lguest: documentation IV: Launcher

Documentation: The Launcher

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 555 insertions, 44 deletions

diff --git a/Documentation/lguest/lguest.c b/Documentation/lguest/lguest.c
index fc1bf70abfb..d7e26f02595 100644
--- a/Documentation/lguest/lguest.c
+++ b/Documentation/lguest/lguest.c
@@ -34,12 +34,20 @@
 #include <termios.h>
 #include <getopt.h>
 #include <zlib.h>
+/*L:110 We can ignore the 28 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 and the header we need uses them, so we define them
+ * here.
+ */
 typedef unsigned long long u64;
 typedef uint32_t u32;
 typedef uint16_t u16;
 typedef uint8_t u8;
 #include "../../include/linux/lguest_launcher.h"
 #include "../../include/asm-i386/e820.h"
+/*:*/
 
 #define PAGE_PRESENT 0x7 	/* Present, RW, Execute */
 #define NET_PEERNUM 1
@@ -48,33 +56,52 @@ typedef uint8_t u8;
 #define SIOCBRADDIF	0x89a2		/* add interface to bridge      */
 #endif
 
+/*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)
+/*:*/
+
+/* The pipe to send commands to the waker process */
 static int waker_fd;
+/* The top of guest physical memory. */
 static u32 top;
 
+/* This is our list of devices. */
 struct device_list
 {
+	/* Summary information about the devices in our list: ready to pass to
+	 * select() to ask which need servicing.*/
 	fd_set infds;
 	int max_infd;
 
+	/* The descriptor page for the devices. */
 	struct lguest_device_desc *descs;
+
+	/* A single linked list of devices. */
 	struct device *dev;
+	/* ... And an end pointer so we can easily append new devices */
 	struct device **lastdev;
 };
 
+/* The device structure describes a single device. */
 struct device
 {
+	/* The linked-list pointer. */
 	struct device *next;
+	/* The descriptor for this device, as mapped into the Guest. */
 	struct lguest_device_desc *desc;
+	/* The memory page(s) of this device, if any.  Also mapped in Guest. */
 	void *mem;
 
-	/* Watch this fd if handle_input non-NULL. */
+	/* If handle_input is set, it wants to be called when this file
+	 * descriptor is ready. */
 	int fd;
 	bool (*handle_input)(int fd, struct device *me);
 
-	/* Watch DMA to this key if handle_input non-NULL. */
+	/* If handle_output is set, it wants to be called when the Guest sends
+	 * DMA to this key. */
 	unsigned long watch_key;
 	u32 (*handle_output)(int fd, const struct iovec *iov,
 			     unsigned int num, struct device *me);
@@ -83,6 +110,11 @@ struct device
 	void *priv;
 };
 
+/*L:130
+ * Loading the Kernel.
+ *
+ * We start with couple of simple helper routines.  open_or_die() avoids
+ * error-checking code cluttering the callers: */
 static int open_or_die(const char *name, int flags)
 {
 	int fd = open(name, flags);
@@ -91,26 +123,38 @@ static int open_or_die(const char *name, int flags)
 	return fd;
 }
 
+/* map_zeroed_pages() takes a (page-aligned) address and a number of pages. */
 static void *map_zeroed_pages(unsigned long addr, unsigned int num)
 {
+	/* We cache the /dev/zero file-descriptor so we only open it once. */
 	static int fd = -1;
 
 	if (fd == -1)
 		fd = open_or_die("/dev/zero", O_RDONLY);
 
+	/* We use a private mapping (ie. if we write to the page, it will be
+	 * copied), and obviously we insist that it be mapped where we ask. */
 	if (mmap((void *)addr, getpagesize() * num,
 		 PROT_READ|PROT_WRITE|PROT_EXEC, MAP_FIXED|MAP_PRIVATE, fd, 0)
 	    != (void *)addr)
 		err(1, "Mmaping %u pages of /dev/zero @%p", num, (void *)addr);
+
+	/* Returning the address is just a courtesy: can simplify callers. */
 	return (void *)addr;
 }
 
