lguest: comment documentation update.

Took some cycles to re-read the Lguest Journey end-to-end, fix some
rot and tighten some phrases.

Only comments change.  No new jokes, but a couple of recycled old jokes.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Documentation/lguest/lguest.c

 /*P:100 This is the Launcher code, a simple program which lays out the
- * "physical" memory for the new Guest by mapping the kernel image and the
- * virtual devices, then reads repeatedly from /dev/lguest to run the Guest.
-:*/
+ * "physical" memory for the new Guest by mapping the kernel image and
+ * the virtual devices, then opens /dev/lguest to tell the kernel
+ * about the Guest and control it. :*/
 #define _LARGEFILE64_SOURCE
 #define _GNU_SOURCE
 #include <stdio.h>
@@ -43,7 +43,7 @@
 #include "linux/virtio_console.h"
 #include "linux/virtio_ring.h"
 #include "asm-x86/bootparam.h"
-/*L:110 We can ignore the 38 include files we need for this program, but I do
+/*L:110 We can ignore the 39 include files we need for this program, but I do
  * want to draw attention to the use of kernel-style types.
  *
  * As Linus said, "C is a Spartan language, and so should your naming be."  I
@@ -320,7 +320,7 @@ static unsigned long map_elf(int elf_fd, const Elf32_Ehdr *ehdr)
 		err(1, "Reading program headers");
 
 	/* Try all the headers: there are usually only three.  A read-only one,
-	 * a read-write one, and a "note" section which isn't loadable. */
+	 * a read-write one, and a "note" section which we don't load. */
 	for (i = 0; i < ehdr->e_phnum; i++) {
 		/* If this isn't a loadable segment, we ignore it */
 		if (phdr[i].p_type != PT_LOAD)
@@ -387,7 +387,7 @@ static unsigned long load_kernel(int fd)
 	if (memcmp(hdr.e_ident, ELFMAG, SELFMAG) == 0)
 		return map_elf(fd, &hdr);
 
-	/* Otherwise we assume it's a bzImage, and try to unpack it */
+	/* Otherwise we assume it's a bzImage, and try to load it. */
 	return load_bzimage(fd);
 }
 
@@ -433,12 +433,12 @@ static unsigned long load_initrd(const char *name, unsigned long mem)
 	return len;
 }
 
-/* Once we know how much memory we have, we can construct simple linear page
+/* Once we know how much memory we have we can construct simple linear page
  * tables which set virtual == physical which will get the Guest far enough
  * into the boot to create its own.
  *
  * We lay them out of the way, just below the initrd (which is why we need to
- * know its size). */
+ * know its size here). */
 static unsigned long setup_pagetables(unsigned long mem,
 				      unsigned long initrd_size)
 {
@@ -850,7 +850,8 @@ static void handle_console_output(int fd, struct virtqueue *vq)
  *
  * Handling output for network is also simple: we get all the output buffers
  * and write them (ignoring the first element) to this device's file descriptor
- * (stdout). */
+ * (/dev/net/tun).
+ */
 static void handle_net_output(int fd, struct virtqueue *vq)
 {
 	unsigned int head, out, in;
@@ -924,7 +925,7 @@ static void enable_fd(int fd, struct virtqueue *vq)
 	write(waker_fd, &vq->dev->fd, sizeof(vq->dev->fd));
 }
 
-/* Resetting a device is fairly easy. */
+/* When the Guest asks us to reset a device, it's is fairly easy. */
 static void reset_device(struct device *dev)
 {
 	struct virtqueue *vq;
@@ -1003,8 +1004,8 @@ static void handle_input(int fd)
 		if (select(devices.max_infd+1, &fds, NULL, NULL, &poll) == 0)
 			break;
 
