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>

1 files changed, 286 insertions, 28 deletions

diff --git a/drivers/lguest/page_tables.c b/drivers/lguest/page_tables.c
index f9ca50d8046..cd047e81cd6 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 @@ static unsigned long gpte_addr(struct lguest *lg,
 	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(&current->mm->mmap_sem);
 	if (get_user_pages(current, current->mm, virtpfn << PAGE_SHIFT,
 			   1, write, 1, &page, NULL) == 1)
@@ -75,28 +140,47 @@ static unsigned long get_pfn(unsigned long virtpfn, int write)
 	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 @@ static void check_gpgd(struct lguest *lg, gpgd_t gpgd)
 		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 @@ int demand_page(struct lguest *lg, unsigned long vaddr, int errcode)
 	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 @@ static unsigned int find_pgdir(struct lguest *lg, unsigned long pgtable)
 	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 @@ static unsigned int new_pgdir(struct lguest *lg,
 	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 @@ int init_guest_pagetable(struct lguest *lg, unsigned long pgtable)
 	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 @@ static void free_switcher_pte_pages(void)
 		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 @@ static __init void populate_switcher_pte_page(unsigned int cpu,
 	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 @@ __init int init_pagetables(struct page **switcher_page, unsigned int pages)
 	}
 	return 0;
 }
+/*:*/
 
+/* Cleaning up simply involves freeing the PTE page for each CPU. */
 void free_pagetables(void)
 {
 	free_switcher_pte_pages();