-/* Find magic string marking entry point, return entry point. */
+/* To find out where to start we look for the magic Guest string, which marks
+ * the code we see in lguest_asm.S.  This is a hack which we are currently
+ * plotting to replace with the normal Linux entry point. */
 static unsigned long entry_point(void *start, void *end,
 				 unsigned long page_offset)
 {
 	void *p;
 
+	/* The scan gives us the physical starting address.  We want the
+	 * virtual address in this case, and fortunately, we already figured
+	 * out the physical-virtual difference and passed it here in
+	 * "page_offset". */
 	for (p = start; p < end; p++)
 		if (memcmp(p, "GenuineLguest", strlen("GenuineLguest")) == 0)
 			return (long)p + strlen("GenuineLguest") + page_offset;
@@ -118,7 +162,17 @@ static unsigned long entry_point(void *start, void *end,
 	err(1, "Is this image a genuine lguest?");
 }
 
-/* Returns the entry point */
+/* 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.
+ *
+ * The ELF headers give *two* addresses: a physical address, and a virtual
+ * address.  The Guest kernel expects to be placed in memory at the physical
+ * address, and the page tables set up so it will correspond to that virtual
+ * address.  We return the difference between the virtual and physical
+ * addresses in the "page_offset" pointer.
+ *
+ * We return the starting address. */
 static unsigned long map_elf(int elf_fd, const Elf32_Ehdr *ehdr,
 			     unsigned long *page_offset)
 {
@@ -127,40 +181,61 @@ static unsigned long map_elf(int elf_fd, const Elf32_Ehdr *ehdr,
 	unsigned int i;
 	unsigned long start = -1UL, end = 0;
 
-	/* Sanity checks. */
+	/* 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"
+	 * headers which indicate which parts ("segments") of the program to
+	 * load where. */
+
+	/* We read in all the program headers at once: */
 	if (lseek(elf_fd, ehdr->e_phoff, SEEK_SET) < 0)
 		err(1, "Seeking to program headers");
 	if (read(elf_fd, phdr, sizeof(phdr)) != sizeof(phdr))
 		err(1, "Reading program headers");
 
+	/* We don't know page_offset yet. */
 	*page_offset = 0;
-	/* We map the loadable segments at virtual addresses corresponding
-	 * to their physical addresses (our virtual == guest physical). */
+
+	/* 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. */
 	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)
 			continue;
 
 		verbose("Section %i: size %i addr %p\n",
 			i, phdr[i].p_memsz, (void *)phdr[i].p_paddr);
 
-		/* We expect linear address space. */
+		/* We expect a simple linear address space: every segment must
+		 * have the same difference between virtual (p_vaddr) and
+		 * physical (p_paddr) address. */
 		if (!*page_offset)
 			*page_offset = phdr[i].p_vaddr - phdr[i].p_paddr;
 		else if (*page_offset != phdr[i].p_vaddr - phdr[i].p_paddr)
 			errx(1, "Page offset of section %i different", i);
 
+		/* We track the first and last address we mapped, so we can
+		 * tell entry_point() where to scan. */
 		if (phdr[i].p_paddr < start)
 			start = phdr[i].p_paddr;
 		if (phdr[i].p_paddr + phdr[i].p_filesz > end)
 			end = phdr[i].p_paddr + phdr[i].p_filesz;
 
-		/* We map everything private, writable. */
+		/* We map this section of the file at its physical address.  We
+		 * map it read & write even if the header says this segment is
+		 * read-only.  The kernel really wants to be writable: it
+		 * patches its own instructions which would normally be
+		 * read-only.
+		 *
+		 * MAP_PRIVATE means that the page won't be copied until a
+		 * write is done to it.  This allows us to share much of the
+		 * kernel memory between Guests. */
 		addr = mmap((void *)phdr[i].p_paddr,
 			    phdr[i].p_filesz,
 			    PROT_READ|PROT_WRITE|PROT_EXEC,
@@ -174,7 +249,31 @@ static unsigned long map_elf(int elf_fd, const Elf32_Ehdr *ehdr,
 	return entry_point((void *)start, (void *)end, *page_offset);
 }
 
