's nothing to release. */
330 if (pgd_flags(*spgd) & _PAGE_PRESENT) {
331 unsigned int i;
332 /* Converting the pfn to find the actual PTE page is easy: turn
333 * the page number into a physical address, then convert to a
334 * virtual address (easy for kernel pages like this one). */
335 pte_t *ptepage = __va(pgd_pfn(*spgd) << PAGE_SHIFT);
336 /* For each entry in the page, we might need to release it. */
337 for (i = 0; i < PTRS_PER_PTE; i++)
338 release_pte(ptepage[i]);
339 /* Now we can free the page of PTEs */
340 free_page((long)ptepage);
341 /* And zero out the PGD entry so we never release it twice. */
342 *spgd = __pgd(0);
346 /*H:445 We saw flush_user_mappings() twice: once from the flush_user_mappings()
347 * hypercall and once in new_pgdir() when we re-used a top-level pgdir page.
348 * It simply releases every PTE page from 0 up to the Guest's kernel address. */
349 static void flush_user_mappings(struct lguest *lg, int idx)
351 unsigned int i;
352 /* Release every pgd entry up to the kernel's address. */
353 for (i = 0; i < pgd_index(lg->kernel_address); i++)
354 release_pgd(lg, lg->pgdirs[idx].pgdir + i);
357 /*H:440 (v) Flushing (throwing away) page tables,
359 * The Guest has a hypercall to throw away the page tables: it's used when a
360 * large number of mappings have been changed. */
361 void guest_pagetable_flush_user(struct lg_cpu *cpu)
363 /* Drop the userspace part of the current page table. */
364 flush_user_mappings(cpu->lg, cpu->cpu_pgd);
366 /*:*/
368 /* We walk down the guest page tables to get a guest-physical address */
369 unsigned long guest_pa(struct lg_cpu *cpu, unsigned long vaddr)
371 pgd_t gpgd;
372 pte_t gpte;
374 /* First step: get the top-level Guest page table entry. */
375 gpgd = lgread(cpu, gpgd_addr(cpu, vaddr), pgd_t);
376 /* Toplevel not present? We can't map it in. */
377 if (!(pgd_flags(gpgd) & _PAGE_PRESENT))
378 kill_guest(cpu, "Bad address %#lx", vaddr);
380 gpte = lgread(cpu, gpte_addr(gpgd, vaddr), pte_t);
381 if (!(pte_flags(gpte) & _PAGE_PRESENT))
382 kill_guest(cpu, "Bad address %#lx", vaddr);
384 return pte_pfn(gpte) * PAGE_SIZE | (vaddr & ~PAGE_MASK);
387 /* We keep several page tables. This is a simple routine to find the page
388 * table (if any) corresponding to this top-level address the Guest has given
389 * us. */
390 static unsigned int find_pgdir(struct lguest *lg, unsigned long pgtable)
392 unsigned int i;
393 for (i = 0; i < ARRAY_SIZE(lg->pgdirs); i++)
394 if (lg->pgdirs[i].pgdir && lg->pgdirs[i].gpgdir == pgtable)
395 break;
396 return i;
399 /*H:435 And this is us, creating the new page directory. If we really do
400 * allocate a new one (and so the kernel parts are not there), we set
401 * blank_pgdir. */
402 static unsigned int new_pgdir(struct lg_cpu *cpu,
403 unsigned long gpgdir,
404 int *blank_pgdir)
406 unsigned int next;
408 /* We pick one entry at random to throw out. Choosing the Least
409 * Recently Used might be better, but this is easy. */
410 next = random32() % ARRAY_SIZE(cpu->lg->pgdirs);
411 /* If it's never been allocated at all before, try now. */
412 if (!cpu->lg->pgdirs[next].pgdir) {
413 cpu->lg->pgdirs[next].pgdir =
414 (pgd_t *)get_zeroed_page(GFP_KERNEL);
415 /* If the allocation fails, just keep using the one we have */
416 if (!cpu->lg->pgdirs[next].pgdir)
417 next = cpu->cpu_pgd;
418 else
419 /* This is a blank page, so there are no kernel
420 * mappings: caller must map the stack! */
421 *blank_pgdir = 1;
423 /* Record which Guest toplevel this shadows. */
424 cpu->lg->pgdirs[next].gpgdir = gpgdir;
425 /* Release all the non-kernel mappings. */
426 flush_user_mappings(cpu->lg, next);
428 return next;
431 /*H:430 (iv) Switching page tables
433 * Now we've seen all the page table setting and manipulation, let's see what
434 * what happens when the Guest changes page tables (ie. changes the top-level
