lguest: documentation V: Host
Documentation: The Host
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/drivers/lguest/page_tables.c b/drivers/lguest/page_tables.c
index f9ca50d..cd047e8 100644
--- a/drivers/lguest/page_tables.c
+++ b/drivers/lguest/page_tables.c
@@ -15,38 +15,91 @@
#include <asm/tlbflush.h>
#include "lg.h"
+/*H:300
+ * The Page Table Code
+ *
+ * We use two-level page tables for the Guest. If you're not entirely
+ * comfortable with virtual addresses, physical addresses and page tables then
+ * I recommend you review lguest.c's "Page Table Handling" (with diagrams!).
+ *
+ * The Guest keeps page tables, but we maintain the actual ones here: these are
+ * called "shadow" page tables. Which is a very Guest-centric name: these are
+ * the real page tables the CPU uses, although we keep them up to date to
+ * reflect the Guest's. (See what I mean about weird naming? Since when do
+ * shadows reflect anything?)
+ *
+ * Anyway, this is the most complicated part of the Host code. There are seven
+ * parts to this:
+ * (i) Setting up a page table entry for the Guest when it faults,
+ * (ii) Setting up the page table entry for the Guest stack,
+ * (iii) Setting up a page table entry when the Guest tells us it has changed,
+ * (iv) Switching page tables,
+ * (v) Flushing (thowing away) page tables,
+ * (vi) Mapping the Switcher when the Guest is about to run,
+ * (vii) Setting up the page tables initially.
+ :*/
+
+/* Pages a 4k long, and each page table entry is 4 bytes long, giving us 1024
+ * (or 2^10) entries per page. */
#define PTES_PER_PAGE_SHIFT 10
#define PTES_PER_PAGE (1 << PTES_PER_PAGE_SHIFT)
+
+/* 1024 entries in a page table page maps 1024 pages: 4MB. The Switcher is
+ * conveniently placed at the top 4MB, so it uses a separate, complete PTE
+ * page. */
#define SWITCHER_PGD_INDEX (PTES_PER_PAGE - 1)
+/* We actually need a separate PTE page for each CPU. Remember that after the
+ * Switcher code itself comes two pages for each CPU, and we don't want this
+ * CPU's guest to see the pages of any other CPU. */
static DEFINE_PER_CPU(spte_t *, switcher_pte_pages);
#define switcher_pte_page(cpu) per_cpu(switcher_pte_pages, cpu)
+/*H:320 With our shadow and Guest types established, we need to deal with
+ * them: the page table code is curly enough to need helper functions to keep
+ * it clear and clean.
+ *
+ * The first helper takes a virtual address, and says which entry in the top
+ * level page table deals with that address. Since each top level entry deals
+ * with 4M, this effectively divides by 4M. */
static unsigned vaddr_to_pgd_index(unsigned long vaddr)
{
return vaddr >> (PAGE_SHIFT + PTES_PER_PAGE_SHIFT);
}
-/* These access the shadow versions (ie. the ones used by the CPU). */
+/* There are two functions which return pointers to the shadow (aka "real")
+ * page tables.
