lguest: update commentry

Every so often, after code shuffles, I need to go through and unbitrot
the Lguest Journey (see drivers/lguest/README).  Since we now use RCU in
a simple form in one place I took the opportunity to expand that explanation.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>

7 files changed, 176 insertions, 40 deletions

diff --git a/drivers/lguest/core.c b/drivers/lguest/core.c
index cd058bc903ff..1e2cb846b3c9 100644
--- a/drivers/lguest/core.c
+++ b/drivers/lguest/core.c
@@ -217,10 +217,15 @@ int run_guest(struct lg_cpu *cpu, unsigned long __user *user)
 
 		/*
 		 * It's possible the Guest did a NOTIFY hypercall to the
-		 * Launcher, in which case we return from the read() now.
+		 * Launcher.
 		 */
 		if (cpu->pending_notify) {
+			/*
+			 * Does it just needs to write to a registered
+			 * eventfd (ie. the appropriate virtqueue thread)?
+			 */
 			if (!send_notify_to_eventfd(cpu)) {
+				/* OK, we tell the main Laucher. */
 				if (put_user(cpu->pending_notify, user))
 					return -EFAULT;
 				return sizeof(cpu->pending_notify);
diff --git a/drivers/lguest/hypercalls.c b/drivers/lguest/hypercalls.c
index 787ab4bc09f0..83511eb0923d 100644
--- a/drivers/lguest/hypercalls.c
+++ b/drivers/lguest/hypercalls.c
@@ -59,7 +59,7 @@ static void do_hcall(struct lg_cpu *cpu, struct hcall_args *args)
 	case LHCALL_SHUTDOWN: {
 		char msg[128];
 		/*
-		 * Shutdown is such a trivial hypercall that we do it in four
+		 * Shutdown is such a trivial hypercall that we do it in five
 		 * lines right here.
 		 *
 		 * If the lgread fails, it will call kill_guest() itself; the
@@ -245,6 +245,10 @@ static void initialize(struct lg_cpu *cpu)
  * device), the Guest will still see the old page.  In practice, this never
  * happens: why would the Guest read a page which it has never written to?  But
  * a similar scenario might one day bite us, so it's worth mentioning.
+ *
+ * Note that if we used a shared anonymous mapping in the Launcher instead of
+ * mapping /dev/zero private, we wouldn't worry about cop-on-write.  And we
+ * need that to switch the Launcher to processes (away from threads) anyway.
 :*/
 
 /*H:100
diff --git a/drivers/lguest/lguest_device.c b/drivers/lguest/lguest_device.c
index cc000e79c3d1..1401c1ace1ec 100644
--- a/drivers/lguest/lguest_device.c
+++ b/drivers/lguest/lguest_device.c
@@ -236,7 +236,7 @@ static void lg_notify(struct virtqueue *vq)
 extern void lguest_setup_irq(unsigned int irq);
 
 /*
- * This routine finds the first virtqueue described in the configuration of
+ * This routine finds the Nth virtqueue described in the configuration of
  * this device and sets it up.
  *
  * This is kind of an ugly duckling.  It'd be nicer to have a standard
@@ -244,9 +244,6 @@ extern void lguest_setup_irq(unsigned int irq);
  * everyone wants to do it differently.  The KVM coders want the Guest to
  * allocate its own pages and tell the Host where they are, but for lguest it's
  * simpler for the Host to simply tell us where the pages are.
- *
- * So we provide drivers with a "find the Nth virtqueue and set it up"
- * function.
  */
 static struct virtqueue *lg_find_vq(struct virtio_device *vdev,
 				    unsigned index,
@@ -422,7 +419,11 @@ static void add_lguest_device(struct lguest_device_desc *d,
 
