1 files changed, 483 insertions, 238 deletions

diff --git a/Documentation/lguest/lguest.c b/Documentation/lguest/lguest.c
index 9ebcd6ef361b..950cde6d6e58 100644
--- a/Documentation/lguest/lguest.c
+++ b/Documentation/lguest/lguest.c
@@ -1,7 +1,9 @@
-/*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 opens /dev/lguest to tell the kernel
- * about the Guest and control it. :*/
+/*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 opens /dev/lguest to tell the kernel about the Guest and
+ * control it.
+:*/
 #define _LARGEFILE64_SOURCE
 #define _GNU_SOURCE
 #include <stdio.h>
@@ -46,13 +48,15 @@
 #include "linux/virtio_rng.h"
 #include "linux/virtio_ring.h"
 #include "asm/bootparam.h"
-/*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.
+/*L:110
+ * We can ignore the 42 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
  * like these abbreviations, so we define them here.  Note that u64 is always
  * unsigned long long, which works on all Linux systems: this means that we can
- * use %llu in printf for any u64. */
+ * use %llu in printf for any u64.
+ */
 typedef unsigned long long u64;
 typedef uint32_t u32;
 typedef uint16_t u16;
@@ -69,8 +73,10 @@ typedef uint8_t u8;
 /* This will occupy 3 pages: it must be a power of 2. */
 #define VIRTQUEUE_NUM 256
 
-/*L:120 verbose is both a global flag and a macro.  The C preprocessor allows
- * this, and although I wouldn't recommend it, it works quite nicely here. */
+/*L:120
+ * verbose is both a global flag and a macro.  The C preprocessor allows
+ * this, and although I wouldn't recommend it, it works quite nicely here.
+ */
 static bool verbose;
 #define verbose(args...) \
         do { if (verbose) printf(args); } while(0)
@@ -87,8 +93,7 @@ static int lguest_fd;
 static unsigned int __thread cpu_id;
 
 /* This is our list of devices. */
-struct device_list
-{
+struct device_list {
         /* Counter to assign interrupt numbers. */
         unsigned int next_irq;
 
@@ -100,8 +105,7 @@ struct device_list
 
         /* A single linked list of devices. */
         struct device *dev;
-        /* And a pointer to the last device for easy append and also for
-         * configuration appending. */
+        /* And a pointer to the last device for easy append. */
         struct device *lastdev;
 };
 
@@ -109,8 +113,7 @@ struct device_list
 static struct device_list devices;
 
 /* The device structure describes a single device. */
-struct device
-{
+struct device {
         /* The linked-list pointer. */
         struct device *next;
 
@@ -135,8 +138,7 @@ struct device
 };
 
 /* The virtqueue structure describes a queue attached to a device. */
-struct virtqueue
-{
+struct virtqueue {
         struct virtqueue *next;
 
         /* Which device owns me. */
@@ -168,20 +170,24 @@ static char **main_args;
 /* The original tty settings to restore on exit. */
 static struct termios orig_term;
 
-/* We have to be careful with barriers: our devices are all run in separate
+/*
+ * We have to be careful with barriers: our devices are all run in separate
  * threads and so we need to make sure that changes visible to the Guest happen
- * in precise order. */
+ * in precise order.
+ */
 #define wmb() __asm__ __volatile__("" : : : "memory")
 #define mb() __asm__ __volatile__("" : : : "memory")
 
-/* Convert an iovec element to the given type.
+/*
+ * Convert an iovec element to the given type.
  *
  * This is a fairly ugly trick: we need to know the size of the type and
  * alignment requirement to check the pointer is kosher.  It's also nice to
  * have the name of the type in case we report failure.
  *
  * Typing those three things all the time is cumbersome and error prone, so we
- * have a macro which sets them all up and passes to the real function. */
+ * have a macro which sets them all up and passes to the real function.
+ */
 #define convert(iov, type) \
         ((type *)_convert((iov), sizeof(type), __alignof__(type), #type))
 
@@ -198,8 +204,10 @@ static void *_convert(struct iovec *iov, size_t size, size_t align,
 /* Wrapper for the last available index.  Makes it easier to change. */
 #define lg_last_avail(vq)       ((vq)->last_avail_idx)
 
-/* The virtio configuration space is defined to be little-endian.  x86 is
- * little-endian too, but it's nice to be explicit so we have these helpers. */
+/*
+ * The virtio configuration space is defined to be little-endian.  x86 is
+ * little-endian too, but it's nice to be explicit so we have these helpers.
+ */
 #define cpu_to_le16(v16) (v16)
 #define cpu_to_le32(v32) (v32)
 #define cpu_to_le64(v64) (v64)
@@ -241,11 +249,12 @@ static u8 *get_feature_bits(struct device *dev)
                 + dev->num_vq * sizeof(struct lguest_vqconfig);
 }
 
-/*L:100 The Launcher code itself takes us out into userspace, that scary place
- * where pointers run wild and free!  Unfortunately, like most userspace
- * programs, it's quite boring (which is why everyone likes to hack on the
- * kernel!).  Perhaps if you make up an Lguest Drinking Game at this point, it
- * will get you through this section.  Or, maybe not.
+/*L:100
+ * The Launcher code itself takes us out into userspace, that scary place where
+ * pointers run wild and free!  Unfortunately, like most userspace programs,
+ * it's quite boring (which is why everyone likes to hack on the kernel!).
+ * Perhaps if you make up an Lguest Drinking Game at this point, it will get
+ * you through this section.  Or, maybe not.
  *
  * The Launcher sets up a big chunk of memory to be the Guest's "physical"
  * memory and stores it in "guest_base".  In other words, Guest physical ==
@@ -253,7 +262,8 @@ static u8 *get_feature_bits(struct device *dev)
  *
  * This can be tough to get your head around, but usually it just means that we
  * use these trivial conversion functions when the Guest gives us it's
- * "physical" addresses: */
+ * "physical" addresses:
+ */
 static void *from_guest_phys(unsigned long addr)
 {
         return guest_base + addr;
@@ -268,7 +278,8 @@ static unsigned long to_guest_phys(const void *addr)
  * Loading the Kernel.
  *
  * We start with couple of simple helper routines.  open_or_die() avoids
- * error-checking code cluttering the callers: */
+ * error-checking code cluttering the callers:
+ */
 static int open_or_die(const char *name, int flags)
 {
         int fd = open(name, flags);
@@ -283,12 +294,19 @@ static void *map_zeroed_pages(unsigned int num)
         int fd = open_or_die("/dev/zero", O_RDONLY);
         void *addr;
 
-        /* We use a private mapping (ie. if we write to the page, it will be
-         * copied). */
+        /*
+         * We use a private mapping (ie. if we write to the page, it will be
+         * copied).
+         */
         addr = mmap(NULL, getpagesize() * num,
                     PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE, fd, 0);
         if (addr == MAP_FAILED)
                 err(1, "Mmaping %u pages of /dev/zero", num);
+
+        /*
+         * One neat mmap feature is that you can close the fd, and it
+         * stays mapped.
+         */
         close(fd);
 
         return addr;
@@ -305,20 +323,24 @@ static void *get_pages(unsigned int num)
         return addr;
 }
 
-/* This routine is used to load the kernel or initrd.  It tries mmap, but if
+/*
+ * This routine is used to load the kernel or initrd.  It tries mmap, but if
  * that fails (Plan 9's kernel file isn't nicely aligned on page boundaries),
- * it falls back to reading the memory in. */
+ * it falls back to reading the memory in.
+ */
 static void map_at(int fd, void *addr, unsigned long offset, unsigned long len)
 {
         ssize_t r;
 