-		/* Otherwise, call the device(s) which have readable
-		 * file descriptors and a method of handling them.  */
+		/* Otherwise, call the device(s) which have readable file
+		 * descriptors and a method of handling them.  */
 		for (i = devices.dev; i; i = i->next) {
 			if (i->handle_input && FD_ISSET(i->fd, &fds)) {
 				int dev_fd;
@@ -1015,8 +1016,7 @@ static void handle_input(int fd)
 				 * should no longer service it.  Networking and
 				 * console do this when there's no input
 				 * buffers to deliver into.  Console also uses
-				 * it when it discovers that stdin is
-				 * closed. */
+				 * it when it discovers that stdin is closed. */
 				FD_CLR(i->fd, &devices.infds);
 				/* Tell waker to ignore it too, by sending a
 				 * negative fd number (-1, since 0 is a valid
@@ -1033,7 +1033,8 @@ static void handle_input(int fd)
  *
  * All devices need a descriptor so the Guest knows it exists, and a "struct
  * device" so the Launcher can keep track of it.  We have common helper
- * routines to allocate and manage them. */
+ * routines to allocate and manage them.
+ */
 
 /* The layout of the device page is a "struct lguest_device_desc" followed by a
  * number of virtqueue descriptors, then two sets of feature bits, then an
@@ -1078,7 +1079,7 @@ static void add_virtqueue(struct device *dev, unsigned int num_descs,
 	struct virtqueue **i, *vq = malloc(sizeof(*vq));
 	void *p;
 
-	/* First we need some pages for this virtqueue. */
+	/* First we need some memory for this virtqueue. */
 	pages = (vring_size(num_descs, getpagesize()) + getpagesize() - 1)
 		/ getpagesize();
 	p = get_pages(pages);
@@ -1122,7 +1123,7 @@ static void add_virtqueue(struct device *dev, unsigned int num_descs,
 }
 
 /* The first half of the feature bitmask is for us to advertise features.  The
- * second half if for the Guest to accept features. */
+ * second half is for the Guest to accept features. */
 static void add_feature(struct device *dev, unsigned bit)
 {
 	u8 *features = get_feature_bits(dev);
@@ -1151,7 +1152,9 @@ static void set_config(struct device *dev, unsigned len, const void *conf)
 }
 
 /* This routine does all the creation and setup of a new device, including
- * calling new_dev_desc() to allocate the descriptor and device memory. */
+ * calling new_dev_desc() to allocate the descriptor and device memory.
+ *
+ * See what I mean about userspace being boring? */
 static struct device *new_device(const char *name, u16 type, int fd,
 				 bool (*handle_input)(int, struct device *))
 {
@@ -1492,7 +1495,10 @@ static int io_thread(void *_dev)
 	while (read(vblk->workpipe[0], &c, 1) == 1) {
 		/* We acknowledge each request immediately to reduce latency,
 		 * rather than waiting until we've done them all.  I haven't
-		 * measured to see if it makes any difference. */
+		 * measured to see if it makes any difference.
+		 *
+		 * That would be an interesting test, wouldn't it?  You could
+		 * also try having more than one I/O thread. */
 		while (service_io(dev))
 			write(vblk->done_fd, &c, 1);
 	}
@@ -1500,7 +1506,7 @@ static int io_thread(void *_dev)
 }
 
 /* Now we've seen the I/O thread, we return to the Launcher to see what happens
- * when the thread tells us it's completed some I/O. */
+ * when that thread tells us it's completed some I/O. */
 static bool handle_io_finish(int fd, struct device *dev)
 {
 	char c;
@@ -1572,7 +1578,8 @@ static void setup_block_file(const char *filename)
 	 * more work. */
 	pipe(vblk->workpipe);
 
-	/* Create stack for thread and run it */
+	/* Create stack for thread and run it.  Since stack grows upwards, we
+	 * point the stack pointer to the end of this region. */
 	stack = malloc(32768);
 	/* SIGCHLD - We dont "wait" for our cloned thread, so prevent it from
 	 * becoming a zombie. */
@@ -1586,14 +1593,14 @@ static void setup_block_file(const char *filename)
 	verbose("device %u: virtblock %llu sectors\n",
 		devices.device_num, le64_to_cpu(conf.capacity));
 }
-/* That's the end of device setup. :*/
+/* That's the end of device setup. */
 