435 * pgdir). This occurs on almost every context switch. */
436 void guest_new_pagetable(struct lg_cpu *cpu, unsigned long pgtable)
438 int newpgdir, repin = 0;
440 /* Look to see if we have this one already. */
441 newpgdir = find_pgdir(cpu->lg, pgtable);
442 /* If not, we allocate or mug an existing one: if it's a fresh one,
443 * repin gets set to 1. */
444 if (newpgdir == ARRAY_SIZE(cpu->lg->pgdirs))
445 newpgdir = new_pgdir(cpu, pgtable, &repin);
446 /* Change the current pgd index to the new one. */
447 cpu->cpu_pgd = newpgdir;
448 /* If it was completely blank, we map in the Guest kernel stack */
449 if (repin)
450 pin_stack_pages(cpu);
453 /*H:470 Finally, a routine which throws away everything: all PGD entries in all
454 * the shadow page tables, including the Guest's kernel mappings. This is used
455 * when we destroy the Guest. */
456 static void release_all_pagetables(struct lguest *lg)
458 unsigned int i, j;
460 /* Every shadow pagetable this Guest has */
461 for (i = 0; i < ARRAY_SIZE(lg->pgdirs); i++)
462 if (lg->pgdirs[i].pgdir)
463 /* Every PGD entry except the Switcher at the top */
464 for (j = 0; j < SWITCHER_PGD_INDEX; j++)
465 release_pgd(lg, lg->pgdirs[i].pgdir + j);
468 /* We also throw away everything when a Guest tells us it's changed a kernel
469 * mapping. Since kernel mappings are in every page table, it's easiest to
470 * throw them all away. This traps the Guest in amber for a while as
471 * everything faults back in, but it's rare. */
472 void guest_pagetable_clear_all(struct lg_cpu *cpu)
474 release_all_pagetables(cpu->lg);
475 /* We need the Guest kernel stack mapped again. */
476 pin_stack_pages(cpu);
478 /*:*/
479 /*M:009 Since we throw away all mappings when a kernel mapping changes, our
480 * performance sucks for guests using highmem. In fact, a guest with
481 * PAGE_OFFSET 0xc0000000 (the default) and more than about 700MB of RAM is
482 * usually slower than a Guest with less memory.
484 * This, of course, cannot be fixed. It would take some kind of... well, I
485 * don't know, but the term "puissant code-fu" comes to mind. :*/
487 /*H:420 This is the routine which actually sets the page table entry for then
488 * "idx"'th shadow page table.
490 * Normally, we can just throw out the old entry and replace it with 0: if they
491 * use it demand_page() will put the new entry in. We need to do this anyway:
492 * The Guest expects _PAGE_ACCESSED to be set on its PTE the first time a page
493 * is read from, and _PAGE_DIRTY when it's written to.
495 * But Avi Kivity pointed out that most Operating Systems (Linux included) set
496 * these bits on PTEs immediately anyway. This is done to save the CPU from
497 * having to update them, but it helps us the same way: if they set
498 * _PAGE_ACCESSED then we can put a read-only PTE entry in immediately, and if
499 * they set _PAGE_DIRTY then we can put a writable PTE entry in immediately.
501 static void do_set_pte(struct lg_cpu *cpu, int idx,
502 unsigned long vaddr, pte_t gpte)
504 /* Look up the matching shadow page directory entry. */
505 pgd_t *spgd = spgd_addr(cpu, idx, vaddr);
507 /* If the top level isn't present, there's no entry to update. */
508 if (pgd_flags(*spgd) & _PAGE_PRESENT) {
509 /* Otherwise, we start by releasing the existing entry. */
510 pte_t *spte = spte_addr(*spgd, vaddr);
511 release_pte(*spte);
513 /* If they're setting this entry as dirty or accessed, we might
514 * as well put that entry they've given us in now. This shaves
515 * 10% off a copy-on-write micro-benchmark. */
516 if (pte_flags(gpte) & (_PAGE_DIRTY | _PAGE_ACCESSED)) {
517 check_gpte(cpu, gpte);
518 *spte = gpte_to_spte(cpu, gpte,
519 pte_flags(gpte) & _PAGE_DIRTY);
520 } else
521 /* Otherwise kill it and we can demand_page() it in
522 * later. */
523 *spte = __pte(0);
527 /*H:410 Updating a PTE entry is a little trickier.