-/* This is amazingly reliable. */
+/*L:170 Prepare to be SHOCKED and AMAZED.  And possibly a trifle nauseated.
+ *
+ * We know that CONFIG_PAGE_OFFSET sets what virtual address the kernel expects
+ * to be.  We don't know what that option was, but we can figure it out
+ * approximately by looking at the addresses in the code.  I chose the common
+ * case of reading a memory location into the %eax register:
+ *
+ *  movl <some-address>, %eax
+ *
+ * This gets encoded as five bytes: "0xA1 <4-byte-address>".  For example,
+ * "0xA1 0x18 0x60 0x47 0xC0" reads the address 0xC0476018 into %eax.
+ *
+ * In this example can guess that the kernel was compiled with
+ * CONFIG_PAGE_OFFSET set to 0xC0000000 (it's always a round number).  If the
+ * kernel were larger than 16MB, we might see 0xC1 addresses show up, but our
+ * kernel isn't that bloated yet.
+ *
+ * Unfortunately, x86 has variable-length instructions, so finding this
+ * particular instruction properly involves writing a disassembler.  Instead,
+ * we rely on statistics.  We look for "0xA1" and tally the different bytes
+ * which occur 4 bytes later (the "0xC0" in our example above).  When one of
+ * those bytes appears three times, we can be reasonably confident that it
+ * forms the start of CONFIG_PAGE_OFFSET.
+ *
+ * This is amazingly reliable. */
 static unsigned long intuit_page_offset(unsigned char *img, unsigned long len)
 {
 	unsigned int i, possibilities[256] = { 0 };
@@ -187,30 +286,52 @@ static unsigned long intuit_page_offset(unsigned char *img, unsigned long len)
 	errx(1, "could not determine page offset");
 }
 
+/*L:160 Unfortunately the entire ELF image isn't compressed: the segments
+ * which need loading are extracted and compressed raw.  This denies us the
+ * information we need to make a fully-general loader. */
 static unsigned long unpack_bzimage(int fd, unsigned long *page_offset)
 {
 	gzFile f;
 	int ret, len = 0;
+	/* A bzImage always gets loaded at physical address 1M.  This is
+	 * actually configurable as CONFIG_PHYSICAL_START, but as the comment
+	 * there says, "Don't change this unless you know what you are doing".
+	 * Indeed. */
 	void *img = (void *)0x100000;
 
+	/* gzdopen takes our file descriptor (carefully placed at the start of
+	 * the GZIP header we found) and returns a gzFile. */
 	f = gzdopen(fd, "rb");
+	/* We read it into memory in 64k chunks until we hit the end. */
 	while ((ret = gzread(f, img + len, 65536)) > 0)
 		len += ret;
 	if (ret < 0)
 		err(1, "reading image from bzImage");
 
 	verbose("Unpacked size %i addr %p\n", len, img);
+
+	/* Without the ELF header, we can't tell virtual-physical gap.  This is
+	 * CONFIG_PAGE_OFFSET, and people do actually change it.  Fortunately,
+	 * I have a clever way of figuring it out from the code itself.  */
 	*page_offset = intuit_page_offset(img, len);
 
 	return entry_point(img, img + len, *page_offset);
 }
 
+/*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 can't do that
+ * because the Guest can't run the unpacking code, and adding features to
+ * lguest kills puppies, so we don't want to.
+ *
+ * The bzImage is formed by putting the decompressing code in front of the
+ * compressed kernel code.  So we can simple scan through it looking for the
+ * first "gzip" header, and start decompressing from there. */
 static unsigned long load_bzimage(int fd, unsigned long *page_offset)
 {
 	unsigned char c;
 	int state = 0;
 
-	/* Ugly brute force search for gzip header. */
+	/* GZIP header is 0x1F 0x8B <method> <flags>... <compressed-by>. */
 	while (read(fd, &c, 1) == 1) {
 		switch (state) {
 		case 0:
@@ -227,8 +348,10 @@ static unsigned long load_bzimage(int fd, unsigned long *page_offset)
 			state++;
 			break;
 		case 9:
+			/* Seek back to the start of the gzip header. */
 			lseek(fd, -10, SEEK_CUR);
-			if (c != 0x03) /* Compressed under UNIX. */
+			/* One final check: "compressed under UNIX". */
+			if (c != 0x03)
 				state = -1;
 			else
 				return unpack_bzimage(fd, page_offset);
@@ -237,25 +360,43 @@ static unsigned long load_bzimage(int fd, unsigned long *page_offset)
 	errx(1, "Could not find kernel in bzImage");
 }
 
