/*P:700 The pagetable code, on the other hand, still shows the scars of * previous encounters. It's functional, and as neat as it can be in the * circumstances, but be wary, for these things are subtle and break easily. * The Guest provides a virtual to physical mapping, but we can neither trust * it nor use it: we verify and convert it here to point the hardware to the * actual Guest pages when running the Guest. :*/ /* Copyright (C) Rusty Russell IBM Corporation 2006. * GPL v2 and any later version */ #include <linux/mm.h> #include <linux/types.h> #include <linux/spinlock.h> #include <linux/random.h> #include <linux/percpu.h> #include <asm/tlbflush.h> #include <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. If you're not entirely * comfortable with virtual addresses, physical addresses and page tables then * I recommend you review arch/x86/lguest/boot.c's "Page Table Handling" (with * diagrams!). * * The Guest keeps page tables, but we maintain the actual ones here: these are * called "shadow" page tables. Which is a very Guest-centric name: these are * the real page tables the CPU uses, although we keep them up to date to * reflect the Guest's. (See what I mean about weird naming? Since when do * shadows reflect anything?) * * Anyway, this is the most complicated part of the Host code. There are seven * parts to this: * (i) Looking up a page table entry when the Guest faults, * (ii) Making sure the Guest stack is mapped, * (iii) Setting up a page table entry when the Guest tells us one has changed, * (iv) Switching page tables, * (v) Flushing (throwing away) page tables, * (vi) Mapping the Switcher when the Guest is about to run, * (vii) Setting up the page tables initially. :*/ /* 1024 entries in a page table page maps 1024 pages: 4MB. The Switcher is * conveniently placed at the top 4MB, so it uses a separate, complete PTE * page. */ #define SWITCHER_PGD_INDEX (PTRS_PER_PGD - 1) /* We actually need a separate PTE page for each CPU. Remember that after the * Switcher code itself comes two pages for each CPU, and we don't want this * CPU's guest to see the pages of any other CPU. */ static DEFINE_PER_CPU(pte_t *, switcher_pte_pages); #define switcher_pte_page(cpu) per_cpu(switcher_pte_pages, cpu) /*H:320 The page table code is curly enough to need helper functions to keep it * clear and clean. * * There are two functions which return pointers to the shadow (aka "real") * page tables. * * spgd_addr() takes the virtual address and returns a pointer to the top-level * page directory entry (PGD) for that address. Since we keep track of several * page tables, the "i" argument tells us which one we're interested in (it's * usually the current one). */ static pgd_t *spgd_addr(struct lg_cpu *cpu, u32 i, unsigned long vaddr) { unsigned int index = pgd_index(vaddr); /* We kill any Guest trying to touch the Switcher addresses. */ if (index >= SWITCHER_PGD_INDEX) { kill_guest(cpu, "attempt to access switcher pages"); index = 0; } /* Return a pointer index'th pgd entry for the i'th page table. */ return &cpu->lg->pgdirs[i].pgdir[index]; } /* This routine then takes the page directory entry returned above, which * contains the address of the page table entry (PTE) page. It then returns a * pointer to the PTE entry for the given address. */ static pte_t *spte_addr(pgd_t spgd, unsigned long vaddr) { pte_t *page = __va(pgd_pfn(spgd) << PAGE_SHIFT); /* You should never call this if the PGD entry wasn't valid */ BUG_ON(!(pgd_flags(spgd) & _PAGE_PRESENT)); return &page[(vaddr >> PAGE_SHIFT) % PTRS_PER_PTE]; } /* These two functions just like the above two, except they access the Guest * page tables. Hence they return a Guest address. */ static unsigned long gpgd_addr(struct lg_cpu *cpu, unsigned long vaddr) { unsigned int index = vaddr >> (PGDIR_SHIFT); return cpu->lg->pgdirs[cpu->cpu_pgd].gpgdir + index * sizeof(pgd_t); } static unsigned long gpte_addr(pgd_t gpgd, unsigned long vaddr) { unsigned long gpage = pgd_pfn(gpgd) << PAGE_SHIFT; BUG_ON(!