-        /* We map writable even though for some segments are marked read-only.
+        /*
+         * We map writable even though for some segments are marked read-only.
          * The kernel really wants to be writable: it patches its own
          * instructions.
          *
          * MAP_PRIVATE means that the page won't be copied until a write is
          * done to it.  This allows us to share untouched memory between
-         * Guests. */
+         * Guests.
+         */
         if (mmap(addr, len, PROT_READ|PROT_WRITE|PROT_EXEC,
                  MAP_FIXED|MAP_PRIVATE, fd, offset) != MAP_FAILED)
                 return;
@@ -329,7 +351,8 @@ static void map_at(int fd, void *addr, unsigned long offset, unsigned long len)
                 err(1, "Reading offset %lu len %lu gave %zi", offset, len, r);
 }
 
-/* This routine takes an open vmlinux image, which is in ELF, and maps it into
+/*
+ * This routine takes an open vmlinux image, which is in ELF, and maps it into
  * the Guest memory.  ELF = Embedded Linking Format, which is the format used
  * by all modern binaries on Linux including the kernel.
  *
@@ -337,23 +360,28 @@ static void map_at(int fd, void *addr, unsigned long offset, unsigned long len)
  * address.  We use the physical address; the Guest will map itself to the
  * virtual address.
  *
- * We return the starting address. */
+ * We return the starting address.
+ */
 static unsigned long map_elf(int elf_fd, const Elf32_Ehdr *ehdr)
 {
         Elf32_Phdr phdr[ehdr->e_phnum];
         unsigned int i;
 
-        /* Sanity checks on the main ELF header: an x86 executable with a
-         * reasonable number of correctly-sized program headers. */
+        /*
+         * Sanity checks on the main ELF header: an x86 executable with a
+         * reasonable number of correctly-sized program headers.
+         */
         if (ehdr->e_type != ET_EXEC
             || ehdr->e_machine != EM_386
             || ehdr->e_phentsize != sizeof(Elf32_Phdr)
             || ehdr->e_phnum < 1 || ehdr->e_phnum > 65536U/sizeof(Elf32_Phdr))
                 errx(1, "Malformed elf header");
 
-        /* An ELF executable contains an ELF header and a number of "program"
+        /*
+         * An ELF executable contains an ELF header and a number of "program"
          * headers which indicate which parts ("segments") of the program to
-         * load where. */
+         * load where.
+         */
 
         /* We read in all the program headers at once: */
         if (lseek(elf_fd, ehdr->e_phoff, SEEK_SET) < 0)
@@ -361,8 +389,10 @@ static unsigned long map_elf(int elf_fd, const Elf32_Ehdr *ehdr)
         if (read(elf_fd, phdr, sizeof(phdr)) != sizeof(phdr))
                 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 we don't load. */
+        /*
+         * Try all the headers: there are usually only three.  A read-only one,
+         * 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)
@@ -380,13 +410,15 @@ static unsigned long map_elf(int elf_fd, const Elf32_Ehdr *ehdr)
         return ehdr->e_entry;
 }
 
-/*L:150 A bzImage, unlike an ELF file, is not meant to be loaded.  You're
- * supposed to jump into it and it will unpack itself.  We used to have to
- * perform some hairy magic because the unpacking code scared me.
+/*L:150
+ * A bzImage, unlike an ELF file, is not meant to be loaded.  You're supposed
+ * to jump into it and it will unpack itself.  We used to have to perform some
+ * hairy magic because the unpacking code scared me.
  *
  * Fortunately, Jeremy Fitzhardinge convinced me it wasn't that hard and wrote
  * a small patch to jump over the tricky bits in the Guest, so now we just read
- * the funky header so we know where in the file to load, and away we go! */
+ * the funky header so we know where in the file to load, and away we go!
+ */
 static unsigned long load_bzimage(int fd)
 {
         struct boot_params boot;
@@ -394,8 +426,10 @@ static unsigned long load_bzimage(int fd)
         /* Modern bzImages get loaded at 1M. */
         void *p = from_guest_phys(0x100000);
 
-        /* Go back to the start of the file and read the header.  It should be
-         * a Linux boot header (see Documentation/x86/i386/boot.txt) */
+        /*
+         * Go back to the start of the file and read the header.  It should be
+         * a Linux boot header (see Documentation/x86/i386/boot.txt)
+         */
         lseek(fd, 0, SEEK_SET);
         read(fd, &boot, sizeof(boot));
 
@@ -414,9 +448,11 @@ static unsigned long load_bzimage(int fd)
         return boot.hdr.code32_start;
 }
 
-/*L:140 Loading the kernel is easy when it's a "vmlinux", but most kernels
+/*L:140
+ * Loading the kernel is easy when it's a "vmlinux", but most kernels
  * come wrapped up in the self-decompressing "bzImage" format.  With a little
- * work, we can load those, too. */
+ * work, we can load those, too.
+ */
 static unsigned long load_kernel(int fd)
 {
         Elf32_Ehdr hdr;
@@ -433,24 +469,28 @@ static unsigned long load_kernel(int fd)
         return load_bzimage(fd);
 }
 
-/* This is a trivial little helper to align pages.  Andi Kleen hated it because
+/*
+ * This is a trivial little helper to align pages.  Andi Kleen hated it because
  * it calls getpagesize() twice: "it's dumb code."
  *
  * Kernel guys get really het up about optimization, even when it's not
- * necessary.  I leave this code as a reaction against that. */
+ * necessary.  I leave this code as a reaction against that.
+ */
 static inline unsigned long page_align(unsigned long addr)
 {
         /* Add upwards and truncate downwards. */
         return ((addr + getpagesize()-1) & ~(getpagesize()-1));
 }
 
-/*L:180 An "initial ram disk" is a disk image loaded into memory along with
- * the kernel which the kernel can use to boot from without needing any
- * drivers.  Most distributions now use this as standard: the initrd contains
- * the code to load the appropriate driver modules for the current machine.
+/*L:180
+ * An "initial ram disk" is a disk image loaded into memory along with the
+ * kernel which the kernel can use to boot from without needing any drivers.
+ * Most distributions now use this as standard: the initrd contains the code to
+ * load the appropriate driver modules for the current machine.
  *
  * Importantly, James Morris works for RedHat, and Fedora uses initrds for its
- * kernels.  He sent me this (and tells me when I break it). */
+ * kernels.  He sent me this (and tells me when I break it).
+ */
 static unsigned long load_initrd(const char *name, unsigned long mem)
 {
         int ifd;
@@ -462,12 +502,16 @@ static unsigned long load_initrd(const char *name, unsigned long mem)
         if (fstat(ifd, &st) < 0)
                 err(1, "fstat() on initrd '%s'", name);
 
-        /* We map the initrd at the top of memory, but mmap wants it to be
-         * page-aligned, so we round the size up for that. */
+        /*
+         * We map the initrd at the top of memory, but mmap wants it to be
+         * page-aligned, so we round the size up for that.
+         */
         len = page_align(st.st_size);
         map_at(ifd, from_guest_phys(mem - len), 0, st.st_size);
-        /* Once a file is mapped, you can close the file descriptor.  It's a
-         * little odd, but quite useful. */
+        /*
+         * Once a file is mapped, you can close the file descriptor.  It's a
+         * little odd, but quite useful.
+         */
         close(ifd);
         verbose("mapped initrd %s size=%lu @ %p\n", name, len, (void*)mem-len);
 