+/*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 some funky
+ * coding, we can load those, too. */
 static unsigned long load_kernel(int fd, unsigned long *page_offset)
 {
 	Elf32_Ehdr hdr;
 
+	/* Read in the first few bytes. */
 	if (read(fd, &hdr, sizeof(hdr)) != sizeof(hdr))
 		err(1, "Reading kernel");
 
+	/* If it's an ELF file, it starts with "\177ELF" */
 	if (memcmp(hdr.e_ident, ELFMAG, SELFMAG) == 0)
 		return map_elf(fd, &hdr, page_offset);
 
+	/* Otherwise we assume it's a bzImage, and try to unpack it */
 	return load_bzimage(fd, page_offset);
 }
 
+/* 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. */
 static inline unsigned long page_align(unsigned long addr)
 {
+	/* Add upwards and truncate downwards. */
 	return ((addr + getpagesize()-1) & ~(getpagesize()-1));
 }
 
-/* initrd gets loaded at top of memory: return length. */
+/*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). */
 static unsigned long load_initrd(const char *name, unsigned long mem)
 {
 	int ifd;
@@ -264,21 +405,35 @@ static unsigned long load_initrd(const char *name, unsigned long mem)
 	void *iaddr;
 
 	ifd = open_or_die(name, O_RDONLY);
+	/* fstat() is needed to get the file size. */
 	if (fstat(ifd, &st) < 0)
 		err(1, "fstat() on initrd '%s'", name);
 
+	/* The length needs to be rounded up to a page size: mmap needs the
+	 * address to be page aligned. */
 	len = page_align(st.st_size);
+	/* We map the initrd at the top of memory. */
 	iaddr = mmap((void *)mem - len, st.st_size,
 		     PROT_READ|PROT_EXEC|PROT_WRITE,
 		     MAP_FIXED|MAP_PRIVATE, ifd, 0);
 	if (iaddr != (void *)mem - len)
 		err(1, "Mmaping initrd '%s' returned %p not %p",
 		    name, iaddr, (void *)mem - len);
+	/* 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, st.st_size, iaddr);
+
+	/* We return the initrd size. */
 	return len;
 }
 
+/* Once we know how much memory we have, and the address the Guest kernel
+ * expects, we can construct simple linear page tables 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). */
 static unsigned long setup_pagetables(unsigned long mem,
 				      unsigned long initrd_size,
 				      unsigned long page_offset)
@@ -287,23 +442,32 @@ static unsigned long setup_pagetables(unsigned long mem,
 	unsigned int mapped_pages, i, linear_pages;
 	unsigned int ptes_per_page = getpagesize()/sizeof(u32);
 
-	/* If we can map all of memory above page_offset, we do so. */
+	/* Ideally we map all physical memory starting at page_offset.
+	 * However, if page_offset is 0xC0000000 we can only map 1G of physical
+	 * (0xC0000000 + 1G overflows). */
 	if (mem <= -page_offset)
 		mapped_pages = mem/getpagesize();
 	else
 		mapped_pages = -page_offset/getpagesize();
 
-	/* Each linear PTE page can map ptes_per_page pages. */
+	/* Each PTE page can map ptes_per_page pages: how many do we need? */
 	linear_pages = (mapped_pages + ptes_per_page-1)/ptes_per_page;
 
-	/* We lay out top-level then linear mapping immediately below initrd */
+	/* We put the toplevel page directory page at the top of memory. */
 	pgdir = (void *)mem - initrd_size - getpagesize();
+
+	/* Now we use the next linear_pages pages as pte pages */
 	linear = (void *)pgdir - linear_pages*getpagesize();
 
+	/* Linear mapping is easy: put every page's address into the mapping in
+	 * order.  PAGE_PRESENT contains the flags Present, Writable and
+	 * Executable. */
 	for (i = 0; i < mapped_pages; i++)
 		linear[i] = ((i * getpagesize()) | PAGE_PRESENT);
 
