diff options
Diffstat (limited to 'drivers/lguest/page_tables.c')
| -rw-r--r-- | drivers/lguest/page_tables.c | 329 |
1 files changed, 299 insertions, 30 deletions
diff --git a/drivers/lguest/page_tables.c b/drivers/lguest/page_tables.c index 1b0ba09b1269..b7a924ace684 100644 --- a/drivers/lguest/page_tables.c +++ b/drivers/lguest/page_tables.c @@ -1,5 +1,11 @@ -/* Shadow page table operations. - * Copyright (C) Rusty Russell IBM Corporation 2006. +/*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> @@ -9,38 +15,96 @@ #include <asm/tlbflush.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 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); } -/* These access the shadow versions (ie. the ones used by the CPU). */ +/* 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 access the guest versions. */ +/* 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); @@ -55,12 +119,24 @@ static unsigned long gpte_addr(struct lguest *lg, return gpage + ((vaddr>>PAGE_SHIFT) % PTES_PER_PAGE) * sizeof(gpte_t); } -/* Do a virtual -> physical mapping on a user page. */ +/*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(¤t->mm->mmap_sem); if (get_user_pages(current, current->mm, virtpfn << PAGE_SHIFT, 1, write, 1, &page, NULL) == 1) @@ -69,28 +145,47 @@ static unsigned long get_pfn(unsigned long virtpfn, int write) 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; - /* We ignore the global flag. */ + /* 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); - /* Must not put_page() bogus page on cleanup. */ + /* 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) { @@ -104,11 +199,16 @@ static void check_gpgd(struct lguest *lg, gpgd_t gpgd) kill_guest(lg, "bad page directory entry"); } -/* FIXME: We hold reference to pages, which prevents them from being - swapped. It'd be nice to have a callback when Linux wants to swap out. */ - -/* We fault pages in, which allows us to update accessed/dirty bits. - * Return true if we got page. */ +/*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; @@ -117,106 +217,161 @@ int demand_page(struct lguest *lg, unsigned long vaddr, int errcode) 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)) { - /* Get a page of PTEs for them. */ + /* No shadow entry: allocate a new shadow PTE page. */ unsigned long ptepage = get_zeroed_page(GFP_KERNEL); - /* FIXME: Steal from self in this case? */ + /* 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)); - /* No page? */ + /* If this page isn't in the Guest page tables, we can't page it in. */ if (!(gpte.flags & _PAGE_PRESENT)) return 0; - /* Write to read-only page? */ + /* 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 non-user page? */ + /* 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; - /* We're done with the old pte. */ + /* 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); - /* We don't make it writable if this isn't a write: later - * write will fault so we can set dirty bit in guest. */ + /* 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); } - /* Now we update dirty/accessed on guest. */ + /* 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; } -/* This is much faster than the full demand_page logic. */ +/*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; @@ -226,21 +381,30 @@ static unsigned int find_pgdir(struct lguest *lg, unsigned long pgtable) 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 - /* There are no mappings: you'll need to re-pin */ + /* 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); @@ -248,82 +412,161 @@ static unsigned int new_pgdir(struct lguest *lg, 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. */ + /* 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) { - /* We assume this in flush_user_mappings, so check now */ + /* 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); @@ -332,33 +575,48 @@ int init_guest_pagetable(struct lguest *lg, unsigned long pgtable) 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); } -/* Caller must be preempt-safe */ +/*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; - /* Since switcher less that 4MB, we simply mug top pte page. */ + /* 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; - /* Map our regs page over stack page. */ + /* 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) { @@ -368,6 +626,10 @@ static void free_switcher_pte_pages(void) 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) @@ -375,21 +637,26 @@ static __init void populate_switcher_pte_page(unsigned int cpu, 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; } - /* We only map this CPU's pages, so guest can't see others. */ + /* The only other thing we map is this CPU's pair of pages. */ i = pages + cpu*2; - /* First page (regs) is rw, second (state) is ro. */ + /* 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; @@ -404,7 +671,9 @@ __init int init_pagetables(struct page **switcher_page, unsigned int pages) } return 0; } +/*:*/ +/* Cleaning up simply involves freeing the PTE page for each CPU. */ void free_pagetables(void) { free_switcher_pte_pages(); |