-/* Reboot */
+/*L:230 Reboot is pretty easy: clean up and exec() the Launcher afresh. */
 static void __attribute__((noreturn)) restart_guest(void)
 {
 	unsigned int i;
 
-	/* Closing pipes causes the waker thread and io_threads to die, and
+	/* Closing pipes causes the Waker thread and io_threads to die, and
 	 * closing /dev/lguest cleans up the Guest.  Since we don't track all
 	 * open fds, we simply close everything beyond stderr. */
 	for (i = 3; i < FD_SETSIZE; i++)
@@ -1602,7 +1609,7 @@ static void __attribute__((noreturn)) restart_guest(void)
 	err(1, "Could not exec %s", main_args[0]);
 }
 
-/*L:220 Finally we reach the core of the Launcher, which runs the Guest, serves
+/*L:220 Finally we reach the core of the Launcher which runs the Guest, serves
  * its input and output, and finally, lays it to rest. */
 static void __attribute__((noreturn)) run_guest(int lguest_fd)
 {
@@ -1643,7 +1650,7 @@ static void __attribute__((noreturn)) run_guest(int lguest_fd)
 			err(1, "Resetting break");
 	}
 }
-/*
+/*L:240
  * This is the end of the Launcher.  The good news: we are over halfway
  * through!  The bad news: the most fiendish part of the code still lies ahead
  * of us.
@@ -1690,8 +1697,8 @@ int main(int argc, char *argv[])
 	 * device receive input from a file descriptor, we keep an fdset
 	 * (infds) and the maximum fd number (max_infd) with the head of the
 	 * list.  We also keep a pointer to the last device.  Finally, we keep
-	 * the next interrupt number to hand out (1: remember that 0 is used by
-	 * the timer). */
+	 * the next interrupt number to use for devices (1: remember that 0 is
+	 * used by the timer). */
 	FD_ZERO(&devices.infds);
 	devices.max_infd = -1;
 	devices.lastdev = NULL;
@@ -1792,8 +1799,8 @@ int main(int argc, char *argv[])
 	lguest_fd = tell_kernel(pgdir, start);
 
 	/* We fork off a child process, which wakes the Launcher whenever one
-	 * of the input file descriptors needs attention.  Otherwise we would
-	 * run the Guest until it tries to output something. */
+	 * of the input file descriptors needs attention.  We call this the
+	 * Waker, and we'll cover it in a moment. */
 	waker_fd = setup_waker(lguest_fd);
 
 	/* Finally, run the Guest.  This doesn't return. */

arch/x86/lguest/i386_head.S

@@ -5,13 +5,20 @@
 #include <asm/thread_info.h>
 #include <asm/processor-flags.h>
 
-/*G:020 This is where we begin: head.S notes that the boot header's platform
- * type field is "1" (lguest), so calls us here.
+/*G:020 Our story starts with the kernel booting into startup_32 in
+ * arch/x86/kernel/head_32.S.  It expects a boot header, which is created by
+ * the bootloader (the Launcher in our case).
+ *
+ * The startup_32 function does very little: it clears the uninitialized global
+ * C variables which we expect to be zero (ie. BSS) and then copies the boot
+ * header and kernel command line somewhere safe.  Finally it checks the
+ * 'hardware_subarch' field.  This was introduced in 2.6.24 for lguest and Xen:
+ * if it's set to '1' (lguest's assigned number), then it calls us here.
  *
  * WARNING: be very careful here!  We're running at addresses equal to physical
  * addesses (around 0), not above PAGE_OFFSET as most code expectes
  * (eg. 0xC0000000).  Jumps are relative, so they're OK, but we can't touch any
- * data.
+ * data without remembering to subtract __PAGE_OFFSET!
  *
  * The .section line puts this code in .init.text so it will be discarded after
  * boot. */
@@ -24,7 +31,7 @@ ENTRY(lguest_entry)
 	int $LGUEST_TRAP_ENTRY
 