@@ -476,8 +520,10 @@ static unsigned long load_initrd(const char *name, unsigned long mem)
 }
 /*:*/
 
-/* Simple routine to roll all the commandline arguments together with spaces
- * between them. */
+/*
+ * Simple routine to roll all the commandline arguments together with spaces
+ * between them.
+ */
 static void concat(char *dst, char *args[])
 {
         unsigned int i, len = 0;
@@ -494,10 +540,12 @@ static void concat(char *dst, char *args[])
         dst[len] = '\0';
 }
 
-/*L:185 This is where we actually tell the kernel to initialize the Guest.  We
+/*L:185
+ * This is where we actually tell the kernel to initialize the Guest.  We
  * saw the arguments it expects when we looked at initialize() in lguest_user.c:
  * the base of Guest "physical" memory, the top physical page to allow and the
- * entry point for the Guest. */
+ * entry point for the Guest.
+ */
 static void tell_kernel(unsigned long start)
 {
         unsigned long args[] = { LHREQ_INITIALIZE,
@@ -511,7 +559,7 @@ static void tell_kernel(unsigned long start)
 }
 /*:*/
 
-/*
+/*L:200
  * Device Handling.
  *
  * When the Guest gives us a buffer, it sends an array of addresses and sizes.
@@ -522,20 +570,26 @@ static void tell_kernel(unsigned long start)
 static void *_check_pointer(unsigned long addr, unsigned int size,
                             unsigned int line)
 {
-        /* We have to separately check addr and addr+size, because size could
-         * be huge and addr + size might wrap around. */
+        /*
+         * We have to separately check addr and addr+size, because size could
+         * be huge and addr + size might wrap around.
+         */
         if (addr >= guest_limit || addr + size >= guest_limit)
                 errx(1, "%s:%i: Invalid address %#lx", __FILE__, line, addr);
-        /* We return a pointer for the caller's convenience, now we know it's
-         * safe to use. */
+        /*
+         * We return a pointer for the caller's convenience, now we know it's
+         * safe to use.
+         */
         return from_guest_phys(addr);
 }
 /* A macro which transparently hands the line number to the real function. */
 #define check_pointer(addr,size) _check_pointer(addr, size, __LINE__)
 
-/* Each buffer in the virtqueues is actually a chain of descriptors.  This
+/*
+ * Each buffer in the virtqueues is actually a chain of descriptors.  This
  * function returns the next descriptor in the chain, or vq->vring.num if we're
- * at the end. */
+ * at the end.
+ */
 static unsigned next_desc(struct vring_desc *desc,
                           unsigned int i, unsigned int max)
 {
@@ -556,7 +610,10 @@ static unsigned next_desc(struct vring_desc *desc,
         return next;
 }
 
-/* This actually sends the interrupt for this virtqueue */
+/*
+ * This actually sends the interrupt for this virtqueue, if we've used a
+ * buffer.
+ */
 static void trigger_irq(struct virtqueue *vq)
 {
         unsigned long buf[] = { LHREQ_IRQ, vq->config.irq };
@@ -576,12 +633,14 @@ static void trigger_irq(struct virtqueue *vq)
                 err(1, "Triggering irq %i", vq->config.irq);
 }
 
-/* This looks in the virtqueue and for the first available buffer, and converts
+/*
+ * This looks in the virtqueue for the first available buffer, and converts
  * it to an iovec for convenient access.  Since descriptors consist of some
  * number of output then some number of input descriptors, it's actually two
  * iovecs, but we pack them into one and note how many of each there were.
  *
- * This function returns the descriptor number found. */
+ * This function waits if necessary, and returns the descriptor number found.
+ */
 static unsigned wait_for_vq_desc(struct virtqueue *vq,
                                  struct iovec iov[],
                                  unsigned int *out_num, unsigned int *in_num)
@@ -590,17 +649,23 @@ static unsigned wait_for_vq_desc(struct virtqueue *vq,
         struct vring_desc *desc;
         u16 last_avail = lg_last_avail(vq);
 
+        /* There's nothing available? */
         while (last_avail == vq->vring.avail->idx) {
                 u64 event;
 
-                /* OK, tell Guest about progress up to now. */
+                /*
+                 * Since we're about to sleep, now is a good time to tell the
+                 * Guest about what we've used up to now.
+                 */
                 trigger_irq(vq);
 
                 /* OK, now we need to know about added descriptors. */
                 vq->vring.used->flags &= ~VRING_USED_F_NO_NOTIFY;
 
-                /* They could have slipped one in as we were doing that: make
-                 * sure it's written, then check again. */
+                /*
+                 * They could have slipped one in as we were doing that: make
+                 * sure it's written, then check again.
+                 */
                 mb();
                 if (last_avail != vq->vring.avail->idx) {
                         vq->vring.used->flags |= VRING_USED_F_NO_NOTIFY;
@@ -620,8 +685,10 @@ static unsigned wait_for_vq_desc(struct virtqueue *vq,
                 errx(1, "Guest moved used index from %u to %u",
                      last_avail, vq->vring.avail->idx);
 
-        /* Grab the next descriptor number they're advertising, and increment
-         * the index we've seen. */
+        /*
+         * Grab the next descriptor number they're advertising, and increment
+         * the index we've seen.
+         */
         head = vq->vring.avail->ring[last_avail % vq->vring.num];
         lg_last_avail(vq)++;
 
@@ -636,8 +703,10 @@ static unsigned wait_for_vq_desc(struct virtqueue *vq,
         desc = vq->vring.desc;
         i = head;
 
-        /* If this is an indirect entry, then this buffer contains a descriptor
-         * table which we handle as if it's any normal descriptor chain. */
+        /*
+         * If this is an indirect entry, then this buffer contains a descriptor
+         * table which we handle as if it's any normal descriptor chain.
+         */
         if (desc[i].flags & VRING_DESC_F_INDIRECT) {
                 if (desc[i].len % sizeof(struct vring_desc))
                         errx(1, "Invalid size for indirect buffer table");
@@ -656,8 +725,10 @@ static unsigned wait_for_vq_desc(struct virtqueue *vq,
                 if (desc[i].flags & VRING_DESC_F_WRITE)
                         (*in_num)++;
                 else {
-                        /* If it's an output descriptor, they're all supposed
-                         * to come before any input descriptors. */
+                        /*
+                         * If it's an output descriptor, they're all supposed
+                         * to come before any input descriptors.
+                         */
                         if (*in_num)
                                 errx(1, "Descriptor has out after in");
                         (*out_num)++;
@@ -671,14 +742,19 @@ static unsigned wait_for_vq_desc(struct virtqueue *vq,
         return head;
 }
 
-/* After we've used one of their buffers, we tell them about it.  We'll then
- * want to send them an interrupt, using trigger_irq(). */
+/*
+ * After we've used one of their buffers, we tell the Guest about it.  Sometime
+ * later we'll want to send them an interrupt using trigger_irq(); note that
+ * wait_for_vq_desc() does that for us if it has to wait.
+ */
 static void add_used(struct virtqueue *vq, unsigned int head, int len)
 {
         struct vring_used_elem *used;
 