-	/* Now set up pgd so that this memory is at page_offset */
+	/* The top level points to the linear page table pages above.  The
+	 * entry representing page_offset points to the first one, and they
+	 * continue from there. */
 	for (i = 0; i < mapped_pages; i += ptes_per_page) {
 		pgdir[(i + page_offset/getpagesize())/ptes_per_page]
 			= (((u32)linear + i*sizeof(u32)) | PAGE_PRESENT);
@@ -312,9 +476,13 @@ static unsigned long setup_pagetables(unsigned long mem,
 	verbose("Linear mapping of %u pages in %u pte pages at %p\n",
 		mapped_pages, linear_pages, linear);
 
+	/* We return the top level (guest-physical) address: the kernel needs
+	 * to know where it is. */
 	return (unsigned long)pgdir;
 }
 
+/* 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;
@@ -328,6 +496,10 @@ static void concat(char *dst, char *args[])
 	dst[len] = '\0';
 }
 
+/* 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 top physical page to allow, the top level pagetable, the entry point and
+ * the page_offset constant for the Guest. */
 static int tell_kernel(u32 pgdir, u32 start, u32 page_offset)
 {
 	u32 args[] = { LHREQ_INITIALIZE,
@@ -337,8 +509,11 @@ static int tell_kernel(u32 pgdir, u32 start, u32 page_offset)
 	fd = open_or_die("/dev/lguest", O_RDWR);
 	if (write(fd, args, sizeof(args)) < 0)
 		err(1, "Writing to /dev/lguest");
+
+	/* We return the /dev/lguest file descriptor to control this Guest */
 	return fd;
 }
+/*:*/
 
 static void set_fd(int fd, struct device_list *devices)
 {
@@ -347,61 +522,108 @@ static void set_fd(int fd, struct device_list *devices)
 		devices->max_infd = fd;
 }
 
-/* When input arrives, we tell the kernel to kick lguest out with -EAGAIN. */
+/*L:200
+ * The Waker.
+ *
+ * With a console and network devices, we can have lots of input which we need
+ * to process.  We could try to tell the kernel what file descriptors to watch,
+ * but handing a file descriptor mask through to the kernel is fairly icky.
+ *
+ * Instead, we fork off a process which watches the file descriptors and writes
+ * the LHREQ_BREAK command to the /dev/lguest filedescriptor to tell the Host
+ * loop to stop running the Guest.  This causes it to return from the
+ * /dev/lguest read with -EAGAIN, where it will write to /dev/lguest to reset
+ * the LHREQ_BREAK and wake us up again.
+ *
+ * This, of course, is merely a different *kind* of icky.
+ */
 static void wake_parent(int pipefd, int lguest_fd, struct device_list *devices)
 {
+	/* Add the pipe from the Launcher to the fdset in the device_list, so
+	 * we watch it, too. */
 	set_fd(pipefd, devices);
 
 	for (;;) {
 		fd_set rfds = devices->infds;
 		u32 args[] = { LHREQ_BREAK, 1 };
 
+		/* Wait until input is ready from one of the devices. */
 		select(devices->max_infd+1, &rfds, NULL, NULL, NULL);
+		/* Is it a message from the Launcher? */
 		if (FD_ISSET(pipefd, &rfds)) {
 			int ignorefd;
+			/* If read() returns 0, it means the Launcher has
+			 * exited.  We silently follow. */
 			if (read(pipefd, &ignorefd, sizeof(ignorefd)) == 0)
 				exit(0);
+			/* Otherwise it's telling us there's a problem with one
+			 * of the devices, and we should ignore that file
+			 * descriptor from now on. */
 			FD_CLR(ignorefd, &devices->infds);
-		} else
+		} else /* Send LHREQ_BREAK command. */
 			write(lguest_fd, args, sizeof(args));
 	}
 }
 
+/* This routine just sets up a pipe to the Waker process. */
 static int setup_waker(int lguest_fd, struct device_list *device_list)
 {
 	int pipefd[2], child;
 