 	/* The Host put the toplevel pagetable in lguest_data.pgdir.  The movsl
-	 * instruction uses %esi implicitly as the source for the copy we'
+	 * instruction uses %esi implicitly as the source for the copy we're
 	 * about to do. */
 	movl lguest_data - __PAGE_OFFSET + LGUEST_DATA_pgdir, %esi
 

drivers/lguest/core.c

 /*P:400 This contains run_guest() which actually calls into the Host<->Guest
  * Switcher and analyzes the return, such as determining if the Guest wants the
- * Host to do something.  This file also contains useful helper routines, and a
- * couple of non-obvious setup and teardown pieces which were implemented after
- * days of debugging pain. :*/
+ * Host to do something.  This file also contains useful helper routines. :*/
 #include <linux/module.h>
 #include <linux/stringify.h>
 #include <linux/stddef.h>
@@ -49,8 +47,8 @@ static __init int map_switcher(void)
 	 * easy.
 	 */
 
-	/* We allocate an array of "struct page"s.  map_vm_area() wants the
-	 * pages in this form, rather than just an array of pointers. */
+	/* We allocate an array of struct page pointers.  map_vm_area() wants
+	 * this, rather than just an array of pages. */
 	switcher_page = kmalloc(sizeof(switcher_page[0])*TOTAL_SWITCHER_PAGES,
 				GFP_KERNEL);
 	if (!switcher_page) {
@@ -172,7 +170,7 @@ void __lgread(struct lg_cpu *cpu, void *b, unsigned long addr, unsigned bytes)
 	}
 }
 
-/* This is the write (copy into guest) version. */
+/* This is the write (copy into Guest) version. */
 void __lgwrite(struct lg_cpu *cpu, unsigned long addr, const void *b,
 	       unsigned bytes)
 {
@@ -209,9 +207,9 @@ int run_guest(struct lg_cpu *cpu, unsigned long __user *user)
 		if (cpu->break_out)
 			return -EAGAIN;
 
-		/* Check if there are any interrupts which can be delivered
-		 * now: if so, this sets up the hander to be executed when we
-		 * next run the Guest. */
+		/* Check if there are any interrupts which can be delivered now:
+		 * if so, this sets up the hander to be executed when we next
+		 * run the Guest. */
 		maybe_do_interrupt(cpu);
 
 		/* All long-lived kernel loops need to check with this horrible
@@ -246,8 +244,10 @@ int run_guest(struct lg_cpu *cpu, unsigned long __user *user)
 		lguest_arch_handle_trap(cpu);
 	}
 
+	/* Special case: Guest is 'dead' but wants a reboot. */
 	if (cpu->lg->dead == ERR_PTR(-ERESTART))
 		return -ERESTART;
+
 	/* The Guest is dead => "No such file or directory" */
 	return -ENOENT;
 }

drivers/lguest/hypercalls.c

@@ -29,7 +29,7 @@
 #include "lg.h"
 
 /*H:120 This is the core hypercall routine: where the Guest gets what it wants.
- * Or gets killed.  Or, in the case of LHCALL_CRASH, both. */
+ * Or gets killed.  Or, in the case of LHCALL_SHUTDOWN, both. */
 static void do_hcall(struct lg_cpu *cpu, struct hcall_args *args)
 {
 	switch (args->arg0) {
@@ -190,6 +190,13 @@ static void initialize(struct lg_cpu *cpu)
 	 * pagetable. */
 	guest_pagetable_clear_all(cpu);
 }
+/*:*/
+
+/*M:013 If a Guest reads from a page (so creates a mapping) that it has never
+ * written to, and then the Launcher writes to it (ie. the output of a virtual
+ * 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. :*/
 