 	/* This devices' parent is the lguest/ dir. */
 	ldev->vdev.dev.parent = lguest_root;
-	/* We have a unique device index thanks to the dev_index counter. */
+	/*
+	 * The device type comes straight from the descriptor.  There's also a
+	 * device vendor field in the virtio_device struct, which we leave as
+	 * 0.
+	 */
 	ldev->vdev.id.device = d->type;
 	/*
 	 * We have a simple set of routines for querying the device's
diff --git a/drivers/lguest/lguest_user.c b/drivers/lguest/lguest_user.c
index 7e92017103dc..b4d3f7ca554f 100644
--- a/drivers/lguest/lguest_user.c
+++ b/drivers/lguest/lguest_user.c
@@ -1,9 +1,8 @@
-/*P:200
- * This contains all the /dev/lguest code, whereby the userspace launcher
+/*P:200 This contains all the /dev/lguest code, whereby the userspace launcher
  * controls and communicates with the Guest.  For example, the first write will
- * tell us the Guest's memory layout, pagetable, entry point and kernel address
- * offset.  A read will run the Guest until something happens, such as a signal
- * or the Guest doing a NOTIFY out to the Launcher.
+ * tell us the Guest's memory layout and entry point.  A read will run the
+ * Guest until something happens, such as a signal or the Guest doing a NOTIFY
+ * out to the Launcher.
 :*/
 #include <linux/uaccess.h>
 #include <linux/miscdevice.h>
@@ -13,14 +12,41 @@
 #include <linux/file.h>
 #include "lg.h"
 
+/*L:056
+ * Before we move on, let's jump ahead and look at what the kernel does when
+ * it needs to look up the eventfds.  That will complete our picture of how we
+ * use RCU.
+ *
+ * The notification value is in cpu->pending_notify: we return true if it went
+ * to an eventfd.
+ */
 bool send_notify_to_eventfd(struct lg_cpu *cpu)
 {
 	unsigned int i;
 	struct lg_eventfd_map *map;
 
-	/* lg->eventfds is RCU-protected */
+	/*
+	 * This "rcu_read_lock()" helps track when someone is still looking at
+	 * the (RCU-using) eventfds array.  It's not actually a lock at all;
+	 * indeed it's a noop in many configurations.  (You didn't expect me to
+	 * explain all the RCU secrets here, did you?)
+	 */
 	rcu_read_lock();
+	/*
+	 * rcu_dereference is the counter-side of rcu_assign_pointer(); it
+	 * makes sure we don't access the memory pointed to by
+	 * cpu->lg->eventfds before cpu->lg->eventfds is set.  Sounds crazy,
+	 * but Alpha allows this!  Paul McKenney points out that a really
+	 * aggressive compiler could have the same effect:
+	 *   http://lists.ozlabs.org/pipermail/lguest/2009-July/001560.html
+	 *
+	 * So play safe, use rcu_dereference to get the rcu-protected pointer:
+	 */
 	map = rcu_dereference(cpu->lg->eventfds);
+	/*
+	 * Simple array search: even if they add an eventfd while we do this,
+	 * we'll continue to use the old array and just won't see the new one.
+	 */
 	for (i = 0; i < map->num; i++) {
 		if (map->map[i].addr == cpu->pending_notify) {
 			eventfd_signal(map->map[i].event, 1);
@@ -28,14 +54,43 @@ bool send_notify_to_eventfd(struct lg_cpu *cpu)
 			break;
 		}
 	}
+	/* We're done with the rcu-protected variable cpu->lg->eventfds. */
 	rcu_read_unlock();
+
+	/* If we cleared the notification, it's because we found a match. */
 	return cpu->pending_notify == 0;
 }
 
+/*L:055
+ * One of the more tricksy tricks in the Linux Kernel is a technique called
+ * Read Copy Update.  Since one point of lguest is to teach lguest journeyers
+ * about kernel coding, I use it here.  (In case you're curious, other purposes
+ * include learning about virtualization and instilling a deep appreciation for
+ * simplicity and puppies).
+ *
+ * We keep a simple array which maps LHCALL_NOTIFY values to eventfds, but we
+ * add new eventfds without ever blocking readers from accessing the array.
+ * The current Launcher only does this during boot, so that never happens.  But
+ * Read Copy Update is cool, and adding a lock risks damaging even more puppies
+ * than this code does.
+ *
+ * We allocate a brand new one-larger array, copy the old one and add our new
+ * element.  Then we make the lg eventfd pointer point to the new array.
+ * That's the easy part: now we need to free the old one, but we need to make
+ * sure no slow CPU somewhere is still looking at it.  That's what
+ * synchronize_rcu does for us: waits until every CPU has indicated that it has
+ * moved on to know it's no longer using the old one.
+ *
+ * If that's unclear, see http://en.wikipedia.org/wiki/Read-copy-update.
+ */
 static int add_eventfd(struct lguest *lg, unsigned long addr, int fd)
 {
 	struct lg_eventfd_map *new, *old = lg->eventfds;
 