+	/* We create a pipe to talk to the waker, and also so it knows when the
+	 * Launcher dies (and closes pipe). */
 	pipe(pipefd);
 	child = fork();
 	if (child == -1)
 		err(1, "forking");
 
 	if (child == 0) {
+		/* Close the "writing" end of our copy of the pipe */
 		close(pipefd[1]);
 		wake_parent(pipefd[0], lguest_fd, device_list);
 	}
+	/* Close the reading end of our copy of the pipe. */
 	close(pipefd[0]);
 
+	/* Here is the fd used to talk to the waker. */
 	return pipefd[1];
 }
 
+/*L:210
+ * Device Handling.
+ *
+ * When the Guest sends DMA to us, it sends us an array of addresses and sizes.
+ * We need to make sure it's not trying to reach into the Launcher itself, so
+ * we have a convenient routine which check it and exits with an error message
+ * if something funny is going on:
+ */
 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. */
 	if (addr >= top || addr + size >= top)
 		errx(1, "%s:%i: Invalid address %li", __FILE__, line, addr);
+	/* We return a pointer for the caller's convenience, now we know it's
+	 * safe to use. */
 	return (void *)addr;
 }
+/* A macro which transparently hands the line number to the real function. */
 #define check_pointer(addr,size) _check_pointer(addr, size, __LINE__)
 
-/* Returns pointer to dma->used_len */
+/* The Guest has given us the address of a "struct lguest_dma".  We check it's
+ * OK and convert it to an iovec (which is a simple array of ptr/size
+ * pairs). */
 static u32 *dma2iov(unsigned long dma, struct iovec iov[], unsigned *num)
 {
 	unsigned int i;
 	struct lguest_dma *udma;
 
+	/* First we make sure that the array memory itself is valid. */
 	udma = check_pointer(dma, sizeof(*udma));
+	/* Now we check each element */
 	for (i = 0; i < LGUEST_MAX_DMA_SECTIONS; i++) {
+		/* A zero length ends the array. */
 		if (!udma->len[i])
 			break;
 
@@ -409,9 +631,15 @@ static u32 *dma2iov(unsigned long dma, struct iovec iov[], unsigned *num)
 		iov[i].iov_len = udma->len[i];
 	}
 	*num = i;
+
+	/* We return the pointer to where the caller should write the amount of
+	 * the buffer used. */
 	return &udma->used_len;
 }
 
+/* This routine gets a DMA buffer from the Guest for a given key, and converts
+ * it to an iovec array.  It returns the interrupt the Guest wants when we're
+ * finished, and a pointer to the "used_len" field to fill in. */
 static u32 *get_dma_buffer(int fd, void *key,
 			   struct iovec iov[], unsigned int *num, u32 *irq)
 {
@@ -419,16 +647,21 @@ static u32 *get_dma_buffer(int fd, void *key,
 	unsigned long udma;
 	u32 *res;
 
+	/* Ask the kernel for a DMA buffer corresponding to this key. */
 	udma = write(fd, buf, sizeof(buf));
+	/* They haven't registered any, or they're all used? */
 	if (udma == (unsigned long)-1)
 		return NULL;
 
-	/* Kernel stashes irq in ->used_len. */
+	/* Convert it into our iovec array */
 	res = dma2iov(udma, iov, num);
+	/* The kernel stashes irq in ->used_len to get it out to us. */
 	*irq = *res;
+	/* Return a pointer to ((struct lguest_dma *)udma)->used_len. */
 	return res;
 }
 
+/* This is a convenient routine to send the Guest an interrupt. */
 static void trigger_irq(int fd, u32 irq)
 {
 	u32 buf[] = { LHREQ_IRQ, irq };
@@ -436,6 +669,10 @@ static void trigger_irq(int fd, u32 irq)
 		err(1, "Triggering irq %i", irq);
 }
 
+/* This simply sets up an iovec array where we can put data to be discarded.
+ * This happens when the Guest doesn't want or can't handle the input: we have
+ * to get rid of it somewhere, and if we bury it in the ceiling space it will
+ * start to smell after a week. */
 static void discard_iovec(struct iovec *iov, unsigned int *num)
 {
 	static char discard_buf[1024];
@@ -444,19 +681,24 @@ static void discard_iovec(struct iovec *iov, unsigned int *num)
 	iov->iov_len = sizeof(discard_buf);
 }
 
