/*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 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. */ bool demand_page(struct lg_cpu *cpu, unsigned long vaddr, int errcode) { unsigned long gpte_ptr; pte_t gpte; pte_t *spte; pmd_t gpmd; pgd_t gpgd; /* 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; /* * 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) { if (!page_writable(cpu, vaddr) && !demand_page(cpu, vaddr, 2)) 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 */ unsigned long guest_pa(struct lg_cpu *cpu, unsigned long vaddr) { 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)) return vaddr; /* 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); return -1UL; } #ifdef CONFIG_X86_PAE gpmd = lgread(cpu, gpmd_addr(gpgd, vaddr), pmd_t); if (!(pmd_flags(gpmd) & _PAGE_PRESENT)) { kill_guest(cpu, "Bad address %#lx", vaddr); return -1UL; } 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)) 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].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)) { 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. */