*
* We use two-level page tables for the Guest. If you're not entirely
* comfortable with virtual addresses, physical addresses and page tables then
- * I recommend you review lguest.c's "Page Table Handling" (with diagrams!).
+ * 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
*
* Anyway, this is the most complicated part of the Host code. There are seven
* parts to this:
- * (i) Setting up a page table entry for the Guest when it faults,
- * (ii) Setting up the page table entry for the Guest stack,
- * (iii) Setting up a page table entry when the Guest tells us it has changed,
+ * (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 (thowing away) 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.
:*/
static DEFINE_PER_CPU(pte_t *, switcher_pte_pages);
#define switcher_pte_page(cpu) per_cpu(switcher_pte_pages, cpu)
-/*H:320 With our shadow and Guest types established, we need to deal with
- * them: the page table code is curly enough to need helper functions to keep
- * it clear and clean.
+/*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 for that address. Since we keep track of several page
- * tables, the "i" argument tells us which one we're interested in (it's
+ * 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 lguest *lg, u32 i, unsigned long vaddr)
{
return &lg->pgdirs[i].pgdir[index];
}
-/* This routine then takes the PGD entry given above, which contains the
- * address of the PTE page. It then returns a pointer to the PTE entry for the
- * given address. */
+/* 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 lguest *lg, pgd_t spgd, unsigned long vaddr)
{
pte_t *page = __va(pgd_pfn(spgd) << PAGE_SHIFT);
/* These two functions just like the above two, except they access the Guest
* page tables. Hence they return a Guest address. */
-static unsigned long gpgd_addr(struct lguest *lg, unsigned long vaddr)
+static unsigned long gpgd_addr(struct lg_cpu *cpu, unsigned long vaddr)
{
unsigned int index = vaddr >> (PGDIR_SHIFT);
- return lg->pgdirs[lg->pgdidx].gpgdir + index * sizeof(pgd_t);
+ return cpu->lg->pgdirs[cpu->cpu_pgd].gpgdir + index * sizeof(pgd_t);
}
static unsigned long gpte_addr(struct lguest *lg,
}
/*H:330
- * (i) Setting up a page table entry for the Guest when it faults
+ * (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
* and return to the Guest without it knowing.
*
* If we fixed up the fault (ie. we mapped the address), this routine returns
- * true. */
-int demand_page(struct lguest *lg, unsigned long vaddr, int errcode)
+ * 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;
+ struct lguest *lg = cpu->lg;
/* First step: get the top-level Guest page table entry. */
- gpgd = lgread(lg, gpgd_addr(lg, vaddr), pgd_t);
+ gpgd = lgread(lg, 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(lg, lg->pgdidx, vaddr);
+ spgd = spgd_addr(lg, 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);
if ((errcode & 2) && !(pte_flags(gpte) & _PAGE_RW))
return 0;
- /* User access to a kernel page? (bit 3 == user access) */
+ /* 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(lg, gpte);
+
/* Add the _PAGE_ACCESSED and (for a write) _PAGE_DIRTY flag */
gpte = pte_mkyoung(gpte);
-
if (errcode & 2)
gpte = pte_mkdirty(gpte);
else
/* If this is a read, don't set the "writable" bit in the page
* table entry, even if the Guest says it's writable. That way
- * we come back here when a write does actually ocur, so we can
- * update the Guest's _PAGE_DIRTY flag. */
+ * 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(lg, 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(lg, gpte_ptr, pte_t, gpte);
- /* We succeeded in mapping the page! */
+ /* 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) Setting up the page table entry for the Guest stack.
+/*H:360
+ * (ii) Making sure the Guest stack is mapped.
*
- * Remember pin_stack_pages() which makes sure the stack is mapped? It could
- * simply call demand_page(), but as we've seen that logic is quite long, and
- * usually the stack pages are already mapped anyway, so it's not required.
+ * 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 lguest *lg, unsigned long vaddr)
+static int page_writable(struct lg_cpu *cpu, unsigned long vaddr)
{
pgd_t *spgd;
unsigned long flags;
- /* Look at the top level entry: is it present? */
- spgd = spgd_addr(lg, lg->pgdidx, vaddr);
+ /* Look at the current top level entry: is it present? */
+ spgd = spgd_addr(cpu->lg, 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(lg, *spgd, vaddr)));
+ flags = pte_flags(*(spte_addr(cpu->lg, *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 lguest *lg, unsigned long vaddr)
+void pin_page(struct lg_cpu *cpu, unsigned long vaddr)
{
- if (!page_writable(lg, vaddr) && !demand_page(lg, vaddr, 2))
- kill_guest(lg, "bad stack page %#lx", vaddr);
+ if (!page_writable(cpu, vaddr) && !demand_page(cpu, vaddr, 2))
+ kill_guest(cpu->lg, "bad stack page %#lx", vaddr);
}
/*H:450 If we chase down the release_pgd() code, it looks like this: */
release_pte(ptepage[i]);
/* Now we can free the page of PTEs */
free_page((long)ptepage);
- /* And zero out the PGD entry we we never release it twice. */
+ /* And zero out the PGD entry so we never release it twice. */
*spgd = __pgd(0);
}
}
-/*H:440 (v) Flushing (thowing away) page tables,
- *
- * We saw flush_user_mappings() called when we re-used a top-level pgdir page.