+	/*
+	 * We don't allow notifications on value 0 anyway (pending_notify of
+	 * 0 means "nothing pending").
+	 */
 	if (!addr)
 		return -EINVAL;
 
@@ -62,12 +117,20 @@ static int add_eventfd(struct lguest *lg, unsigned long addr, int fd)
 	}
 	new->num++;
 
-	/* Now put new one in place. */
+	/*
+	 * Now put new one in place: rcu_assign_pointer() is a fancy way of
+	 * doing "lg->eventfds = new", but it uses memory barriers to make
+	 * absolutely sure that the contents of "new" written above is nailed
+	 * down before we actually do the assignment.
+	 *
+	 * We have to think about these kinds of things when we're operating on
+	 * live data without locks.
+	 */
 	rcu_assign_pointer(lg->eventfds, new);
 
 	/*
 	 * We're not in a big hurry.  Wait until noone's looking at old
-	 * version, then delete it.
+	 * version, then free it.
 	 */
 	synchronize_rcu();
 	kfree(old);
@@ -75,6 +138,14 @@ static int add_eventfd(struct lguest *lg, unsigned long addr, int fd)
 	return 0;
 }
 
+/*L:052
+ * Receiving notifications from the Guest is usually done by attaching a
+ * particular LHCALL_NOTIFY value to an event filedescriptor.  The eventfd will
+ * become readable when the Guest does an LHCALL_NOTIFY with that value.
+ *
+ * This is really convenient for processing each virtqueue in a separate
+ * thread.
+ */
 static int attach_eventfd(struct lguest *lg, const unsigned long __user *input)
 {
 	unsigned long addr, fd;
@@ -86,6 +157,11 @@ static int attach_eventfd(struct lguest *lg, const unsigned long __user *input)
 	if (get_user(fd, input) != 0)
 		return -EFAULT;
 
+	/*
+	 * Just make sure two callers don't add eventfds at once.  We really
+	 * only need to lock against callers adding to the same Guest, so using
+	 * the Big Lguest Lock is overkill.  But this is setup, not a fast path.
+	 */
 	mutex_lock(&lguest_lock);
 	err = add_eventfd(lg, addr, fd);
 	mutex_unlock(&lguest_lock);
@@ -106,6 +182,10 @@ static int user_send_irq(struct lg_cpu *cpu, const unsigned long __user *input)
 	if (irq >= LGUEST_IRQS)
 		return -EINVAL;
 
+	/*
+	 * Next time the Guest runs, the core code will see if it can deliver
+	 * this interrupt.
+	 */
 	set_interrupt(cpu, irq);
 	return 0;
 }
@@ -307,10 +387,10 @@ unlock:
  * The first operation the Launcher does must be a write.  All writes
  * start with an unsigned long number: for the first write this must be
  * LHREQ_INITIALIZE to set up the Guest.  After that the Launcher can use
- * writes of other values to send interrupts.
+ * writes of other values to send interrupts or set up receipt of notifications.
  *
  * Note that we overload the "offset" in the /dev/lguest file to indicate what
- * CPU number we're dealing with.  Currently this is always 0, since we only
+ * CPU number we're dealing with.  Currently this is always 0 since we only
  * support uniprocessor Guests, but you can see the beginnings of SMP support
  * here.
  */
diff --git a/drivers/lguest/page_tables.c b/drivers/lguest/page_tables.c
index 3da902e4b4cb..a8d0aee3bc0e 100644
--- a/drivers/lguest/page_tables.c
+++ b/drivers/lguest/page_tables.c
@@ -29,10 +29,10 @@
 /*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 arch/x86/lguest/boot.c's "Page Table Handling" (with
- * diagrams!).
+ * 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
@@ -52,9 +52,8 @@
 :*/
 
 /*
- * 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.
+ * 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)
 
@@ -81,7 +80,8 @@ static DEFINE_PER_CPU(pte_t *, switcher_pte_pages);
 
 /*H:320
  * The page table code is curly enough to need helper functions to keep it
- * clear and clean.
+ * 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.
@@ -155,7 +155,7 @@ static pte_t *spte_addr(struct lg_cpu *cpu, pgd_t spgd, unsigned long vaddr)
 }
 
 /*
- * These two functions just like the above two, except they access the Guest
+ * These functions are just like the above two, 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)
@@ -165,6 +165,7 @@ static unsigned long gpgd_addr(struct lg_cpu *cpu, unsigned long vaddr)
 }
 