+ *
+ * spgd_addr() takes the virtual address and returns a pointer to the top-level
+ * page directory entry for that address. Since we keep track of several page
+ * tables, the "i" argument tells us which one we're interested in (it's
+ * usually the current one). */
static spgd_t *spgd_addr(struct lguest *lg, u32 i, unsigned long vaddr)
{
unsigned int index = vaddr_to_pgd_index(vaddr);
+ /* We kill any Guest trying to touch the Switcher addresses. */
if (index >= SWITCHER_PGD_INDEX) {
kill_guest(lg, "attempt to access switcher pages");
index = 0;
}
+ /* Return a pointer index'th pgd entry for the i'th page table. */
return &lg->pgdirs[i].pgdir[index];
}
+/* This routine then takes the PGD entry given above, which contains the
+ * address of the PTE page. It then returns a pointer to the PTE entry for the
+ * given address. */
static spte_t *spte_addr(struct lguest *lg, spgd_t spgd, unsigned long vaddr)
{
spte_t *page = __va(spgd.pfn << PAGE_SHIFT);
+ /* You should never call this if the PGD entry wasn't valid */
BUG_ON(!(spgd.flags & _PAGE_PRESENT));
return &page[(vaddr >> PAGE_SHIFT) % PTES_PER_PAGE];
}
-/* These access the guest versions. */
+/* These two functions just like the above two, except they access the Guest
+ * page tables. Hence they return a Guest address. */
static unsigned long gpgd_addr(struct lguest *lg, unsigned long vaddr)
{
unsigned int index = vaddr >> (PAGE_SHIFT + PTES_PER_PAGE_SHIFT);
@@ -61,12 +114,24 @@
return gpage + ((vaddr>>PAGE_SHIFT) % PTES_PER_PAGE) * sizeof(gpte_t);
}
-/* Do a virtual -> physical mapping on a user page. */
+/*H:350 This routine takes a page number given by the Guest and converts it to
+ * an actual, physical page number. It can fail for several reasons: the
+ * virtual address might not be mapped by the Launcher, the write flag is set
+ * and the page is read-only, or the write flag was set and the page was
+ * shared so had to be copied, but we ran out of memory.
+ *
+ * This holds a reference to the page, so release_pte() is careful to
+ * put that back. */
static unsigned long get_pfn(unsigned long virtpfn, int write)
{
struct page *page;
+ /* This value indicates failure. */
unsigned long ret = -1UL;
+ /* get_user_pages() is a complex interface: it gets the "struct
+ * vm_area_struct" and "struct page" assocated with a range of pages.
+ * It also needs the task's mmap_sem held, and is not very quick.
+ * It returns the number of pages it got. */
down_read(¤t->mm->mmap_sem);
if (get_user_pages(current, current->mm, virtpfn << PAGE_SHIFT,
1, write, 1, &page, NULL) == 1)
@@ -75,28 +140,47 @@
return ret;
}
+/*H:340 Converting a Guest page table entry to a shadow (ie. real) page table
+ * entry can be a little tricky. The flags are (almost) the same, but the
+ * Guest PTE contains a virtual page number: the CPU needs the real page
+ * number. */
static spte_t gpte_to_spte(struct lguest *lg, gpte_t gpte, int write)
{
spte_t spte;
unsigned long pfn;
- /* We ignore the global flag. */
+ /* The Guest sets the global flag, because it thinks that it is using
+ * PGE. We only told it to use PGE so it would tell us whether it was
+ * flushing a kernel mapping or a userspace mapping. We don't actually
+ * use the global bit, so throw it away. */
spte.flags = (gpte.flags & ~_PAGE_GLOBAL);
+
+ /* We need a temporary "unsigned long" variable to hold the answer from
+ * get_pfn(), because it returns 0xFFFFFFFF on failure, which wouldn't
+ * fit in spte.pfn. get_pfn() finds the real physical number of the
+ * page, given the virtual number. */
pfn = get_pfn(gpte.pfn, write);
if (pfn == -1UL) {
kill_guest(lg, "failed to get page %u", gpte.pfn);
- /* Must not put_page() bogus page on cleanup. */
+ /* When we destroy the Guest, we'll go through the shadow page
+ * tables and release_pte() them. Make sure we don't think
+ * this one is valid! */
spte.flags = 0;
}
+ /* Now we assign the page number, and our shadow PTE is complete. */
spte.pfn = pfn;
return spte;
}
+/*H:460 And to complete the chain, release_pte() looks like this: */
static void release_pte(spte_t pte)
{
+ /* Remember that get_user_pages() took a reference to the page, in
+ * get_pfn()? We have to put it back now. */
if (pte.flags & _PAGE_PRESENT)
put_page(pfn_to_page(pte.pfn));
}
+/*:*/
static void check_gpte(struct lguest *lg, gpte_t gpte)
{
@@ -110,11 +194,16 @@
kill_guest(lg, "bad page directory entry");
}
-/* FIXME: We hold reference to pages, which prevents them from being
- swapped. It'd be nice to have a callback when Linux wants to swap out. */
-
-/* We fault pages in, which allows us to update accessed/dirty bits.