529 * We keep track of several different page tables (the Guest uses one for each
530 * process, so it makes sense to cache at least a few). Each of these have
531 * identical kernel parts: ie. every mapping above PAGE_OFFSET is the same for
532 * all processes. So when the page table above that address changes, we update
533 * all the page tables, not just the current one. This is rare.
535 * The benefit is that when we have to track a new page table, we can copy keep
536 * all the kernel mappings. This speeds up context switch immensely. */
537 void guest_set_pte(struct lg_cpu *cpu,
538 unsigned long gpgdir, unsigned long vaddr, pte_t gpte)
540 /* Kernel mappings must be changed on all top levels. Slow, but
541 * doesn't happen often. */
542 if (vaddr >= cpu->lg->kernel_address) {
543 unsigned int i;
544 for (i = 0; i < ARRAY_SIZE(cpu->lg->pgdirs); i++)
545 if (cpu->lg->pgdirs[i].pgdir)
546 do_set_pte(cpu, i, vaddr, gpte);
547 } else {
548 /* Is this page table one we have a shadow for? */
549 int pgdir = find_pgdir(cpu->lg, gpgdir);
550 if (pgdir != ARRAY_SIZE(cpu->lg->pgdirs))
551 /* If so, do the update. */
552 do_set_pte(cpu, pgdir, vaddr, gpte);
556 /*H:400
557 * (iii) Setting up a page table entry when the Guest tells us one has changed.
559 * Just like we did in interrupts_and_traps.c, it makes sense for us to deal
560 * with the other side of page tables while we're here: what happens when the
561 * Guest asks for a page table to be updated?
563 * We already saw that demand_page() will fill in the shadow page tables when
564 * needed, so we can simply remove shadow page table entries whenever the Guest
565 * tells us they've changed. When the Guest tries to use the new entry it will
566 * fault and demand_page() will fix it up.
568 * So with that in mind here's our code to to update a (top-level) PGD entry:
570 void guest_set_pmd(struct lguest *lg, unsigned long gpgdir, u32 idx)
572 int pgdir;
574 /* The kernel seems to try to initialize this early on: we ignore its
575 * attempts to map over the Switcher. */
576 if (idx >= SWITCHER_PGD_INDEX)
577 return;
579 /* If they're talking about a page table we have a shadow for... */
580 pgdir = find_pgdir(lg, gpgdir);
581 if (pgdir < ARRAY_SIZE(lg->pgdirs))
582 /* ... throw it away. */
583 release_pgd(lg, lg->pgdirs[pgdir].pgdir + idx);
586 /*H:500 (vii) Setting up the page tables initially.
588 * When a Guest is first created, the Launcher tells us where the toplevel of
589 * its first page table is. We set some things up here: */
590 int init_guest_pagetable(struct lguest *lg, unsigned long pgtable)
592 /* We start on the first shadow page table, and give it a blank PGD
593 * page. */
594 lg->pgdirs[0].gpgdir = pgtable;
595 lg->pgdirs[0].pgdir = (pgd_t *)get_zeroed_page(GFP_KERNEL);
596 if (!lg->pgdirs[0].pgdir)
597 return -ENOMEM;
598 lg->cpus[0].cpu_pgd = 0;
599 return 0;
602 /* When the Guest calls LHCALL_LGUEST_INIT we do more setup. */
603 void page_table_guest_data_init(struct lg_cpu *cpu)
605 /* We get the kernel address: above this is all kernel memory. */
606 if (get_user(cpu->lg->kernel_address,
607 &cpu->lg->lguest_data->kernel_address)
608 /* We tell the Guest that it can't use the top 4MB of virtual
609 * addresses used by the Switcher. */
610 || put_user(4U*1024*1024, &cpu->lg->lguest_data->reserve_mem)
611 || put_user(cpu->lg->pgdirs[0].gpgdir, &cpu->lg->lguest_data->pgdir))
612 kill_guest(cpu, "bad guest page %p", cpu->lg->lguest_data);
614 /* In flush_user_mappings() we loop from 0 to
615 * "pgd_index(lg->kernel_address)". This assumes it won't hit the
616 * Switcher mappings, so check that now. */
617 if (pgd_index(cpu->lg->kernel_address) >= SWITCHER_PGD_INDEX)
618 kill_guest(cpu, "bad kernel address %#lx",
619 cpu->lg->kernel_address);
622 /* When a Guest dies, our cleanup is fairly simple. */
623 void free_guest_pagetable(struct lguest *lg)
625 unsigned int i;
627 /* Throw away all page table pages. */
628 release_all_pagetables(lg);
629 /* Now free the top levels: free_page() can handle 0 just fine. */
630 for (i = 0; i < ARRAY_SIZE(lg->pgdirs); i++)
631 free_page((long)lg->pgdirs[i].pgdir);
634 /*H:480 (vi) Mapping the Switcher when the Guest is about to run.