 /*H:100
  * Hypercalls
@@ -227,7 +234,7 @@ void do_hypercalls(struct lg_cpu *cpu)
 		 * However, if we are signalled or the Guest sends I/O to the
 		 * Launcher, the run_guest() loop will exit without running the
 		 * Guest.  When it comes back it would try to re-run the
-		 * hypercall. */
+		 * hypercall.  Finding that bug sucked. */
 		cpu->hcall = NULL;
 	}
 }

drivers/lguest/interrupts_and_traps.c

@@ -144,7 +144,6 @@ void maybe_do_interrupt(struct lg_cpu *cpu)
 	if (copy_from_user(&blk, cpu->lg->lguest_data->blocked_interrupts,
 			   sizeof(blk)))
 		return;
-
 	bitmap_andnot(blk, cpu->irqs_pending, blk, LGUEST_IRQS);
 
 	/* Find the first interrupt. */
@@ -237,9 +236,9 @@ void free_interrupts(void)
 		clear_bit(syscall_vector, used_vectors);
 }
 
-/*H:220 Now we've got the routines to deliver interrupts, delivering traps
- * like page fault is easy.  The only trick is that Intel decided that some
- * traps should have error codes: */
+/*H:220 Now we've got the routines to deliver interrupts, delivering traps like
+ * page fault is easy.  The only trick is that Intel decided that some traps
+ * should have error codes: */
 static int has_err(unsigned int trap)
 {
 	return (trap == 8 || (trap >= 10 && trap <= 14) || trap == 17);

drivers/lguest/lguest_device.c

 /*P:050 Lguest guests use a very simple method to describe devices.  It's a
- * series of device descriptors contained just above the top of normal
+ * series of device descriptors contained just above the top of normal Guest
  * memory.
  *
  * We use the standard "virtio" device infrastructure, which provides us with a
  * console, a network and a block driver.  Each one expects some configuration
- * information and a "virtqueue" mechanism to send and receive data. :*/
+ * information and a "virtqueue" or two to send and receive data. :*/
 #include <linux/init.h>
 #include <linux/bootmem.h>
 #include <linux/lguest_launcher.h>
@@ -53,7 +53,7 @@ struct lguest_device {
  * Device configurations
  *
  * The configuration information for a device consists of one or more
- * virtqueues, a feature bitmaks, and some configuration bytes.  The
+ * virtqueues, a feature bitmap, and some configuration bytes.  The
  * configuration bytes don't really matter to us: the Launcher sets them up, and
  * the driver will look at them during setup.
  *
@@ -179,7 +179,7 @@ struct lguest_vq_info
 };
 
 /* When the virtio_ring code wants to prod the Host, it calls us here and we
- * make a hypercall.  We hand the page number of the virtqueue so the Host
+ * make a hypercall.  We hand the physical address of the virtqueue so the Host
  * knows which virtqueue we're talking about. */
 static void lg_notify(struct virtqueue *vq)
 {
@@ -199,7 +199,8 @@ static void lg_notify(struct virtqueue *vq)
  * 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 devices with a "find virtqueue and set it up" function. */
+ * 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,
 				    void (*callback)(struct virtqueue *vq))

drivers/lguest/lguest_user.c

@@ -73,7 +73,7 @@ static ssize_t read(struct file *file, char __user *user, size_t size,loff_t*o)
 	if (current != cpu->tsk)
 		return -EPERM;
 
-	/* If the guest is already dead, we indicate why */
+	/* If the Guest is already dead, we indicate why */
 	if (lg->dead) {
 		size_t len;
 
@@ -88,7 +88,7 @@ static ssize_t read(struct file *file, char __user *user, size_t size,loff_t*o)
 		return len;
 	}
 