- * Return true if we got page. */
+/*H:330
+ * (i) Setting up a page table entry for the Guest when it faults
+ *
+ * We saw this call in run_guest(): when we see a page fault in the Guest, we
+ * come here. That's because we only set up the shadow page tables lazily as
+ * they're needed, so we get page faults all the time and quietly fix them up
+ * and return to the Guest without it knowing.
+ *
+ * If we fixed up the fault (ie. we mapped the address), this routine returns
+ * true. */
int demand_page(struct lguest *lg, unsigned long vaddr, int errcode)
{
gpgd_t gpgd;
@@ -123,106 +212,161 @@
gpte_t gpte;
spte_t *spte;
+ /* First step: get the top-level Guest page table entry. */
gpgd = mkgpgd(lgread_u32(lg, gpgd_addr(lg, vaddr)));
+ /* Toplevel not present? We can't map it in. */
if (!(gpgd.flags & _PAGE_PRESENT))
return 0;
+ /* Now look at the matching shadow entry. */
spgd = spgd_addr(lg, lg->pgdidx, vaddr);
if (!(spgd->flags & _PAGE_PRESENT)) {
- /* Get a page of PTEs for them. */
+ /* No shadow entry: allocate a new shadow PTE page. */
unsigned long ptepage = get_zeroed_page(GFP_KERNEL);
- /* FIXME: Steal from self in this case? */
+ /* This is not really the Guest's fault, but killing it is
+ * simple for this corner case. */
if (!ptepage) {
kill_guest(lg, "out of memory allocating pte page");
return 0;
}
+ /* We check that the Guest pgd is OK. */
check_gpgd(lg, gpgd);
+ /* And we copy the flags to the shadow PGD entry. The page
+ * number in the shadow PGD is the page we just allocated. */
spgd->raw.val = (__pa(ptepage) | gpgd.flags);
}
+ /* OK, now we look at the lower level in the Guest page table: keep its
+ * address, because we might update it later. */
gpte_ptr = gpte_addr(lg, gpgd, vaddr);
gpte = mkgpte(lgread_u32(lg, gpte_ptr));
- /* No page? */
+ /* If this page isn't in the Guest page tables, we can't page it in. */
if (!(gpte.flags & _PAGE_PRESENT))
return 0;
- /* Write to read-only page? */
+ /* Check they're not trying to write to a page the Guest wants
+ * read-only (bit 2 of errcode == write). */
if ((errcode & 2) && !(gpte.flags & _PAGE_RW))
return 0;
- /* User access to a non-user page? */
+ /* User access to a kernel page? (bit 3 == user access) */
if ((errcode & 4) && !(gpte.flags & _PAGE_USER))
return 0;
+ /* Check that the Guest PTE flags are OK, and the page number is below
+ * the pfn_limit (ie. not mapping the Launcher binary). */
check_gpte(lg, gpte);
+ /* Add the _PAGE_ACCESSED and (for a write) _PAGE_DIRTY flag */
gpte.flags |= _PAGE_ACCESSED;
if (errcode & 2)
gpte.flags |= _PAGE_DIRTY;
- /* We're done with the old pte. */
+ /* Get the pointer to the shadow PTE entry we're going to set. */
spte = spte_addr(lg, *spgd, vaddr);
+ /* If there was a valid shadow PTE entry here before, we release it.
+ * This can happen with a write to a previously read-only entry. */
release_pte(*spte);
- /* We don't make it writable if this isn't a write: later
- * write will fault so we can set dirty bit in guest. */
+ /* If this is a write, we insist that the Guest page is writable (the
+ * final arg to gpte_to_spte()). */
if (gpte.flags & _PAGE_DIRTY)
*spte = gpte_to_spte(lg, gpte, 1);
else {
+ /* If this is a read, don't set the "writable" bit in the page
+ * table entry, even if the Guest says it's writable. That way
+ * we come back here when a write does actually ocur, so we can
+ * update the Guest's _PAGE_DIRTY flag. */
gpte_t ro_gpte = gpte;
ro_gpte.flags &= ~_PAGE_RW;
*spte = gpte_to_spte(lg, ro_gpte, 0);
}
- /* Now we update dirty/accessed on guest. */
+ /* Finally, we write the Guest PTE entry back: we've set the
+ * _PAGE_ACCESSED and maybe the _PAGE_DIRTY flags. */
lgwrite_u32(lg, gpte_ptr, gpte.raw.val);
+
+ /* We succeeded in mapping the page! */
return 1;
}
-/* This is much faster than the full demand_page logic. */
+/*H:360 (ii) Setting up the page table entry for the Guest stack.
