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Diffstat (limited to 'drivers/lguest/page_tables.c')
| -rw-r--r-- | drivers/lguest/page_tables.c | 1152 |
1 files changed, 1152 insertions, 0 deletions
diff --git a/drivers/lguest/page_tables.c b/drivers/lguest/page_tables.c new file mode 100644 index 00000000..3b62be16 --- /dev/null +++ b/drivers/lguest/page_tables.c @@ -0,0 +1,1152 @@ +/*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 2006. + * 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 SWITCHER_PMD_INDEX (PTRS_PER_PMD - 1) +#define RESERVE_MEM 2U +#define CHECK_GPGD_MASK _PAGE_PRESENT +#else +#define RESERVE_MEM 4U +#define CHECK_GPGD_MASK _PAGE_TABLE +#endif + +/* + * 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. The kernel itself provides many of them; one advantage + * of insisting that the Guest and Host use the same CONFIG_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); + +#ifndef CONFIG_X86_PAE + /* 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; + } +#endif + /* 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; + + /* We kill any Guest trying to touch the Switcher addresses. */ + if (pgd_index(vaddr) == SWITCHER_PGD_INDEX && + index >= SWITCHER_PMD_INDEX) { + kill_guest(cpu, "attempt to access switcher pages"); + index = 0; + } + + /* 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 void 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"); +} + +static void 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"); +} + +#ifdef CONFIG_X86_PAE +static void 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"); +} +#endif + +/*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) +{ + pgd_t gpgd; + pgd_t *spgd; + unsigned long gpte_ptr; + pte_t gpte; + pte_t *spte; + + /* Mid level for PAE. */ +#ifdef CONFIG_X86_PAE + pmd_t *spmd; + pmd_t gpmd; +#endif + + /* 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; + } + + /* 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 false; + } + /* 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. + */ + set_pgd(spgd, __pgd(__pa(ptepage) | pgd_flags(gpgd))); + } + +#ifdef CONFIG_X86_PAE + if (unlikely(cpu->linear_pages)) { + /* Faking up a linear mapping. */ + gpmd = __pmd(_PAGE_TABLE); + } else { + 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; + } + + /* Now look at the matching 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 = 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 false; + } + + /* We check that the Guest pmd is OK. */ + check_gpmd(cpu, gpmd); + + /* + * 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(gpmd))); + } + + /* + * 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). + */ + 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(cpu, *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. + */ + 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) +{ + pgd_t *spgd; + unsigned long flags; + +#ifdef CONFIG_X86_PAE + pmd_t *spmd; +#endif + /* 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 false; + +#ifdef CONFIG_X86_PAE + spmd = spmd_addr(cpu, *spgd, vaddr); + if (!(pmd_flags(*spmd) & _PAGE_PRESENT)) + return false; +#endif + + /* + * Check the flags on the pte entry itself: it must be present and + * writable. + */ + flags = pte_flags(*(spte_addr(cpu, *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); +} +/*:*/ + +#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); + 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; +#ifdef CONFIG_X86_PAE + pmd_t *pmd_table; +#endif + + /* + * 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 { +#ifdef CONFIG_X86_PAE + /* + * In PAE mode, allocate a pmd page and populate the + * last pgd entry. + */ + pmd_table = (pmd_t *)get_zeroed_page(GFP_KERNEL); + if (!pmd_table) { + free_page((long)cpu->lg->pgdirs[next].pgdir); + set_pgd(cpu->lg->pgdirs[next].pgdir, __pgd(0)); + next = cpu->cpu_pgd; + } else { + set_pgd(cpu->lg->pgdirs[next].pgdir + + SWITCHER_PGD_INDEX, + __pgd(__pa(pmd_table) | _PAGE_PRESENT)); + /* + * This is a blank page, so there are no kernel + * mappings: caller must map the stack! + */ + *blank_pgdir = 1; + } +#else + *blank_pgdir = 1; +#endif + } + } + /* 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: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) { +#ifdef CONFIG_X86_PAE + pgd_t *spgd; + pmd_t *pmdpage; + unsigned int k; + + /* Get the last pmd page. */ + spgd = lg->pgdirs[i].pgdir + SWITCHER_PGD_INDEX; + pmdpage = __va(pgd_pfn(*spgd) << PAGE_SHIFT); + + /* + * And release the pmd entries of that pmd page, + * except for the switcher pmd. + */ + for (k = 0; k < SWITCHER_PMD_INDEX; k++) + release_pmd(&pmdpage[k]); +#endif + /* Every PGD entry except the Switcher at the top */ + for (j = 0; j < SWITCHER_PGD_INDEX; j++) + release_pgd(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); +} + +/*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 */ + if (repin) + 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); +#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)) { + check_gpte(cpu, gpte); + 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) +{ + /* + * 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 update a (top-level) PGD entry: + */ +void guest_set_pgd(struct lguest *lg, unsigned long gpgdir, u32 idx) +{ + int pgdir; + + 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->pgdirs[pgdir].pgdir + idx); +} + +#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. + */ +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; + 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 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 2 or 4 MB + * of virtual addresses used by the Switcher. + */ + || put_user(RESERVE_MEM * 1024 * 1024, + &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. + */ +#ifdef CONFIG_X86_PAE + if (pgd_index(cpu->lg->kernel_address) == SWITCHER_PGD_INDEX && + pmd_index(cpu->lg->kernel_address) == SWITCHER_PMD_INDEX) +#else + if (pgd_index(cpu->lg->kernel_address) >= SWITCHER_PGD_INDEX) +#endif + 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 = __this_cpu_read(switcher_pte_pages); + pte_t regs_pte; + +#ifdef CONFIG_X86_PAE + pmd_t switcher_pmd; + pmd_t *pmd_table; + + switcher_pmd = pfn_pmd(__pa(switcher_pte_page) >> PAGE_SHIFT, + PAGE_KERNEL_EXEC); + + /* Figure out where the pmd page is, by reading the PGD, and converting + * it to a virtual address. */ + pmd_table = __va(pgd_pfn(cpu->lg-> + pgdirs[cpu->cpu_pgd].pgdir[SWITCHER_PGD_INDEX]) + << PAGE_SHIFT); + /* Now write it into the shadow page table. */ + set_pmd(&pmd_table[SWITCHER_PMD_INDEX], switcher_pmd); +#else + pgd_t switcher_pgd; + + /* + * 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_EXEC); + + cpu->lg->pgdirs[cpu->cpu_pgd].pgdir[SWITCHER_PGD_INDEX] = switcher_pgd; + +#endif + /* + * 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_pte(__pa(cpu->regs_page) >> PAGE_SHIFT, PAGE_KERNEL); + set_pte(&switcher_pte_page[pte_index((unsigned long)pages)], 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++) { + set_pte(&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 */ + set_pte(&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. + */ + set_pte(&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(); +} |