-	/* If we returned from read() last time because the Guest notified,
+	/* If we returned from read() last time because the Guest sent I/O,
 	 * clear the flag. */
 	if (cpu->pending_notify)
 		cpu->pending_notify = 0;
@@ -97,14 +97,20 @@ static ssize_t read(struct file *file, char __user *user, size_t size,loff_t*o)
 	return run_guest(cpu, (unsigned long __user *)user);
 }
 
+/*L:025 This actually initializes a CPU.  For the moment, a Guest is only
+ * uniprocessor, so "id" is always 0. */
 static int lg_cpu_start(struct lg_cpu *cpu, unsigned id, unsigned long start_ip)
 {
+	/* We have a limited number the number of CPUs in the lguest struct. */
 	if (id >= NR_CPUS)
 		return -EINVAL;
 
+	/* Set up this CPU's id, and pointer back to the lguest struct. */
 	cpu->id = id;
 	cpu->lg = container_of((cpu - id), struct lguest, cpus[0]);
 	cpu->lg->nr_cpus++;
+
+	/* Each CPU has a timer it can set. */
 	init_clockdev(cpu);
 
 	/* We need a complete page for the Guest registers: they are accessible
@@ -120,11 +126,11 @@ static int lg_cpu_start(struct lg_cpu *cpu, unsigned id, unsigned long start_ip)
 	 * address. */
 	lguest_arch_setup_regs(cpu, start_ip);
 
-	/* Initialize the queue for the waker to wait on */
+	/* Initialize the queue for the Waker to wait on */
 	init_waitqueue_head(&cpu->break_wq);
 
 	/* We keep a pointer to the Launcher task (ie. current task) for when
-	 * other Guests want to wake this one (inter-Guest I/O). */
+	 * other Guests want to wake this one (eg. console input). */
 	cpu->tsk = current;
 
 	/* We need to keep a pointer to the Launcher's memory map, because if
@@ -136,6 +142,7 @@ static int lg_cpu_start(struct lg_cpu *cpu, unsigned id, unsigned long start_ip)
 	 * when the same Guest runs on the same CPU twice. */
 	cpu->last_pages = NULL;
 
+	/* No error == success. */
 	return 0;
 }
 
@@ -185,14 +192,13 @@ static int initialize(struct file *file, const unsigned long __user *input)
 	lg->mem_base = (void __user *)(long)args[0];
 	lg->pfn_limit = args[1];
 
-	/* This is the first cpu */
+	/* This is the first cpu (cpu 0) and it will start booting at args[3] */
 	err = lg_cpu_start(&lg->cpus[0], 0, args[3]);
 	if (err)
 		goto release_guest;
 
 	/* Initialize the Guest's shadow page tables, using the toplevel
-	 * address the Launcher gave us.  This allocates memory, so can
-	 * fail. */
+	 * address the Launcher gave us.  This allocates memory, so can fail. */
 	err = init_guest_pagetable(lg, args[2]);
 	if (err)
 		goto free_regs;
@@ -218,11 +224,16 @@ unlock:
 /*L:010 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.
+ *
+ * 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
+ * support uniprocessor Guests, but you can see the beginnings of SMP support
+ * here. */
 static ssize_t write(struct file *file, const char __user *in,
 		     size_t size, loff_t *off)
 {
-	/* Once the guest is initialized, we hold the "struct lguest" in the
+	/* Once the Guest is initialized, we hold the "struct lguest" in the
 	 * file private data. */
 	struct lguest *lg = file->private_data;
 	const unsigned long __user *input = (const unsigned long __user *)in;
@@ -230,6 +241,7 @@ static ssize_t write(struct file *file, const char __user *in,
 	struct lg_cpu *uninitialized_var(cpu);
 	unsigned int cpu_id = *off;
 