+ *
+ * Remember pin_stack_pages() which makes sure the stack is mapped? It could
+ * simply call demand_page(), but as we've seen that logic is quite long, and
+ * usually the stack pages are already mapped anyway, so it's not required.
+ *
+ * This is a quick version which answers the question: is this virtual address
+ * mapped by the shadow page tables, and is it writable? */
static int page_writable(struct lguest *lg, unsigned long vaddr)
{
spgd_t *spgd;
unsigned long flags;
+ /* Look at the top level entry: is it present? */
spgd = spgd_addr(lg, lg->pgdidx, vaddr);
if (!(spgd->flags & _PAGE_PRESENT))
return 0;
+ /* Check the flags on the pte entry itself: it must be present and
+ * writable. */
flags = spte_addr(lg, *spgd, vaddr)->flags;
return (flags & (_PAGE_PRESENT|_PAGE_RW)) == (_PAGE_PRESENT|_PAGE_RW);
}
+/* So, when pin_stack_pages() asks us to pin a page, we check if it's already
+ * in the page tables, and if not, we call demand_page() with error code 2
+ * (meaning "write"). */
void pin_page(struct lguest *lg, unsigned long vaddr)
{
if (!page_writable(lg, vaddr) && !demand_page(lg, vaddr, 2))
kill_guest(lg, "bad stack page %#lx", vaddr);
}
+/*H:450 If we chase down the release_pgd() code, it looks like this: */
static void release_pgd(struct lguest *lg, spgd_t *spgd)
{
+ /* If the entry's not present, there's nothing to release. */
if (spgd->flags & _PAGE_PRESENT) {
unsigned int i;
+ /* Converting the pfn to find the actual PTE page is easy: turn
+ * the page number into a physical address, then convert to a
+ * virtual address (easy for kernel pages like this one). */
spte_t *ptepage = __va(spgd->pfn << PAGE_SHIFT);
+ /* For each entry in the page, we might need to release it. */
for (i = 0; i < PTES_PER_PAGE; i++)
release_pte(ptepage[i]);
+ /* Now we can free the page of PTEs */
free_page((long)ptepage);
+ /* And zero out the PGD entry we we never release it twice. */
spgd->raw.val = 0;
}
}
+/*H:440 (v) Flushing (thowing away) page tables,
+ *
+ * We saw flush_user_mappings() called when we re-used a top-level pgdir page.