-        /* The virtqueue contains a ring of used buffers.  Get a pointer to the
-         * next entry in that used ring. */
+        /*
+         * The virtqueue contains a ring of used buffers.  Get a pointer to the
+         * next entry in that used ring.
+         */
         used = &vq->vring.used->ring[vq->vring.used->idx % vq->vring.num];
         used->id = head;
         used->len = len;
@@ -698,9 +774,9 @@ static void add_used_and_trigger(struct virtqueue *vq, unsigned head, int len)
 /*
  * The Console
  *
- * We associate some data with the console for our exit hack. */
-struct console_abort
-{
+ * We associate some data with the console for our exit hack.
+ */
+struct console_abort {
         /* How many times have they hit ^C? */
         int count;
         /* When did they start? */
@@ -715,30 +791,35 @@ static void console_input(struct virtqueue *vq)
         struct console_abort *abort = vq->dev->priv;
         struct iovec iov[vq->vring.num];
 
-        /* Make sure there's a descriptor waiting. */
+        /* Make sure there's a descriptor available. */
         head = wait_for_vq_desc(vq, iov, &out_num, &in_num);
         if (out_num)
                 errx(1, "Output buffers in console in queue?");
 
-        /* Read it in. */
+        /* Read into it.  This is where we usually wait. */
         len = readv(STDIN_FILENO, iov, in_num);
         if (len <= 0) {
                 /* Ran out of input? */
                 warnx("Failed to get console input, ignoring console.");
-                /* For simplicity, dying threads kill the whole Launcher.  So
-                 * just nap here. */
+                /*
+                 * For simplicity, dying threads kill the whole Launcher.  So
+                 * just nap here.
+                 */
                 for (;;)
                         pause();
         }
 
+        /* Tell the Guest we used a buffer. */
         add_used_and_trigger(vq, head, len);
 
-        /* Three ^C within one second?  Exit.
+        /*
+         * Three ^C within one second?  Exit.
          *
          * This is such a hack, but works surprisingly well.  Each ^C has to
          * be in a buffer by itself, so they can't be too fast.  But we check
          * that we get three within about a second, so they can't be too
-         * slow. */
+         * slow.
+         */
         if (len != 1 || ((char *)iov[0].iov_base)[0] != 3) {
                 abort->count = 0;
                 return;
@@ -763,15 +844,23 @@ static void console_output(struct virtqueue *vq)
         unsigned int head, out, in;
         struct iovec iov[vq->vring.num];
 
+        /* We usually wait in here, for the Guest to give us something. */
         head = wait_for_vq_desc(vq, iov, &out, &in);
         if (in)
                 errx(1, "Input buffers in console output queue?");
+
+        /* writev can return a partial write, so we loop here. */
         while (!iov_empty(iov, out)) {
                 int len = writev(STDOUT_FILENO, iov, out);
                 if (len <= 0)
                         err(1, "Write to stdout gave %i", len);
                 iov_consume(iov, out, len);
         }
+
+        /*
+         * We're finished with that buffer: if we're going to sleep,
+         * wait_for_vq_desc() will prod the Guest with an interrupt.
+         */
         add_used(vq, head, 0);
 }
 
@@ -791,15 +880,30 @@ static void net_output(struct virtqueue *vq)
         unsigned int head, out, in;
         struct iovec iov[vq->vring.num];
 
+        /* We usually wait in here for the Guest to give us a packet. */
         head = wait_for_vq_desc(vq, iov, &out, &in);
         if (in)
                 errx(1, "Input buffers in net output queue?");
+        /*
+         * Send the whole thing through to /dev/net/tun.  It expects the exact
+         * same format: what a coincidence!
+         */
         if (writev(net_info->tunfd, iov, out) < 0)
                 errx(1, "Write to tun failed?");
+
+        /*
+         * Done with that one; wait_for_vq_desc() will send the interrupt if
+         * all packets are processed.
+         */
         add_used(vq, head, 0);
 }
 
-/* Will reading from this file descriptor block? */
+/*
+ * Handling network input is a bit trickier, because I've tried to optimize it.
+ *
+ * First we have a helper routine which tells is if from this file descriptor
+ * (ie. the /dev/net/tun device) will block:
+ */
 static bool will_block(int fd)
 {
         fd_set fdset;
@@ -809,8 +913,11 @@ static bool will_block(int fd)
         return select(fd+1, &fdset, NULL, NULL, &zero) != 1;
 }
 
-/* This is where we handle packets coming in from the tun device to our
- * Guest. */
+/*
+ * This handles packets coming in from the tun device to our Guest.  Like all
+ * service routines, it gets called again as soon as it returns, so you don't
+ * see a while(1) loop here.
+ */
 static void net_input(struct virtqueue *vq)
 {
         int len;
@@ -818,21 +925,38 @@ static void net_input(struct virtqueue *vq)
         struct iovec iov[vq->vring.num];
         struct net_info *net_info = vq->dev->priv;
 
+        /*
+         * Get a descriptor to write an incoming packet into.  This will also
+         * send an interrupt if they're out of descriptors.
+         */
         head = wait_for_vq_desc(vq, iov, &out, &in);
         if (out)
                 errx(1, "Output buffers in net input queue?");
 
-        /* Deliver interrupt now, since we're about to sleep. */
+        /*
+         * If it looks like we'll block reading from the tun device, send them
+         * an interrupt.
+         */
         if (vq->pending_used && will_block(net_info->tunfd))
                 trigger_irq(vq);
 
+        /*
+         * Read in the packet.  This is where we normally wait (when there's no
+         * incoming network traffic).
+         */
         len = readv(net_info->tunfd, iov, in);
         if (len <= 0)
                 err(1, "Failed to read from tun.");
+
+        /*
+         * Mark that packet buffer as used, but don't interrupt here.  We want
+         * to wait until we've done as much work as we can.
+         */
         add_used(vq, head, len);
 }
+/*:*/
 
-/* This is the helper to create threads. */
+/* This is the helper to create threads: run the service routine in a loop. */
 static int do_thread(void *_vq)
 {
         struct virtqueue *vq = _vq;
@@ -842,8 +966,10 @@ static int do_thread(void *_vq)
         return 0;
 }
 
-/* When a child dies, we kill our entire process group with SIGTERM.  This
- * also has the side effect that the shell restores the console for us! */
+/*
+ * When a child dies, we kill our entire process group with SIGTERM.  This
+ * also has the side effect that the shell restores the console for us!
+ */
 static void kill_launcher(int signal)
 {
         kill(0, SIGTERM);
@@ -878,11 +1004,15 @@ static void reset_device(struct device *dev)
         signal(SIGCHLD, (void *)kill_launcher);
 }
 
+/*L:216
+ * This actually creates the thread which services the virtqueue for a device.
+ */
 static void create_thread(struct virtqueue *vq)
 {
-        /* Create stack for thread and run it.  Since stack grows
-         * upwards, we point the stack pointer to the end of this
-         * region. */
+        /*
+         * Create stack for thread.  Since the stack grows upwards, we point
+         * the stack pointer to the end of this region.
+         */
         char *stack = malloc(32768);
         unsigned long args[] = { LHREQ_EVENTFD,
                                  vq->config.pfn*getpagesize(), 0 };
@@ -893,17 +1023,22 @@ static void create_thread(struct virtqueue *vq)
                 err(1, "Creating eventfd");
         args[2] = vq->eventfd;
 
-        /* Attach an eventfd to this virtqueue: it will go off
-         * when the Guest does an LHCALL_NOTIFY for this vq. */
+        /*
+         * Attach an eventfd to this virtqueue: it will go off when the Guest
+         * does an LHCALL_NOTIFY for this vq.
+         */
         if (write(lguest_fd, &args, sizeof(args)) != 0)
                 err(1, "Attaching eventfd");
 