+	/* The first value tells us what this request is. */
 	if (get_user(req, input) != 0)
 		return -EFAULT;
 	input++;

drivers/lguest/page_tables.c

@@ -2,8 +2,8 @@
  * 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 to point the hardware to the
- * actual Guest pages when running the Guest. :*/
+ * 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 */
@@ -106,6 +106,11 @@ static unsigned long gpte_addr(pgd_t gpgd, unsigned long vaddr)
 	BUG_ON(!(pgd_flags(gpgd) & _PAGE_PRESENT));
 	return gpage + ((vaddr>>PAGE_SHIFT) % PTRS_PER_PTE) * sizeof(pte_t);
 }
+/*:*/
+
+/*M:014 get_pfn is slow; it takes the mmap sem and calls get_user_pages.  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
@@ -113,8 +118,8 @@ static unsigned long gpte_addr(pgd_t gpgd, unsigned long vaddr)
  * 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. */
+ * 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;
@@ -532,13 +537,13 @@ static void do_set_pte(struct lg_cpu *cpu, int idx,
  * 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. */
+ * 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. */
+	/* 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++)
@@ -704,12 +709,11 @@ static __init void populate_switcher_pte_page(unsigned int cpu,
 /* 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.
+ * 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. */
 

drivers/lguest/x86/core.c

@@ -17,6 +17,13 @@
  * along with this program; if not, write to the Free Software
  * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  */
+/*P:450 This file contains the x86-specific lguest code.  It used to be all
+ * mixed in with drivers/lguest/core.c but several foolhardy code slashers
+ * wrestled most of the dependencies out to here in preparation for porting
+ * lguest to other architectures (see what I mean by foolhardy?).
+ *
+ * This also contains a couple of non-obvious setup and teardown pieces which
+ * were implemented after days of debugging pain. :*/
 #include <linux/kernel.h>
 #include <linux/start_kernel.h>
 #include <linux/string.h>
@@ -157,6 +164,8 @@ 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.
+ *
  * The hooks were designed for KVM, but we can also put them to good use. :*/
 
 /*H:040 This is the i386-specific code to setup and run the Guest.  Interrupts
@@ -182,7 +191,7 @@ void lguest_arch_run_guest(struct lg_cpu *cpu)
 	 * was doing. */
 	run_guest_once(cpu, lguest_pages(raw_smp_processor_id()));
 
-	/* Note that the "regs" pointer contains two extra entries which are
+	/* Note that the "regs" structure contains two extra entries which are
 	 * not really registers: a trap number which says what interrupt or
 	 * trap made the switcher code come back, and an error code which some
 	 * traps set.  */
@@ -293,11 +302,10 @@ void lguest_arch_handle_trap(struct lg_cpu *cpu)
 		break;
 	case 14: /* We've intercepted a Page Fault. */
 		/* The Guest accessed a virtual address that wasn't mapped.
-		 * This happens a lot: we don't actually set up most of the
-		 * page tables for the Guest at all when we start: as it runs
-		 * it asks for more and more, and we set them up as
-		 * required. In this case, we don't even tell the Guest that
-		 * the fault happened.
+		 * This happens a lot: we don't actually set up most of the page
+		 * tables for the Guest at all when we start: as it runs it asks
+		 * for more and more, and we set them up as required. In this
+		 * case, we don't even tell the Guest that the fault happened.
 		 *
 		 * The errcode tells whether this was a read or a write, and
 		 * whether kernel or userspace code. */
@@ -342,7 +350,7 @@ void lguest_arch_handle_trap(struct lg_cpu *cpu)
 	if (!deliver_trap(cpu, cpu->regs->trapnum))
 		/* If the Guest doesn't have a handler (either it hasn't
 		 * registered any yet, or it's one of the faults we don't let
-		 * it handle), it dies with a cryptic error message. */
+		 * it handle), it dies with this cryptic error message. */
 		kill_guest(cpu, "unhandled trap %li at %#lx (%#lx)",
 			   cpu->regs->trapnum, cpu->regs->eip,
 			   cpu->regs->trapnum == 14 ? cpu->arch.last_pagefault
@@ -375,8 +383,8 @@ void __init lguest_arch_host_init(void)
 	 * The only exception is the interrupt handlers in switcher.S: their
 	 * addresses are placed in a table (default_idt_entries), so we need to
 	 * update the table with the new addresses.  switcher_offset() is a
-	 * convenience function which returns the distance between the builtin
-	 * switcher code and the high-mapped copy we just made. */
+	 * convenience function which returns the distance between the
+	 * compiled-in switcher code and the high-mapped copy we just made. */
 	for (i = 0; i < IDT_ENTRIES; i++)
 		default_idt_entries[i] += switcher_offset();
 