+ * It simply releases every PTE page from 0 up to the kernel address. */
static void flush_user_mappings(struct lguest *lg, int idx)
{
unsigned int i;
+ /* Release every pgd entry up to the kernel's address. */
for (i = 0; i < vaddr_to_pgd_index(lg->page_offset); i++)
release_pgd(lg, lg->pgdirs[idx].pgdir + i);
}
+/* The Guest also has a hypercall to do this manually: it's used when a large
+ * number of mappings have been changed. */
void guest_pagetable_flush_user(struct lguest *lg)
{
+ /* Drop the userspace part of the current page table. */
flush_user_mappings(lg, lg->pgdidx);
}
+/*:*/
+/* We keep several page tables. This is a simple routine to find the page
+ * table (if any) corresponding to this top-level address the Guest has given
+ * us. */
static unsigned int find_pgdir(struct lguest *lg, unsigned long pgtable)
{
unsigned int i;
@@ -232,21 +376,30 @@
return i;
}
+/*H:435 And this is us, creating the new page directory. If we really do
+ * allocate a new one (and so the kernel parts are not there), we set
+ * blank_pgdir. */
static unsigned int new_pgdir(struct lguest *lg,
unsigned long cr3,
int *blank_pgdir)
{
unsigned int next;
+ /* We pick one entry at random to throw out. Choosing the Least
+ * Recently Used might be better, but this is easy. */
next = random32() % ARRAY_SIZE(lg->pgdirs);
+ /* If it's never been allocated at all before, try now. */
if (!lg->pgdirs[next].pgdir) {
lg->pgdirs[next].pgdir = (spgd_t *)get_zeroed_page(GFP_KERNEL);
+ /* If the allocation fails, just keep using the one we have */
if (!lg->pgdirs[next].pgdir)
next = lg->pgdidx;
else
- /* There are no mappings: you'll need to re-pin */
+ /* This is a blank page, so there are no kernel
+ * mappings: caller must map the stack! */
*blank_pgdir = 1;
}
+ /* Record which Guest toplevel this shadows. */
lg->pgdirs[next].cr3 = cr3;
/* Release all the non-kernel mappings. */
flush_user_mappings(lg, next);
@@ -254,82 +407,161 @@
return next;
}
+/*H:430 (iv) Switching page tables
+ *
+ * This is what happens when the Guest changes page tables (ie. changes the
+ * top-level pgdir). This happens on almost every context switch. */
void guest_new_pagetable(struct lguest *lg, unsigned long pgtable)
{
int newpgdir, repin = 0;
+ /* Look to see if we have this one already. */
newpgdir = find_pgdir(lg, pgtable);
+ /* If not, we allocate or mug an existing one: if it's a fresh one,
+ * repin gets set to 1. */
if (newpgdir == ARRAY_SIZE(lg->pgdirs))
newpgdir = new_pgdir(lg, pgtable, &repin);
+ /* Change the current pgd index to the new one. */
lg->pgdidx = newpgdir;
+ /* If it was completely blank, we map in the Guest kernel stack */
if (repin)
pin_stack_pages(lg);
}
+/*H:470 Finally, a routine which throws away everything: all PGD entries in all
+ * the shadow page tables. This is used when we destroy the Guest. */
static void release_all_pagetables(struct lguest *lg)
{
unsigned int i, j;
+ /* Every shadow pagetable this Guest has */
for (i = 0; i < ARRAY_SIZE(lg->pgdirs); i++)
if (lg->pgdirs[i].pgdir)
+ /* Every PGD entry except the Switcher at the top */
for (j = 0; j < SWITCHER_PGD_INDEX; j++)
release_pgd(lg, lg->pgdirs[i].pgdir + j);
}
+/* We also throw away everything when a Guest tells us it's changed a kernel
+ * mapping. Since kernel mappings are in every page table, it's easiest to
+ * throw them all away. This is amazingly slow, but thankfully rare. */
void guest_pagetable_clear_all(struct lguest *lg)
{
release_all_pagetables(lg);
+ /* We need the Guest kernel stack mapped again. */
pin_stack_pages(lg);
}
+/*H:420 This is the routine which actually sets the page table entry for then
+ * "idx"'th shadow page table.
+ *
+ * Normally, we can just throw out the old entry and replace it with 0: if they
+ * use it demand_page() will put the new entry in. We need to do this anyway:
+ * The Guest expects _PAGE_ACCESSED to be set on its PTE the first time a page
+ * is read from, and _PAGE_DIRTY when it's written to.
+ *
+ * But Avi Kivity pointed out that most Operating Systems (Linux included) set
+ * these bits on PTEs immediately anyway. This is done to save the CPU from
+ * having to update them, but it helps us the same way: if they set
+ * _PAGE_ACCESSED then we can put a read-only PTE entry in immediately, and if
+ * they set _PAGE_DIRTY then we can put a writable PTE entry in immediately.