-        /* CLONE_VM: because it has to access the Guest memory, and
-         * SIGCHLD so we get a signal if it dies. */
+        /*
+         * CLONE_VM: because it has to access the Guest memory, and SIGCHLD so
+         * we get a signal if it dies.
+         */
         vq->thread = clone(do_thread, stack + 32768, CLONE_VM | SIGCHLD, vq);
         if (vq->thread == (pid_t)-1)
                 err(1, "Creating clone");
-        /* We close our local copy, now the child has it. */
+
+        /* We close our local copy now the child has it. */
         close(vq->eventfd);
 }
 
@@ -955,7 +1090,10 @@ static void update_device_status(struct device *dev)
         }
 }
 
-/* This is the generic routine we call when the Guest uses LHCALL_NOTIFY. */
+/*L:215
+ * This is the generic routine we call when the Guest uses LHCALL_NOTIFY.  In
+ * particular, it's used to notify us of device status changes during boot.
+ */
 static void handle_output(unsigned long addr)
 {
         struct device *i;
@@ -964,25 +1102,42 @@ static void handle_output(unsigned long addr)
         for (i = devices.dev; i; i = i->next) {
                 struct virtqueue *vq;
 
-                /* Notifications to device descriptors update device status. */
+                /*
+                 * Notifications to device descriptors mean they updated the
+                 * device status.
+                 */
                 if (from_guest_phys(addr) == i->desc) {
                         update_device_status(i);
                         return;
                 }
 
-                /* Devices *can* be used before status is set to DRIVER_OK. */
+                /*
+                 * Devices *can* be used before status is set to DRIVER_OK.
+                 * The original plan was that they would never do this: they
+                 * would always finish setting up their status bits before
+                 * actually touching the virtqueues.  In practice, we allowed
+                 * them to, and they do (eg. the disk probes for partition
+                 * tables as part of initialization).
+                 *
+                 * If we see this, we start the device: once it's running, we
+                 * expect the device to catch all the notifications.
+                 */
                 for (vq = i->vq; vq; vq = vq->next) {
                         if (addr != vq->config.pfn*getpagesize())
                                 continue;
                         if (i->running)
                                 errx(1, "Notification on running %s", i->name);
+                        /* This just calls create_thread() for each virtqueue */
                         start_device(i);
                         return;
                 }
         }
 
-        /* Early console write is done using notify on a nul-terminated string
-         * in Guest memory. */
+        /*
+         * Early console write is done using notify on a nul-terminated string
+         * in Guest memory.  It's also great for hacking debugging messages
+         * into a Guest.
+         */
         if (addr >= guest_limit)
                 errx(1, "Bad NOTIFY %#lx", addr);
 
@@ -998,10 +1153,12 @@ static void handle_output(unsigned long addr)
  * routines to allocate and manage them.
  */
 
-/* The layout of the device page is a "struct lguest_device_desc" followed by a
+/*
+ * 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
  * array of configuration bytes.  This routine returns the configuration
- * pointer. */
+ * pointer.
+ */
 static u8 *device_config(const struct device *dev)
 {
         return (void *)(dev->desc + 1)
@@ -1009,9 +1166,11 @@ static u8 *device_config(const struct device *dev)
                 + dev->feature_len * 2;
 }
 
-/* This routine allocates a new "struct lguest_device_desc" from descriptor
+/*
+ * This routine allocates a new "struct lguest_device_desc" from descriptor
  * table page just above the Guest's normal memory.  It returns a pointer to
- * that descriptor. */
+ * that descriptor.
+ */
 static struct lguest_device_desc *new_dev_desc(u16 type)
 {
         struct lguest_device_desc d = { .type = type };
@@ -1032,8 +1191,10 @@ static struct lguest_device_desc *new_dev_desc(u16 type)
         return memcpy(p, &d, sizeof(d));
 }
 
-/* Each device descriptor is followed by the description of its virtqueues.  We
- * specify how many descriptors the virtqueue is to have. */
+/*
+ * Each device descriptor is followed by the description of its virtqueues.  We
+ * specify how many descriptors the virtqueue is to have.
+ */
 static void add_virtqueue(struct device *dev, unsigned int num_descs,
                           void (*service)(struct virtqueue *))
 {
@@ -1050,6 +1211,11 @@ static void add_virtqueue(struct device *dev, unsigned int num_descs,
         vq->next = NULL;
         vq->last_avail_idx = 0;
         vq->dev = dev;
+
+        /*
+         * This is the routine the service thread will run, and its Process ID
+         * once it's running.
+         */
         vq->service = service;
         vq->thread = (pid_t)-1;
 
@@ -1061,10 +1227,12 @@ static void add_virtqueue(struct device *dev, unsigned int num_descs,
         /* Initialize the vring. */
         vring_init(&vq->vring, num_descs, p, LGUEST_VRING_ALIGN);
 
-        /* Append virtqueue to this device's descriptor.  We use
+        /*
+         * Append virtqueue to this device's descriptor.  We use
          * device_config() to get the end of the device's current virtqueues;
          * we check that we haven't added any config or feature information
-         * yet, otherwise we'd be overwriting them. */
+         * yet, otherwise we'd be overwriting them.
+         */
         assert(dev->desc->config_len == 0 && dev->desc->feature_len == 0);
         memcpy(device_config(dev), &vq->config, sizeof(vq->config));
         dev->num_vq++;
@@ -1072,14 +1240,18 @@ static void add_virtqueue(struct device *dev, unsigned int num_descs,
 
         verbose("Virtqueue page %#lx\n", to_guest_phys(p));
 
-        /* Add to tail of list, so dev->vq is first vq, dev->vq->next is
-         * second.  */
+        /*
+         * Add to tail of list, so dev->vq is first vq, dev->vq->next is
+         * second.
+         */
         for (i = &dev->vq; *i; i = &(*i)->next);
         *i = vq;
 }
 
-/* The first half of the feature bitmask is for us to advertise features.  The
- * second half is for the Guest to accept features. */
+/*
+ * The first half of the feature bitmask is for us to advertise features.  The
+ * second half is for the Guest to accept features.
+ */
 static void add_feature(struct device *dev, unsigned bit)
 {
         u8 *features = get_feature_bits(dev);
@@ -1093,9 +1265,11 @@ static void add_feature(struct device *dev, unsigned bit)
         features[bit / CHAR_BIT] |= (1 << (bit % CHAR_BIT));
 }
 
-/* This routine sets the configuration fields for an existing device's
+/*
+ * This routine sets the configuration fields for an existing device's
  * descriptor.  It only works for the last device, but that's OK because that's
- * how we use it. */
+ * how we use it.
+ */
 static void set_config(struct device *dev, unsigned len, const void *conf)
 {
         /* Check we haven't overflowed our single page. */
@@ -1105,12 +1279,18 @@ static void set_config(struct device *dev, unsigned len, const void *conf)
         /* Copy in the config information, and store the length. */
         memcpy(device_config(dev), conf, len);
         dev->desc->config_len = len;
+
+        /* Size must fit in config_len field (8 bits)! */
+        assert(dev->desc->config_len == len);
 }
 