 #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;
@@ -172,6 +173,7 @@ static unsigned long gpmd_addr(pgd_t gpgd, unsigned long vaddr)
 	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)
 {
@@ -181,6 +183,7 @@ static unsigned long gpte_addr(struct lg_cpu *cpu,
 	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)
 {
@@ -314,6 +317,7 @@ bool demand_page(struct lg_cpu *cpu, unsigned long vaddr, int errcode)
 	pte_t gpte;
 	pte_t *spte;
 
+	/* Mid level for PAE. */
 #ifdef CONFIG_X86_PAE
 	pmd_t *spmd;
 	pmd_t gpmd;
@@ -391,6 +395,8 @@ bool demand_page(struct lg_cpu *cpu, unsigned long vaddr, int errcode)
 	 */
 	gpte_ptr = gpte_addr(cpu, gpgd, vaddr);
 #endif
+
+	/* 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. */
@@ -507,6 +513,7 @@ 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)
@@ -543,7 +550,11 @@ static void release_pgd(pgd_t *spgd)
 }
 
 #else /* !CONFIG_X86_PAE */
-/*H:450 If we chase down the release_pgd() code, it looks like this: */
+/*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. */
@@ -898,17 +909,21 @@ void guest_set_pgd(struct lguest *lg, unsigned long gpgdir, u32 idx)
 		/* ... 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
 
-/*
- * Once we know how much memory we have we can construct simple identity (which
+/*H:505
+ * To get through boot, we construct simple identity page mappings (which
  * set virtual == physical) and linear mappings which will get the Guest far
- * enough into the boot to create its own.
+ * enough into the boot to create its own.  The linear mapping means we
+ * simplify the Guest boot, but it makes assumptions about their PAGE_OFFSET,
+ * as you'll see.
  *
  * We lay them out of the way, just below the initrd (which is why we need to
  * know its size here).
@@ -944,6 +959,10 @@ static unsigned long setup_pagetables(struct lguest *lg,
 	linear = (void *)pgdir - linear_pages * PAGE_SIZE;
 
 #ifdef CONFIG_X86_PAE
+	/*
+	 * And the single mid page goes below that.  We only use one, but
+	 * that's enough to map 1G, which definitely gets us through boot.
+	 */
 	pmds = (void *)linear - PAGE_SIZE;
 #endif
 	/*
@@ -957,13 +976,14 @@ static unsigned long setup_pagetables(struct lguest *lg,
 			return -EFAULT;
 	}
 
+#ifdef CONFIG_X86_PAE
 	/*
-	 * The top level points to the linear page table pages above.
-	 * We setup the identity and linear mappings here.
+	 * Make the Guest PMD entries point to the corresponding place in the
+	 * linear mapping (up to one page worth of PMD).
 	 */
-#ifdef CONFIG_X86_PAE
 	for (i = j = 0; i < mapped_pages && j < PTRS_PER_PMD;
 	     i += PTRS_PER_PTE, j++) {
+		/* FIXME: native_set_pmd is overkill here. */
 		native_set_pmd(&pmd, __pmd(((unsigned long)(linear + i)
 		- mem_base) | _PAGE_PRESENT | _PAGE_RW | _PAGE_USER));
 
@@ -971,18 +991,36 @@ static unsigned long setup_pagetables(struct lguest *lg,
 			return -EFAULT;
 	}
 
+	/* One PGD entry, pointing to that PMD page. */
 	set_pgd(&pgd, __pgd(((u32)pmds - mem_base) | _PAGE_PRESENT));
+	/* Copy it in as the first PGD entry (ie. addresses 0-1G). */
 	if (copy_to_user(&pgdir[0], &pgd, sizeof(pgd)) != 0)
 		return -EFAULT;
+	/*
+	 * And the third PGD entry (ie. addresses 3G-4G).
+	 *
+	 * FIXME: This assumes that PAGE_OFFSET for the Guest is 0xC0000000.
+	 */
 	if (copy_to_user(&pgdir[3], &pgd, sizeof(pgd)) != 0)
 		return -EFAULT;
 #else
+	/*
+	 * The top level points to the linear page table pages above.
+	 * We setup the identity and linear mappings here.
+	 */
 	phys_linear = (unsigned long)linear - mem_base;
 	for (i = 0; i < mapped_pages; i += PTRS_PER_PTE) {
 		pgd_t pgd;
+		/*
+		 * Create a PGD entry which points to the right part of the
+		 * linear PTE pages.
+		 */
 		pgd = __pgd((phys_linear + i * sizeof(pte_t)) |
 			    (_PAGE_PRESENT | _PAGE_RW | _PAGE_USER));
 