+ */
static void do_set_pte(struct lguest *lg, int idx,
unsigned long vaddr, gpte_t gpte)
{
+ /* Look up the matching shadow page directot entry. */
spgd_t *spgd = spgd_addr(lg, idx, vaddr);
+
+ /* If the top level isn't present, there's no entry to update. */
if (spgd->flags & _PAGE_PRESENT) {
+ /* Otherwise, we start by releasing the existing entry. */
spte_t *spte = spte_addr(lg, *spgd, vaddr);
release_pte(*spte);
+
+ /* If they're setting this entry as dirty or accessed, we might
+ * as well put that entry they've given us in now. This shaves
+ * 10% off a copy-on-write micro-benchmark. */
if (gpte.flags & (_PAGE_DIRTY | _PAGE_ACCESSED)) {
check_gpte(lg, gpte);
*spte = gpte_to_spte(lg, gpte, gpte.flags&_PAGE_DIRTY);
} else
+ /* Otherwise we can demand_page() it in later. */
spte->raw.val = 0;
}
}
+/*H:410 Updating a PTE entry is a little trickier.
+ *
+ * We keep track of several different page tables (the Guest uses one for each
+ * process, so it makes sense to cache at least a few). Each of these have
+ * identical kernel parts: ie. every mapping above PAGE_OFFSET is the same for
+ * all processes. So when the page table above that address changes, we update
+ * all the page tables, not just the current one. This is rare.
+ *
+ * The benefit is that when we have to track a new page table, we can copy keep
+ * all the kernel mappings. This speeds up context switch immensely. */
void guest_set_pte(struct lguest *lg,
unsigned long cr3, unsigned long vaddr, gpte_t gpte)
{
- /* Kernel mappings must be changed on all top levels. */
+ /* Kernel mappings must be changed on all top levels. Slow, but
+ * doesn't happen often. */
if (vaddr >= lg->page_offset) {
unsigned int i;
for (i = 0; i < ARRAY_SIZE(lg->pgdirs); i++)
if (lg->pgdirs[i].pgdir)
do_set_pte(lg, i, vaddr, gpte);
} else {
+ /* Is this page table one we have a shadow for? */
int pgdir = find_pgdir(lg, cr3);
if (pgdir != ARRAY_SIZE(lg->pgdirs))
+ /* If so, do the update. */
do_set_pte(lg, pgdir, vaddr, gpte);
}
}
+/*H:400
+ * (iii) Setting up a page table entry when the Guest tells us it has changed.
+ *
+ * Just like we did in interrupts_and_traps.c, it makes sense for us to deal
+ * with the other side of page tables while we're here: what happens when the
+ * Guest asks for a page table to be updated?
+ *
+ * We already saw that demand_page() will fill in the shadow page tables when
+ * needed, so we can simply remove shadow page table entries whenever the Guest
+ * tells us they've changed. When the Guest tries to use the new entry it will
+ * fault and demand_page() will fix it up.
+ *
+ * So with that in mind here's our code to to update a (top-level) PGD entry:
+ */
void guest_set_pmd(struct lguest *lg, unsigned long cr3, u32 idx)
{
int pgdir;
+ /* The kernel seems to try to initialize this early on: we ignore its
+ * attempts to map over the Switcher. */
if (idx >= SWITCHER_PGD_INDEX)
return;
+ /* If they're talking about a page table we have a shadow for... */
pgdir = find_pgdir(lg, cr3);
if (pgdir < ARRAY_SIZE(lg->pgdirs))
+ /* ... throw it away. */
release_pgd(lg, lg->pgdirs[pgdir].pgdir + idx);
}
+/*H:500 (vii) Setting up the page tables initially.