-/* This routine does all the creation and setup of a new device, including
- * calling new_dev_desc() to allocate the descriptor and device memory.
+/*
+ * This routine does all the creation and setup of a new device, including
+ * calling new_dev_desc() to allocate the descriptor and device memory.  We
+ * don't actually start the service threads until later.
  *
- * See what I mean about userspace being boring? */
+ * See what I mean about userspace being boring?
+ */
 static struct device *new_device(const char *name, u16 type)
 {
         struct device *dev = malloc(sizeof(*dev));
@@ -1123,10 +1303,12 @@ static struct device *new_device(const char *name, u16 type)
         dev->num_vq = 0;
         dev->running = false;
 
-        /* Append to device list.  Prepending to a single-linked list is
+        /*
+         * Append to device list.  Prepending to a single-linked list is
          * easier, but the user expects the devices to be arranged on the bus
          * in command-line order.  The first network device on the command line
-         * is eth0, the first block device /dev/vda, etc. */
+         * is eth0, the first block device /dev/vda, etc.
+         */
         if (devices.lastdev)
                 devices.lastdev->next = dev;
         else
@@ -1136,8 +1318,10 @@ static struct device *new_device(const char *name, u16 type)
         return dev;
 }
 
-/* Our first setup routine is the console.  It's a fairly simple device, but
- * UNIX tty handling makes it uglier than it could be. */
+/*
+ * Our first setup routine is the console.  It's a fairly simple device, but
+ * UNIX tty handling makes it uglier than it could be.
+ */
 static void setup_console(void)
 {
         struct device *dev;
@@ -1145,8 +1329,10 @@ static void setup_console(void)
         /* If we can save the initial standard input settings... */
         if (tcgetattr(STDIN_FILENO, &orig_term) == 0) {
                 struct termios term = orig_term;
-                /* Then we turn off echo, line buffering and ^C etc.  We want a
-                 * raw input stream to the Guest. */
+                /*
+                 * Then we turn off echo, line buffering and ^C etc: We want a
+                 * raw input stream to the Guest.
+                 */
                 term.c_lflag &= ~(ISIG|ICANON|ECHO);
                 tcsetattr(STDIN_FILENO, TCSANOW, &term);
         }
@@ -1157,10 +1343,12 @@ static void setup_console(void)
         dev->priv = malloc(sizeof(struct console_abort));
         ((struct console_abort *)dev->priv)->count = 0;
 
-        /* The console needs two virtqueues: the input then the output.  When
+        /*
+         * The console needs two virtqueues: the input then the output.  When
          * they put something the input queue, we make sure we're listening to
          * stdin.  When they put something in the output queue, we write it to
-         * stdout. */
+         * stdout.
+         */
         add_virtqueue(dev, VIRTQUEUE_NUM, console_input);
         add_virtqueue(dev, VIRTQUEUE_NUM, console_output);
 
@@ -1168,7 +1356,8 @@ static void setup_console(void)
 }
 /*:*/
 
-/*M:010 Inter-guest networking is an interesting area.  Simplest is to have a
+/*M:010
+ * Inter-guest networking is an interesting area.  Simplest is to have a
  * --sharenet=<name> option which opens or creates a named pipe.  This can be
  * used to send packets to another guest in a 1:1 manner.
  *
@@ -1182,7 +1371,8 @@ static void setup_console(void)
  * multiple inter-guest channels behind one interface, although it would
  * require some manner of hotplugging new virtio channels.
  *
- * Finally, we could implement a virtio network switch in the kernel. :*/
+ * Finally, we could implement a virtio network switch in the kernel.
+:*/
 
 static u32 str2ip(const char *ipaddr)
 {
@@ -1207,11 +1397,13 @@ static void str2mac(const char *macaddr, unsigned char mac[6])
         mac[5] = m[5];
 }
 
-/* This code is "adapted" from libbridge: it attaches the Host end of the
+/*
+ * This code is "adapted" from libbridge: it attaches the Host end of the
  * network device to the bridge device specified by the command line.
  *
  * This is yet another James Morris contribution (I'm an IP-level guy, so I
- * dislike bridging), and I just try not to break it. */
+ * dislike bridging), and I just try not to break it.
+ */
 static void add_to_bridge(int fd, const char *if_name, const char *br_name)
 {
         int ifidx;
@@ -1231,9 +1423,11 @@ static void add_to_bridge(int fd, const char *if_name, const char *br_name)
                 err(1, "can't add %s to bridge %s", if_name, br_name);
 }
 
-/* This sets up the Host end of the network device with an IP address, brings
+/*
+ * This sets up the Host end of the network device with an IP address, brings
  * it up so packets will flow, the copies the MAC address into the hwaddr
- * pointer. */
+ * pointer.
+ */
 static void configure_device(int fd, const char *tapif, u32 ipaddr)
 {
         struct ifreq ifr;
@@ -1260,10 +1454,12 @@ static int get_tun_device(char tapif[IFNAMSIZ])
         /* Start with this zeroed.  Messy but sure. */
         memset(&ifr, 0, sizeof(ifr));
 
-        /* We open the /dev/net/tun device and tell it we want a tap device.  A
+        /*
+         * We open the /dev/net/tun device and tell it we want a tap device.  A
          * tap device is like a tun device, only somehow different.  To tell
          * the truth, I completely blundered my way through this code, but it
-         * works now! */
+         * works now!
+         */
         netfd = open_or_die("/dev/net/tun", O_RDWR);
         ifr.ifr_flags = IFF_TAP | IFF_NO_PI | IFF_VNET_HDR;
         strcpy(ifr.ifr_name, "tap%d");
@@ -1274,18 +1470,22 @@ static int get_tun_device(char tapif[IFNAMSIZ])
                   TUN_F_CSUM|TUN_F_TSO4|TUN_F_TSO6|TUN_F_TSO_ECN) != 0)
                 err(1, "Could not set features for tun device");
 
-        /* We don't need checksums calculated for packets coming in this
-         * device: trust us! */
+        /*
+         * We don't need checksums calculated for packets coming in this
+         * device: trust us!
+         */
         ioctl(netfd, TUNSETNOCSUM, 1);
 
         memcpy(tapif, ifr.ifr_name, IFNAMSIZ);
         return netfd;
 }
 
-/*L:195 Our network is a Host<->Guest network.  This can either use bridging or
+/*L:195
+ * Our network is a Host<->Guest network.  This can either use bridging or
  * routing, but the principle is the same: it uses the "tun" device to inject
  * packets into the Host as if they came in from a normal network card.  We
- * just shunt packets between the Guest and the tun device. */
+ * just shunt packets between the Guest and the tun device.
+ */
 static void setup_tun_net(char *arg)
 {
         struct device *dev;
@@ -1302,13 +1502,14 @@ static void setup_tun_net(char *arg)
         dev = new_device("net", VIRTIO_ID_NET);
         dev->priv = net_info;
 
-        /* Network devices need a receive and a send queue, just like
-         * console. */
+        /* Network devices need a recv and a send queue, just like console. */
         add_virtqueue(dev, VIRTQUEUE_NUM, net_input);
         add_virtqueue(dev, VIRTQUEUE_NUM, net_output);
 