+		/*
+		 * Copy it into the PGD page at 0 and PAGE_OFFSET.
+		 */
 		if (copy_to_user(&pgdir[i / PTRS_PER_PTE], &pgd, sizeof(pgd))
 		    || copy_to_user(&pgdir[pgd_index(PAGE_OFFSET)
 					   + i / PTRS_PER_PTE],
@@ -992,8 +1030,8 @@ static unsigned long setup_pagetables(struct lguest *lg,
 #endif
 
 	/*
-	 * We return the top level (guest-physical) address: remember where
-	 * this is.
+	 * We return the top level (guest-physical) address: we remember where
+	 * this is to write it into lguest_data when the Guest initializes.
 	 */
 	return (unsigned long)pgdir - mem_base;
 }
@@ -1031,7 +1069,9 @@ int init_guest_pagetable(struct lguest *lg)
 	lg->pgdirs[0].pgdir = (pgd_t *)get_zeroed_page(GFP_KERNEL);
 	if (!lg->pgdirs[0].pgdir)
 		return -ENOMEM;
+
 #ifdef CONFIG_X86_PAE
+	/* For PAE, we also create the initial mid-level. */
 	pgd = lg->pgdirs[0].pgdir;
 	pmd_table = (pmd_t *) get_zeroed_page(GFP_KERNEL);
 	if (!pmd_table)
@@ -1040,11 +1080,13 @@ int init_guest_pagetable(struct lguest *lg)
 	set_pgd(pgd + SWITCHER_PGD_INDEX,
 		__pgd(__pa(pmd_table) | _PAGE_PRESENT));
 #endif
+
+	/* This is the current page table. */
 	lg->cpus[0].cpu_pgd = 0;
 	return 0;
 }
 
-/* When the Guest calls LHCALL_LGUEST_INIT we do more setup. */
+/*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. */
@@ -1105,12 +1147,16 @@ void map_switcher_in_guest(struct lg_cpu *cpu, struct lguest_pages *pages)
 	pmd_t switcher_pmd;
 	pmd_t *pmd_table;
 
+	/* FIXME: native_set_pmd is overkill here. */
 	native_set_pmd(&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. */
 	native_set_pmd(&pmd_table[SWITCHER_PMD_INDEX], switcher_pmd);
 #else
 	pgd_t switcher_pgd;
diff --git a/drivers/lguest/x86/core.c b/drivers/lguest/x86/core.c
index 96f7d88ec7f8..6ae388849a3b 100644
--- a/drivers/lguest/x86/core.c
+++ b/drivers/lguest/x86/core.c
@@ -187,7 +187,7 @@ static void run_guest_once(struct lg_cpu *cpu, struct lguest_pages *pages)
  * also simplify copy_in_guest_info().  Note that we'd still need to restore
  * things when we exit to Launcher userspace, but that's fairly easy.
  *
- * We could also try using this hooks for PGE, but that might be too expensive.
+ * We could also try using these hooks for PGE, but that might be too expensive.
  *
  * The hooks were designed for KVM, but we can also put them to good use.
 :*/
diff --git a/drivers/lguest/x86/switcher_32.S b/drivers/lguest/x86/switcher_32.S
index 6dec09793836..40634b0db9f7 100644
--- a/drivers/lguest/x86/switcher_32.S
+++ b/drivers/lguest/x86/switcher_32.S
@@ -1,7 +1,7 @@
 /*P:900
- * This is the Switcher: code which sits at 0xFFC00000 astride both the
- * Host and Guest to do the low-level Guest<->Host switch.  It is as simple as
- * it can be made, but it's naturally very specific to x86.
+ * This is the Switcher: code which sits at 0xFFC00000 (or 0xFFE00000) astride
+ * both the Host and Guest to do the low-level Guest<->Host switch.  It is as
+ * simple as it can be made, but it's naturally very specific to x86.
  *
  * You have now completed Preparation.  If this has whet your appetite; if you
  * are feeling invigorated and refreshed then the next, more challenging stage