+/* Here is the input terminal setting we save, and the routine to restore them
+ * on exit so the user can see what they type next. */
 static struct termios orig_term;
 static void restore_term(void)
 {
 	tcsetattr(STDIN_FILENO, TCSANOW, &orig_term);
 }
 
+/* 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? */
 	struct timeval start;
 };
 
-/* We DMA input to buffer bound at start of console page. */
+/* This is the routine which handles console input (ie. stdin). */
 static bool handle_console_input(int fd, struct device *dev)
 {
 	u32 irq = 0, *lenp;
@@ -465,24 +707,38 @@ static bool handle_console_input(int fd, struct device *dev)
 	struct iovec iov[LGUEST_MAX_DMA_SECTIONS];
 	struct console_abort *abort = dev->priv;
 
+	/* First we get the console buffer from the Guest.  The key is dev->mem
+	 * which was set to 0 in setup_console(). */
 	lenp = get_dma_buffer(fd, dev->mem, iov, &num, &irq);
 	if (!lenp) {
+		/* If it's not ready for input, warn and set up to discard. */
 		warn("console: no dma buffer!");
 		discard_iovec(iov, &num);
 	}
 
+	/* This is why we convert to iovecs: the readv() call uses them, and so
+	 * it reads straight into the Guest's buffer. */
 	len = readv(dev->fd, iov, num);
 	if (len <= 0) {
+		/* This implies that the console is closed, is /dev/null, or
+		 * something went terribly wrong.  We still go through the rest
+		 * of the logic, though, especially the exit handling below. */
 		warnx("Failed to get console input, ignoring console.");
 		len = 0;
 	}
 
+	/* If we read the data into the Guest, fill in the length and send the
+	 * interrupt. */
 	if (lenp) {
 		*lenp = len;
 		trigger_irq(fd, irq);
 	}
 
-	/* 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. */
 	if (len == 1 && ((char *)iov[0].iov_base)[0] == 3) {
 		if (!abort->count++)
 			gettimeofday(&abort->start, NULL);
@@ -490,43 +746,60 @@ static bool handle_console_input(int fd, struct device *dev)
 			struct timeval now;
 			gettimeofday(&now, NULL);
 			if (now.tv_sec <= abort->start.tv_sec+1) {
-				/* Make sure waker is not blocked in BREAK */
 				u32 args[] = { LHREQ_BREAK, 0 };
+				/* Close the fd so Waker will know it has to
+				 * exit. */
 				close(waker_fd);
+				/* Just in case waker is blocked in BREAK, send
+				 * unbreak now. */
 				write(fd, args, sizeof(args));
 				exit(2);
 			}
 			abort->count = 0;
 		}
 	} else
+		/* Any other key resets the abort counter. */
 		abort->count = 0;
 
+	/* Now, if we didn't read anything, put the input terminal back and
+	 * return failure (meaning, don't call us again). */
 	if (!len) {
 		restore_term();
 		return false;
 	}
+	/* Everything went OK! */
 	return true;
 }
 
+/* Handling console output is much simpler than input. */
 static u32 handle_console_output(int fd, const struct iovec *iov,
 				 unsigned num, struct device*dev)
 {
+	/* Whatever the Guest sends, write it to standard output.  Return the
+	 * number of bytes written. */
 	return writev(STDOUT_FILENO, iov, num);
 }
 
+/* Guest->Host network output is also pretty easy. */
 static u32 handle_tun_output(int fd, const struct iovec *iov,
 			     unsigned num, struct device *dev)
 {
-	/* Now we've seen output, we should warn if we can't get buffers. */
+	/* We put a flag in the "priv" pointer of the network device, and set
+	 * it as soon as we see output.  We'll see why in handle_tun_input() */
 	*(bool *)dev->priv = true;
+	/* Whatever packet the Guest sent us, write it out to the tun
+	 * device. */
 	return writev(dev->fd, iov, num);
 }
 