@@ -416,7 +424,7 @@ void __init lguest_arch_host_init(void)
 		state->guest_gdt_desc.address = (long)&state->guest_gdt;
 
 		/* We know where we want the stack to be when the Guest enters
-		 * the switcher: in pages->regs.  The stack grows upwards, so
+		 * the Switcher: in pages->regs.  The stack grows upwards, so
 		 * we start it at the end of that structure. */
 		state->guest_tss.sp0 = (long)(&pages->regs + 1);
 		/* And this is the GDT entry to use for the stack: we keep a
@@ -513,8 +521,8 @@ int lguest_arch_init_hypercalls(struct lg_cpu *cpu)
 {
 	u32 tsc_speed;
 
-	/* The pointer to the Guest's "struct lguest_data" is the only
-	 * argument.  We check that address now. */
+	/* The pointer to the Guest's "struct lguest_data" is the only argument.
+	 * We check that address now. */
 	if (!lguest_address_ok(cpu->lg, cpu->hcall->arg1,
 			       sizeof(*cpu->lg->lguest_data)))
 		return -EFAULT;
@@ -546,6 +554,7 @@ int lguest_arch_init_hypercalls(struct lg_cpu *cpu)
 
 	return 0;
 }
+/*:*/
 
 /*L:030 lguest_arch_setup_regs()
  *

drivers/lguest/x86/switcher_32.S

-/*P:900 This is the Switcher: code which sits at 0xFFC00000 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.
+/*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.
  *
  * You have now completed Preparation.  If this has whet your appetite; if you
  * are feeling invigorated and refreshed then the next, more challenging stage
@@ -189,7 +189,7 @@ ENTRY(switch_to_guest)
 	// Interrupts are turned back on: we are Guest.
 	iret
 
-// We treat two paths to switch back to the Host
+// We tread two paths to switch back to the Host
 // Yet both must save Guest state and restore Host
 // So we put the routine in a macro.
 #define SWITCH_TO_HOST							\

include/asm-x86/lguest_hcall.h

@@ -27,7 +27,7 @@
 #ifndef __ASSEMBLY__
 #include <asm/hw_irq.h>
 
-/*G:031 First, how does our Guest contact the Host to ask for privileged
+/*G:031 But first, how does our Guest contact the Host to ask for privileged
  * operations?  There are two ways: the direct way is to make a "hypercall",
  * to make requests of the Host Itself.
  *

include/linux/lguest_launcher.h

@@ -16,6 +16,10 @@
  * a new device, we simply need to write a new virtio driver and create support
  * for it in the Launcher: this code won't need to change.
  *
+ * Virtio devices are also used by kvm, so we can simply reuse their optimized
+ * device drivers.  And one day when everyone uses virtio, my plan will be
+ * complete.  Bwahahahah!
+ *
  * Devices are described by a simplified ID, a status byte, and some "config"
  * bytes which describe this device's configuration.  This is placed by the
  * Launcher just above the top of physical memory:
@@ -26,7 +30,7 @@ struct lguest_device_desc {
 	/* The number of virtqueues (first in config array) */
 	__u8 num_vq;
 	/* The number of bytes of feature bits.  Multiply by 2: one for host
-	 * features and one for guest acknowledgements. */
+	 * features and one for Guest acknowledgements. */
 	__u8 feature_len;
 	/* The number of bytes of the config array after virtqueues. */
 	__u8 config_len;