-        /* We need a socket to perform the magic network ioctls to bring up the
-         * tap interface, connect to the bridge etc.  Any socket will do! */
+        /*
+         * We need a socket to perform the magic network ioctls to bring up the
+         * tap interface, connect to the bridge etc.  Any socket will do!
+         */
         ipfd = socket(PF_INET, SOCK_DGRAM, IPPROTO_IP);
         if (ipfd < 0)
                 err(1, "opening IP socket");
@@ -1362,39 +1563,31 @@ static void setup_tun_net(char *arg)
                 verbose("device %u: tun %s: %s\n",
                         devices.device_num, tapif, arg);
 }
-
-/* Our block (disk) device should be really simple: the Guest asks for a block
- * number and we read or write that position in the file.  Unfortunately, that
- * was amazingly slow: the Guest waits until the read is finished before
- * running anything else, even if it could have been doing useful work.
- *
- * We could use async I/O, except it's reputed to suck so hard that characters
- * actually go missing from your code when you try to use it.
- *
- * So we farm the I/O out to thread, and communicate with it via a pipe. */
+/*:*/
 
 /* This hangs off device->priv. */
-struct vblk_info
-{
+struct vblk_info {
         /* The size of the file. */
         off64_t len;
 
         /* The file descriptor for the file. */
         int fd;
 
-        /* IO thread listens on this file descriptor [0]. */
-        int workpipe[2];
-
-        /* IO thread writes to this file descriptor to mark it done, then
-         * Launcher triggers interrupt to Guest. */
-        int done_fd;
 };
 
 /*L:210
  * The Disk
  *
- * Remember that the block device is handled by a separate I/O thread.  We head
- * straight into the core of that thread here:
+ * The disk only has one virtqueue, so it only has one thread.  It is really
+ * simple: the Guest asks for a block number and we read or write that position
+ * in the file.
+ *
+ * Before we serviced each virtqueue in a separate thread, that was unacceptably
+ * slow: the Guest waits until the read is finished before running anything
+ * else, even if it could have been doing useful work.
+ *
+ * We could have used async I/O, except it's reputed to suck so hard that
+ * characters actually go missing from your code when you try to use it.
  */
 static void blk_request(struct virtqueue *vq)
 {
@@ -1406,47 +1599,64 @@ static void blk_request(struct virtqueue *vq)
         struct iovec iov[vq->vring.num];
         off64_t off;
 
-        /* Get the next request. */
+        /*
+         * Get the next request, where we normally wait.  It triggers the
+         * interrupt to acknowledge previously serviced requests (if any).
+         */
         head = wait_for_vq_desc(vq, iov, &out_num, &in_num);
 
-        /* Every block request should contain at least one output buffer
+        /*
+         * Every block request should contain at least one output buffer
          * (detailing the location on disk and the type of request) and one
-         * input buffer (to hold the result). */
+         * input buffer (to hold the result).
+         */
         if (out_num == 0 || in_num == 0)
                 errx(1, "Bad virtblk cmd %u out=%u in=%u",
                      head, out_num, in_num);
 
         out = convert(&iov[0], struct virtio_blk_outhdr);
         in = convert(&iov[out_num+in_num-1], u8);
+        /*
+         * For historical reasons, block operations are expressed in 512 byte
+         * "sectors".
+         */
         off = out->sector * 512;
 
-        /* The block device implements "barriers", where the Guest indicates
+        /*
+         * The block device implements "barriers", where the Guest indicates
          * that it wants all previous writes to occur before this write.  We
          * don't have a way of asking our kernel to do a barrier, so we just
-         * synchronize all the data in the file.  Pretty poor, no? */
+         * synchronize all the data in the file.  Pretty poor, no?
+         */
         if (out->type & VIRTIO_BLK_T_BARRIER)
                 fdatasync(vblk->fd);
 
-        /* In general the virtio block driver is allowed to try SCSI commands.
-         * It'd be nice if we supported eject, for example, but we don't. */
+        /*
+         * In general the virtio block driver is allowed to try SCSI commands.
+         * It'd be nice if we supported eject, for example, but we don't.
+         */
         if (out->type & VIRTIO_BLK_T_SCSI_CMD) {
                 fprintf(stderr, "Scsi commands unsupported\n");
                 *in = VIRTIO_BLK_S_UNSUPP;
                 wlen = sizeof(*in);
         } else if (out->type & VIRTIO_BLK_T_OUT) {
-                /* Write */
-
-                /* Move to the right location in the block file.  This can fail
-                 * if they try to write past end. */
+                /*
+                 * Write
+                 *
+                 * Move to the right location in the block file.  This can fail
+                 * if they try to write past end.
+                 */
                 if (lseek64(vblk->fd, off, SEEK_SET) != off)
                         err(1, "Bad seek to sector %llu", out->sector);
 
                 ret = writev(vblk->fd, iov+1, out_num-1);
                 verbose("WRITE to sector %llu: %i\n", out->sector, ret);
 
-                /* Grr... Now we know how long the descriptor they sent was, we
+                /*
+                 * Grr... Now we know how long the descriptor they sent was, we
                  * make sure they didn't try to write over the end of the block
-                 * file (possibly extending it). */
+                 * file (possibly extending it).
+                 */
                 if (ret > 0 && off + ret > vblk->len) {
                         /* Trim it back to the correct length */
                         ftruncate64(vblk->fd, vblk->len);
@@ -1456,10 +1666,12 @@ static void blk_request(struct virtqueue *vq)
                 wlen = sizeof(*in);
                 *in = (ret >= 0 ? VIRTIO_BLK_S_OK : VIRTIO_BLK_S_IOERR);
         } else {
-                /* Read */
-
-                /* Move to the right location in the block file.  This can fail
-                 * if they try to read past end. */
+                /*
+                 * Read
+                 *
+                 * Move to the right location in the block file.  This can fail
+                 * if they try to read past end.
+                 */
                 if (lseek64(vblk->fd, off, SEEK_SET) != off)
                         err(1, "Bad seek to sector %llu", out->sector);
 
@@ -1474,13 +1686,16 @@ static void blk_request(struct virtqueue *vq)
                 }
         }
 
-        /* OK, so we noted that it was pretty poor to use an fdatasync as a
+        /*
+         * OK, so we noted that it was pretty poor to use an fdatasync as a
          * barrier.  But Christoph Hellwig points out that we need a sync
          * *afterwards* as well: "Barriers specify no reordering to the front
-         * or the back."  And Jens Axboe confirmed it, so here we are: */
+         * or the back."  And Jens Axboe confirmed it, so here we are:
+         */
         if (out->type & VIRTIO_BLK_T_BARRIER)
                 fdatasync(vblk->fd);
 
+        /* Finished that request. */
         add_used(vq, head, wlen);
 }
 
@@ -1491,7 +1706,7 @@ static void setup_block_file(const char *filename)
         struct vblk_info *vblk;
         struct virtio_blk_config conf;
 
-        /* The device responds to return from I/O thread. */
+        /* Creat the device. */
         dev = new_device("block", VIRTIO_ID_BLOCK);
 
         /* The device has one virtqueue, where the Guest places requests. */
@@ -1510,27 +1725,32 @@ static void setup_block_file(const char *filename)
         /* Tell Guest how many sectors this device has. */
         conf.capacity = cpu_to_le64(vblk->len / 512);
 
-        /* Tell Guest not to put in too many descriptors at once: two are used
-         * for the in and out elements. */
+        /*
+         * Tell Guest not to put in too many descriptors at once: two are used
+         * for the in and out elements.
+         */
         add_feature(dev, VIRTIO_BLK_F_SEG_MAX);
         conf.seg_max = cpu_to_le32(VIRTQUEUE_NUM - 2);
 