636 * The Switcher and the two pages for this CPU need to be visible in the
637 * Guest (and not the pages for other CPUs). We have the appropriate PTE pages
638 * for each CPU already set up, we just need to hook them in now we know which
639 * Guest is about to run on this CPU. */
640 void map_switcher_in_guest(struct lg_cpu *cpu, struct lguest_pages *pages)
642 pte_t *switcher_pte_page = __get_cpu_var(switcher_pte_pages);
643 pgd_t switcher_pgd;
644 pte_t regs_pte;
645 unsigned long pfn;
647 /* Make the last PGD entry for this Guest point to the Switcher's PTE
648 * page for this CPU (with appropriate flags). */
649 switcher_pgd = __pgd(__pa(switcher_pte_page) | __PAGE_KERNEL);
651 cpu->lg->pgdirs[cpu->cpu_pgd].pgdir[SWITCHER_PGD_INDEX] = switcher_pgd;
653 /* We also change the Switcher PTE page. When we're running the Guest,
654 * we want the Guest's "regs" page to appear where the first Switcher
655 * page for this CPU is. This is an optimization: when the Switcher
656 * saves the Guest registers, it saves them into the first page of this
657 * CPU's "struct lguest_pages": if we make sure the Guest's register
658 * page is already mapped there, we don't have to copy them out
659 * again. */
660 pfn = __pa(cpu->regs_page) >> PAGE_SHIFT;
661 regs_pte = pfn_pte(pfn, __pgprot(__PAGE_KERNEL));
662 switcher_pte_page[(unsigned long)pages/PAGE_SIZE%PTRS_PER_PTE] = regs_pte;
664 /*:*/
666 static void free_switcher_pte_pages(void)
668 unsigned int i;
670 for_each_possible_cpu(i)
671 free_page((long)switcher_pte_page(i));
674 /*H:520 Setting up the Switcher PTE page for given CPU is fairly easy, given
675 * the CPU number and the "struct page"s for the Switcher code itself.
677 * Currently the Switcher is less than a page long, so "pages" is always 1. */
678 static __init void populate_switcher_pte_page(unsigned int cpu,
679 struct page *switcher_page[],
680 unsigned int pages)
682 unsigned int i;
683 pte_t *pte = switcher_pte_page(cpu);
685 /* The first entries are easy: they map the Switcher code. */
686 for (i = 0; i < pages; i++) {
687 pte[i] = mk_pte(switcher_page[i],
688 __pgprot(_PAGE_PRESENT|_PAGE_ACCESSED));
691 /* The only other thing we map is this CPU's pair of pages. */
692 i = pages + cpu*2;
694 /* First page (Guest registers) is writable from the Guest */
695 pte[i] = pfn_pte(page_to_pfn(switcher_page[i]),
696 __pgprot(_PAGE_PRESENT|_PAGE_ACCESSED|_PAGE_RW));
698 /* The second page contains the "struct lguest_ro_state", and is
699 * read-only. */
700 pte[i+1] = pfn_pte(page_to_pfn(switcher_page[i+1]),
701 __pgprot(_PAGE_PRESENT|_PAGE_ACCESSED));
704 /* We've made it through the page table code. Perhaps our tired brains are
705 * still processing the details, or perhaps we're simply glad it's over.
707 * If nothing else, note that all this complexity in juggling shadow page
708 * tables in sync with the Guest's page tables is for one reason: for most
709 * Guests this page table dance determines how bad performance will be. This
710 * is why Xen uses exotic direct Guest pagetable manipulation, and why both
711 * Intel and AMD have implemented shadow page table support directly into
712 * hardware.
714 * There is just one file remaining in the Host. */
716 /*H:510 At boot or module load time, init_pagetables() allocates and populates
717 * the Switcher PTE page for each CPU. */
718 __init int init_pagetables(struct page **switcher_page, unsigned int pages)
720 unsigned int i;
722 for_each_possible_cpu(i) {
723 switcher_pte_page(i) = (pte_t *)get_zeroed_page(GFP_KERNEL);
724 if (!switcher_pte_page(i)) {
725 free_switcher_pte_pages();
726 return -ENOMEM;
728 populate_switcher_pte_page(i, switcher_page, pages);
730 return 0;
732 /*:*/
734 /* Cleaning up simply involves freeing the PTE page for each CPU. */
735 void free_pagetables(void)
737 free_switcher_pte_pages();