(pgd_flags(gpgd) & _PAGE_PRESENT)); return gpage + ((vaddr>>PAGE_SHIFT) % PTRS_PER_PTE) * sizeof(pte_t); } /*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) ret = page_to_pfn(page); up_read(¤t->mm->mmap_sem); return ret; } /*H:340 Converting a Guest page table entry to a shadow (ie. real) page table * entry can be a little tricky. The flags are (almost) the same, but the * Guest PTE contains a virtual page number: the CPU needs the real page * number. */ static 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() took a reference to the page, in * get_pfn()? We have to put it back now. */ if (pte_flags(pte) & _PAGE_PRESENT) put_page(pfn_to_page(pte_pfn(pte))); } /*:*/ static void check_gpte(struct lg_cpu *cpu, pte_t gpte) { if ((pte_flags(gpte) & (_PAGE_PWT|_PAGE_PSE)) || pte_pfn(gpte) >= cpu->lg->pfn_limit) kill_guest(cpu, "bad page table entry"); } static void check_gpgd(struct lg_cpu *cpu, pgd_t gpgd) { if ((pgd_flags(gpgd) & ~_PAGE_TABLE) || (pgd_pfn(gpgd) >= cpu->lg->pfn_limit)) kill_guest(cpu, "bad page directory entry"); } /*H:330 * (i) Looking up a page table entry when the Guest faults. * * We saw this call in run_guest(): when we see a page fault in the Guest, we * come here. That's because we only set up the shadow page tables lazily as * they're needed, so we get page faults all the time and quietly fix them up * and return to the Guest without it knowing. * * If we fixed up the fault (ie. we mapped the address), this routine returns * true. Otherwise, it was a real fault and we need to tell the Guest. */ int demand_page(struct lg_cpu *cpu, unsigned long vaddr, int errcode) { pgd_t gpgd; pgd_t *spgd; unsigned long gpte_ptr; pte_t gpte; pte_t *spte; /* First step: get the top-level Guest page table entry. */ gpgd = lgread(cpu, gpgd_addr(cpu, vaddr), pgd_t); /* Toplevel not present? We can't map it in. */ if (!(pgd_flags(gpgd) & _PAGE_PRESENT)) return 0; /* Now look at the matching shadow 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 = 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 0; } /* We check that the Guest pgd is OK. */ check_gpgd(cpu, 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 = __pgd(__pa(ptepage) | pgd_flags(gpgd)); } /* 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(gpgd, vaddr); 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 0; /* 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 0; /* User access to a kernel-only page? (bit 3 == user access) */ if ((errcode & 4) && !(pte_flags(gpte) & _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(cpu, gpte); /* 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 = spte_addr(*spgd, vaddr); /* If there was a valid shadow PTE entry here before, we release it. * This can happen with a write to a previously read-only entry. */ release_pte(*spte); /* If this is a write, we insist that the Guest page is writable (the * final arg to gpte_to_spte()). */ if (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. */ *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. */ 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 1; } /*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 int page_writable(struct lg_cpu *cpu, unsigned long vaddr) { pgd_t *spgd; unsigned long flags; /* Look at the current top level entry: is it present? */ spgd = spgd_addr(cpu, cpu->cpu_pgd, vaddr); if (!(pgd_flags(*spgd) & _PAGE_PRESENT)) return 0; /* Check the flags on the pte entry itself: it must be present and * writable. */ flags = pte_flags(*(spte_addr(*spgd, vaddr))); 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) { if (!page_writable(cpu, vaddr) && !demand_page(cpu, vaddr, 2)) kill_guest(cpu, "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, 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); } } /*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, 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 */ unsigned long guest_pa(struct lg_cpu *cpu, unsigned long vaddr) { pgd_t gpgd; pte_t gpte; /* 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)) kill_guest(cpu, "Bad address %#lx", vaddr); gpte = lgread(cpu, gpte_addr(gpgd, vaddr), pte_t); if (!(pte_flags(gpte) & _PAGE_PRESENT)) kill_guest(cpu, "Bad address %#lx", vaddr); return pte_pfn(gpte) * PAGE_SIZE | (vaddr & ~PAGE_MASK); } /* 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].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 = random32() % 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); return next; } /*H:430 (iv) Switching page tables * * Now we've seen all the page table setting and manipulation, let's see what * 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; /* 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 */ if (repin) pin_stack_pages(cpu); } /*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) /* 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 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); } /*:*/ /*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 do_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); /* If the top level isn't present, there's no entry to update. */ if (pgd_flags(*spgd) & _PAGE_PRESENT) { /* Otherwise, we start by releasing the existing entry. */ pte_t *spte = spte_addr(*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)) { check_gpte(cpu, gpte); *spte = gpte_to_spte(cpu, gpte, pte_flags(gpte) & _PAGE_DIRTY); } else /* Otherwise kill it and we can demand_page() it in * later. */ *spte = __pte(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 lg_cpu *cpu, unsigned long gpgdir, unsigned long vaddr, pte_t gpte) { /* 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) do_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. */ do_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 to update a (top-level) PGD entry: */ void guest_set_pmd(struct lguest *lg, unsigned long gpgdir, 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, gpgdir); 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 start on the first shadow page table, and give it a blank PGD * page. */ lg->pgdirs[0].gpgdir = pgtable; lg->pgdirs[0].pgdir = (pgd_t *)get_zeroed_page(GFP_KERNEL); if (!lg->pgdirs[0].pgdir) return -ENOMEM; lg->cpus[0].cpu_pgd = 0; return 0; } /* When the Guest calls LHCALL_LGUEST_INIT we do more setup. */ void page_table_guest_data_init(struct lg_cpu *cpu) { /* 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 4MB of virtual * addresses used by the Switcher. */ || put_user(4U*1024*1024, &cpu->lg->lguest_data->reserve_mem) || put_user(cpu->lg->pgdirs[0].gpgdir, &cpu->lg->lguest_data->pgdir)) kill_guest(cpu, "bad guest page %p", cpu->lg->lguest_data); /* 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 (pgd_index(cpu->lg->kernel_address) >= SWITCHER_PGD_INDEX) 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: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). We have the appropriate PTE pages * for each CPU already set up, we just need to hook them in now we know which * Guest is about to run on this CPU. */ void map_switcher_in_guest(struct lg_cpu *cpu, struct lguest_pages *pages) { pte_t *switcher_pte_page = __get_cpu_var(switcher_pte_pages); pgd_t switcher_pgd; pte_t regs_pte; unsigned long pfn; /* Make the last PGD entry for this Guest point to the Switcher's PTE * page for this CPU (with appropriate flags). */ switcher_pgd = __pgd(__pa(switcher_pte_page) | __PAGE_KERNEL); cpu->lg->pgdirs[cpu->cpu_pgd].pgdir[SWITCHER_PGD_INDEX] = switcher_pgd; /* We also change the Switcher PTE page. When we're running the Guest, * we want the Guest's "regs" page to appear where the first Switcher * page for this CPU is. This is an optimization: when the Switcher * saves the Guest registers, it saves them into the first page of this * CPU's "struct lguest_pages": if we make sure the Guest's register * page is already mapped there, we don't have to copy them out * again. */ pfn = __pa(cpu->regs_page) >> PAGE_SHIFT; regs_pte = pfn_pte(pfn, __pgprot(__PAGE_KERNEL)); switcher_pte_page[(unsigned long)pages/PAGE_SIZE%PTRS_PER_PTE] = regs_pte; } /*:*/ static void free_switcher_pte_pages(void) { unsigned int i; for_each_possible_cpu(i) free_page((long)switcher_pte_page(i)); } /*H:520 Setting up the Switcher PTE page for given CPU is fairly easy, given * the CPU number and the "struct page"s for the Switcher code itself. * * Currently the Switcher is less than a page long, so "pages" is always 1. */ static __init void populate_switcher_pte_page(unsigned int cpu, struct page *switcher_page[], unsigned int pages) { unsigned int i; pte_t *pte = switcher_pte_page(cpu); /* The first entries are easy: they map the Switcher code. */ for (i = 0; i < pages; i++) { pte[i] = mk_pte(switcher_page[i], __pgprot(_PAGE_PRESENT|_PAGE_ACCESSED)); } /* The only other thing we map is this CPU's pair of pages. */ i = pages + cpu*2; /* First page (Guest registers) is writable from the Guest */ pte[i] = pfn_pte(page_to_pfn(switcher_page[i]), __pgprot(_PAGE_PRESENT|_PAGE_ACCESSED|_PAGE_RW)); /* The second page contains the "struct lguest_ro_state", and is * read-only. */ pte[i+1] = pfn_pte(page_to_pfn(switcher_page[i+1]), __pgprot(_PAGE_PRESENT|_PAGE_ACCESSED)); } /* 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. */ /*H:510 At boot or module load time, init_pagetables() allocates and populates * the Switcher PTE page for each CPU. */ __init int init_pagetables(struct page **switcher_page, unsigned int pages) { unsigned int i; for_each_possible_cpu(i) { switcher_pte_page(i) = (pte_t *)get_zeroed_page(GFP_KERNEL); if (!switcher_pte_page(i)) { free_switcher_pte_pages(); return -ENOMEM; } populate_switcher_pte_page(i, switcher_page, pages); } return 0; } /*:*/ /* Cleaning up simply involves freeing the PTE page for each CPU. */ void free_pagetables(void) { free_switcher_pte_pages(); }