- * It simply releases every PTE page from 0 up to the kernel address. */
+/*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_pgd(lg, lg->pgdirs[idx].pgdir + i);
}
-/* The Guest also has a hypercall to do this manually: it's used when a large
- * number of mappings have been changed. */
-void guest_pagetable_flush_user(struct lguest *lg)
+/*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(lg, lg->pgdidx);
+ 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 lguest *lg, unsigned long vaddr)
+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(lg, gpgd_addr(lg, vaddr), pgd_t);
+ gpgd = lgread(cpu->lg, gpgd_addr(cpu, vaddr), pgd_t);
/* Toplevel not present? We can't map it in. */
if (!(pgd_flags(gpgd) & _PAGE_PRESENT))
- kill_guest(lg, "Bad address %#lx", vaddr);
+ kill_guest(cpu->lg, "Bad address %#lx", vaddr);
- gpte = lgread(lg, gpte_addr(lg, gpgd, vaddr), pte_t);
+ gpte = lgread(cpu->lg, gpte_addr(cpu->lg, gpgd, vaddr), pte_t);
if (!(pte_flags(gpte) & _PAGE_PRESENT))
- kill_guest(lg, "Bad address %#lx", vaddr);
+ kill_guest(cpu->lg, "Bad address %#lx", vaddr);
return pte_pfn(gpte) * PAGE_SIZE | (vaddr & ~PAGE_MASK);
}
/*H:435 And this is us, creating the new page directory. If we really do
* allocate a new one (and so the kernel parts are not there), we set
* blank_pgdir. */
-static unsigned int new_pgdir(struct lguest *lg,
+static unsigned int new_pgdir(struct lg_cpu *cpu,
unsigned long gpgdir,
int *blank_pgdir)
{
unsigned int next;
+ struct lguest *lg = cpu->lg;
/* We pick one entry at random to throw out. Choosing the Least
* Recently Used might be better, but this is easy. */
lg->pgdirs[next].pgdir = (pgd_t *)get_zeroed_page(GFP_KERNEL);
/* If the allocation fails, just keep using the one we have */
if (!lg->pgdirs[next].pgdir)
- next = lg->pgdidx;
+ next = cpu->cpu_pgd;
else
/* This is a blank page, so there are no kernel
* mappings: caller must map the stack! */
/*H:430 (iv) Switching page tables
*
- * This is what happens when the Guest changes page tables (ie. changes the
- * top-level pgdir). This happens on almost every context switch. */
-void guest_new_pagetable(struct lguest *lg, unsigned long pgtable)
+ * 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;
+ struct lguest *lg = cpu->lg;
/* Look to see if we have this one already. */
newpgdir = find_pgdir(lg, pgtable);
/* If not, we allocate or mug an existing one: if it's a fresh one,
* repin gets set to 1. */
if (newpgdir == ARRAY_SIZE(lg->pgdirs))
- newpgdir = new_pgdir(lg, pgtable, &repin);
+ newpgdir = new_pgdir(cpu, pgtable, &repin);
/* Change the current pgd index to the new one. */
- lg->pgdidx = newpgdir;
+ cpu->cpu_pgd = newpgdir;
/* If it was completely blank, we map in the Guest kernel stack */
if (repin)
- pin_stack_pages(lg);
+ pin_stack_pages(cpu);
}
/*H:470 Finally, a routine which throws away everything: all PGD entries in all
- * the shadow page tables. This is used when we destroy the Guest. */
+ * 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;
/* We also throw away everything when a Guest tells us it's changed a kernel
* mapping. Since kernel mappings are in every page table, it's easiest to
- * throw them all away. This is amazingly slow, but thankfully rare. */
-void guest_pagetable_clear_all(struct lguest *lg)
+ * 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(lg);
+ release_all_pagetables(cpu->lg);
/* We need the Guest kernel stack mapped again. */
- pin_stack_pages(lg);
+ 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.
static void do_set_pte(struct lguest *lg, int idx,
unsigned long vaddr, pte_t gpte)
{
- /* Look up the matching shadow page directot entry. */
+ /* Look up the matching shadow page directory entry. */
pgd_t *spgd = spgd_addr(lg, idx, vaddr);
/* If the top level isn't present, there's no entry to update. */
*spte = gpte_to_spte(lg, gpte,
pte_flags(gpte) & _PAGE_DIRTY);
} else
- /* Otherwise we can demand_page() it in later. */
+ /* Otherwise kill it and we can demand_page() it in
+ * later. */
*spte = __pte(0);
}
}
}
/*H:400
- * (iii) Setting up a page table entry when the Guest tells us it has changed.
+ * (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
{
/* We start on the first shadow page table, and give it a blank PGD
* page. */
- lg->pgdidx = 0;
- lg->pgdirs[lg->pgdidx].gpgdir = pgtable;
- lg->pgdirs[lg->pgdidx].pgdir = (pgd_t*)get_zeroed_page(GFP_KERNEL);
- if (!lg->pgdirs[lg->pgdidx].pgdir)
+ 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;
}
/* We tell the Guest that it can't use the top 4MB of virtual
* addresses used by the Switcher. */
|| put_user(4U*1024*1024, &lg->lguest_data->reserve_mem)
- || put_user(lg->pgdirs[lg->pgdidx].gpgdir,&lg->lguest_data->pgdir))
+ || put_user(lg->pgdirs[0].gpgdir, &lg->lguest_data->pgdir))
kill_guest(lg, "bad guest page %p", lg->lguest_data);
/* In flush_user_mappings() we loop from 0 to
/*H:480 (vi) Mapping the Switcher when the Guest is about to run.
*
- * The Switcher and the two pages for this CPU need to be available to the
+ * 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. */
-void map_switcher_in_guest(struct lguest *lg, struct lguest_pages *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);
- lg->pgdirs[lg->pgdidx].pgdir[SWITCHER_PGD_INDEX] = switcher_pgd;
+ 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
* 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(lg->regs_page) >> PAGE_SHIFT, __pgprot(_PAGE_KERNEL));
+ 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;
}
/*:*/
__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)
projects / karo-tx-linux.git / blobdiff