-        set_config(dev, sizeof(conf), &conf);
+        /* Don't try to put whole struct: we have 8 bit limit. */
+        set_config(dev, offsetof(struct virtio_blk_config, geometry), &conf);
 
         verbose("device %u: virtblock %llu sectors\n",
                 ++devices.device_num, le64_to_cpu(conf.capacity));
 }
 
-struct rng_info {
-        int rfd;
-};
-
-/* Our random number generator device reads from /dev/random into the Guest's
+/*L:211
+ * Our random number generator device reads from /dev/random into the Guest's
  * input buffers.  The usual case is that the Guest doesn't want random numbers
  * and so has no buffers although /dev/random is still readable, whereas
  * console is the reverse.
  *
- * The same logic applies, however. */
+ * The same logic applies, however.
+ */
+struct rng_info {
+        int rfd;
+};
+
 static void rng_input(struct virtqueue *vq)
 {
         int len;
@@ -1543,9 +1763,10 @@ static void rng_input(struct virtqueue *vq)
         if (out_num)
                 errx(1, "Output buffers in rng?");
 
-        /* This is why we convert to iovecs: the readv() call uses them, and so
-         * it reads straight into the Guest's buffer.  We loop to make sure we
-         * fill it. */
+        /*
+         * Just like the console write, we loop to cover the whole iovec.
+         * In this case, short reads actually happen quite a bit.
+         */
         while (!iov_empty(iov, in_num)) {
                 len = readv(rng_info->rfd, iov, in_num);
                 if (len <= 0)
@@ -1558,15 +1779,18 @@ static void rng_input(struct virtqueue *vq)
         add_used(vq, head, totlen);
 }
 
-/* And this creates a "hardware" random number device for the Guest. */
+/*L:199
+ * This creates a "hardware" random number device for the Guest.
+ */
 static void setup_rng(void)
 {
         struct device *dev;
         struct rng_info *rng_info = malloc(sizeof(*rng_info));
 
+        /* Our device's privat info simply contains the /dev/random fd. */
         rng_info->rfd = open_or_die("/dev/random", O_RDONLY);
 
-        /* The device responds to return from I/O thread. */
+        /* Create the new device. */
         dev = new_device("rng", VIRTIO_ID_RNG);
         dev->priv = rng_info;
 
@@ -1582,8 +1806,10 @@ static void __attribute__((noreturn)) restart_guest(void)
 {
         unsigned int i;
 
-        /* Since we don't track all open fds, we simply close everything beyond
-         * stderr. */
+        /*
+         * Since we don't track all open fds, we simply close everything beyond
+         * stderr.
+         */
         for (i = 3; i < FD_SETSIZE; i++)
                 close(i);
 
@@ -1594,8 +1820,10 @@ 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
- * its input and output, and finally, lays it to rest. */
+/*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(void)
 {
         for (;;) {
@@ -1630,7 +1858,7 @@ static void __attribute__((noreturn)) run_guest(void)
  *
  * Are you ready?  Take a deep breath and join me in the core of the Host, in
  * "make Host".
- :*/
+:*/
 
 static struct option opts[] = {
         { "verbose", 0, NULL, 'v' },
@@ -1651,8 +1879,7 @@ static void usage(void)
 /*L:105 The main routine is where the real work begins: */
 int main(int argc, char *argv[])
 {
-        /* Memory, top-level pagetable, code startpoint and size of the
-         * (optional) initrd. */
+        /* Memory, code startpoint and size of the (optional) initrd. */
         unsigned long mem = 0, start, initrd_size = 0;
         /* Two temporaries. */
         int i, c;
@@ -1664,24 +1891,32 @@ int main(int argc, char *argv[])
         /* Save the args: we "reboot" by execing ourselves again. */
         main_args = argv;
 
-        /* First we initialize the device list.  We keep a pointer to the last
+        /*
+         * First we initialize the device list.  We keep a pointer to the last
          * device, and the next interrupt number to use for devices (1:
-         * remember that 0 is used by the timer). */
+         * remember that 0 is used by the timer).
+         */
         devices.lastdev = NULL;
         devices.next_irq = 1;
 
+        /* We're CPU 0.  In fact, that's the only CPU possible right now. */
         cpu_id = 0;
-        /* We need to know how much memory so we can set up the device
+
+        /*
+         * We need to know how much memory so we can set up the device
          * descriptor and memory pages for the devices as we parse the command
          * line.  So we quickly look through the arguments to find the amount
-         * of memory now. */
+         * of memory now.
+         */
         for (i = 1; i < argc; i++) {
                 if (argv[i][0] != '-') {
                         mem = atoi(argv[i]) * 1024 * 1024;
-                        /* We start by mapping anonymous pages over all of
+                        /*
+                         * We start by mapping anonymous pages over all of
                          * guest-physical memory range.  This fills it with 0,
                          * and ensures that the Guest won't be killed when it
-                         * tries to access it. */
+                         * tries to access it.
+                         */
                         guest_base = map_zeroed_pages(mem / getpagesize()
                                                       + DEVICE_PAGES);
                         guest_limit = mem;
@@ -1714,8 +1949,10 @@ int main(int argc, char *argv[])
                         usage();
                 }
         }
-        /* After the other arguments we expect memory and kernel image name,
-         * followed by command line arguments for the kernel. */
+        /*
+         * After the other arguments we expect memory and kernel image name,
+         * followed by command line arguments for the kernel.
+         */
         if (optind + 2 > argc)
                 usage();
 
@@ -1733,20 +1970,26 @@ int main(int argc, char *argv[])
         /* Map the initrd image if requested (at top of physical memory) */
         if (initrd_name) {
                 initrd_size = load_initrd(initrd_name, mem);
-                /* These are the location in the Linux boot header where the
-                 * start and size of the initrd are expected to be found. */
+                /*
+                 * These are the location in the Linux boot header where the
+                 * start and size of the initrd are expected to be found.
+                 */
                 boot->hdr.ramdisk_image = mem - initrd_size;
                 boot->hdr.ramdisk_size = initrd_size;
                 /* The bootloader type 0xFF means "unknown"; that's OK. */
                 boot->hdr.type_of_loader = 0xFF;
         }
 
-        /* The Linux boot header contains an "E820" memory map: ours is a
-         * simple, single region. */
+        /*
+         * The Linux boot header contains an "E820" memory map: ours is a
+         * simple, single region.
+         */
         boot->e820_entries = 1;
         boot->e820_map[0] = ((struct e820entry) { 0, mem, E820_RAM });
-        /* The boot header contains a command line pointer: we put the command
-         * line after the boot header. */
+        /*
+         * The boot header contains a command line pointer: we put the command
+         * line after the boot header.
+         */
         boot->hdr.cmd_line_ptr = to_guest_phys(boot + 1);
         /* We use a simple helper to copy the arguments separated by spaces. */
         concat((char *)(boot + 1), argv+optind+2);
@@ -1760,11 +2003,13 @@ int main(int argc, char *argv[])
         /* Tell the entry path not to try to reload segment registers. */
         boot->hdr.loadflags |= KEEP_SEGMENTS;
 
-        /* We tell the kernel to initialize the Guest: this returns the open
-         * /dev/lguest file descriptor. */
+        /*
+         * We tell the kernel to initialize the Guest: this returns the open
+         * /dev/lguest file descriptor.
+         */
         tell_kernel(start);
 
-        /* Ensure that we terminate if a child dies. */
+        /* Ensure that we terminate if a device-servicing child dies. */
         signal(SIGCHLD, kill_launcher);
 
         /* If we exit via err(), this kills all the threads, restores tty. */