+ *
+ * When a Guest is first created, the Launcher tells us where the toplevel of
+ * its first page table is. We set some things up here: */
int init_guest_pagetable(struct lguest *lg, unsigned long pgtable)
{
- /* We assume this in flush_user_mappings, so check now */
+ /* In flush_user_mappings() we loop from 0 to
+ * "vaddr_to_pgd_index(lg->page_offset)". This assumes it won't hit
+ * the Switcher mappings, so check that now. */
if (vaddr_to_pgd_index(lg->page_offset) >= SWITCHER_PGD_INDEX)
return -EINVAL;
+ /* We start on the first shadow page table, and give it a blank PGD
+ * page. */
lg->pgdidx = 0;
lg->pgdirs[lg->pgdidx].cr3 = pgtable;
lg->pgdirs[lg->pgdidx].pgdir = (spgd_t*)get_zeroed_page(GFP_KERNEL);
@@ -338,33 +570,48 @@
return 0;
}
+/* When a Guest dies, our cleanup is fairly simple. */
void free_guest_pagetable(struct lguest *lg)
{
unsigned int i;
+ /* Throw away all page table pages. */
release_all_pagetables(lg);
+ /* Now free the top levels: free_page() can handle 0 just fine. */
for (i = 0; i < ARRAY_SIZE(lg->pgdirs); i++)
free_page((long)lg->pgdirs[i].pgdir);
}
-/* Caller must be preempt-safe */
+/*H:480 (vi) Mapping the Switcher when the Guest is about to run.
+ *
+ * The Switcher and the two pages for this CPU need to be available to the
+ * Guest (and not the pages for other CPUs). We have the appropriate PTE pages
+ * for each CPU already set up, we just need to hook them in. */
void map_switcher_in_guest(struct lguest *lg, struct lguest_pages *pages)
{
spte_t *switcher_pte_page = __get_cpu_var(switcher_pte_pages);
spgd_t switcher_pgd;
spte_t regs_pte;
- /* Since switcher less that 4MB, we simply mug top pte page. */
+ /* Make the last PGD entry for this Guest point to the Switcher's PTE
+ * page for this CPU (with appropriate flags). */
switcher_pgd.pfn = __pa(switcher_pte_page) >> PAGE_SHIFT;
switcher_pgd.flags = _PAGE_KERNEL;
lg->pgdirs[lg->pgdidx].pgdir[SWITCHER_PGD_INDEX] = switcher_pgd;
- /* Map our regs page over stack page. */
+ /* We also change the Switcher PTE page. When we're running the Guest,
+ * we want the Guest's "regs" page to appear where the first Switcher
+ * page for this CPU is. This is an optimization: when the Switcher
+ * saves the Guest registers, it saves them into the first page of this
+ * CPU's "struct lguest_pages": if we make sure the Guest's register
+ * page is already mapped there, we don't have to copy them out
+ * again. */
regs_pte.pfn = __pa(lg->regs_page) >> PAGE_SHIFT;
regs_pte.flags = _PAGE_KERNEL;
switcher_pte_page[(unsigned long)pages/PAGE_SIZE%PTES_PER_PAGE]
= regs_pte;
}
+/*:*/
static void free_switcher_pte_pages(void)
{
@@ -374,6 +621,10 @@
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)
@@ -381,21 +632,26 @@
unsigned int i;
spte_t *pte = switcher_pte_page(cpu);
+ /* The first entries are easy: they map the Switcher code. */
for (i = 0; i < pages; i++) {
pte[i].pfn = page_to_pfn(switcher_page[i]);
pte[i].flags = _PAGE_PRESENT|_PAGE_ACCESSED;
}
- /* We only map this CPU's pages, so guest can't see others. */
+ /* The only other thing we map is this CPU's pair of pages. */
i = pages + cpu*2;
- /* First page (regs) is rw, second (state) is ro. */
+ /* First page (Guest registers) is writable from the Guest */
pte[i].pfn = page_to_pfn(switcher_page[i]);
pte[i].flags = _PAGE_PRESENT|_PAGE_ACCESSED|_PAGE_RW;
+ /* The second page contains the "struct lguest_ro_state", and is
+ * read-only. */
pte[i+1].pfn = page_to_pfn(switcher_page[i+1]);
pte[i+1].flags = _PAGE_PRESENT|_PAGE_ACCESSED;
}
+/*H:510 At boot or module load time, init_pagetables() allocates and populates
+ * the Switcher PTE page for each CPU. */
__init int init_pagetables(struct page **switcher_page, unsigned int pages)
{
unsigned int i;
@@ -410,7 +666,9 @@
}
return 0;
}
+/*:*/
+/* Cleaning up simply involves freeing the PTE page for each CPU. */
void free_pagetables(void)
{
free_switcher_pte_pages();