+/* This matches the peer_key() in lguest_net.c.  The key for any given slot
+ * is the address of the network device's page plus 4 * the slot number. */
 static unsigned long peer_offset(unsigned int peernum)
 {
 	return 4 * peernum;
 }
 
+/* This is where we handle a packet coming in from the tun device */
 static bool handle_tun_input(int fd, struct device *dev)
 {
 	u32 irq = 0, *lenp;
@@ -534,17 +807,28 @@ static bool handle_tun_input(int fd, struct device *dev)
 	unsigned num;
 	struct iovec iov[LGUEST_MAX_DMA_SECTIONS];
 
+	/* First we get a buffer the Guest has bound to its key. */
 	lenp = get_dma_buffer(fd, dev->mem+peer_offset(NET_PEERNUM), iov, &num,
 			      &irq);
 	if (!lenp) {
+		/* Now, it's expected that if we try to send a packet too
+		 * early, the Guest won't be ready yet.  This is why we set a
+		 * flag when the Guest sends its first packet.  If it's sent a
+		 * packet we assume it should be ready to receive them.
+		 *
+		 * Actually, this is what the status bits in the descriptor are
+		 * for: we should *use* them.  FIXME! */
 		if (*(bool *)dev->priv)
 			warn("network: no dma buffer!");
 		discard_iovec(iov, &num);
 	}
 
+	/* Read the packet from the device directly into the Guest's buffer. */
 	len = readv(dev->fd, iov, num);
 	if (len <= 0)
 		err(1, "reading network");
+
+	/* Write the used_len, and trigger the interrupt for the Guest */
 	if (lenp) {
 		*lenp = len;
 		trigger_irq(fd, irq);
@@ -552,9 +836,13 @@ static bool handle_tun_input(int fd, struct device *dev)
 	verbose("tun input packet len %i [%02x %02x] (%s)\n", len,
 		((u8 *)iov[0].iov_base)[0], ((u8 *)iov[0].iov_base)[1],
 		lenp ? "sent" : "discarded");
+	/* All good. */
 	return true;
 }
 
+/* The last device handling routine is block output: the Guest has sent a DMA
+ * to the block device.  It will have placed the command it wants in the
+ * "struct lguest_block_page". */
 static u32 handle_block_output(int fd, const struct iovec *iov,
 			       unsigned num, struct device *dev)
 {
@@ -564,36 +852,64 @@ static u32 handle_block_output(int fd, const struct iovec *iov,
 	struct iovec reply[LGUEST_MAX_DMA_SECTIONS];
 	off64_t device_len, off = (off64_t)p->sector * 512;
 
+	/* First we extract the device length from the dev->priv pointer. */
 	device_len = *(off64_t *)dev->priv;
 
+	/* We first check that the read or write is within the length of the
+	 * block file. */
 	if (off >= device_len)
 		err(1, "Bad offset %llu vs %llu", off, device_len);
+	/* Move to the right location in the block file.  This shouldn't fail,
+	 * but best to check. */
 	if (lseek64(dev->fd, off, SEEK_SET) != off)
 		err(1, "Bad seek to sector %i", p->sector);
 
 	verbose("Block: %s at offset %llu\n", p->type ? "WRITE" : "READ", off);
 
+	/* They were supposed to bind a reply buffer at key equal to the start
+	 * of the block device memory.  We need this to tell them when the
+	 * request is finished. */
 	lenp = get_dma_buffer(fd, dev->mem, reply, &reply_num, &irq);
 	if (!lenp)
 		err(1, "Block request didn't give us a dma buffer");
 
 	if (p->type) {
+		/* A write request.  The DMA they sent contained the data, so
+		 * write it out. */
 		len = writev(dev->fd, iov, num);
+		/* Grr... Now we know how long the "struct lguest_dma" they
+		 * sent was, we make sure they didn't try to write over the end
+		 * of the block file (possibly extending it). */
 		if (off + len > device_len) {
+			/* Trim it back to the correct length */
 			ftruncate(dev->fd, device_len);
+			/* Die, bad Guest, die. */
 			errx(1, "Write past end %llu+%u", off, len);
 		}
+		/* The reply length is 0: we just send back an empty DMA to
+		 * interrupt them and tell them the write is finished. */
 		*lenp = 0;
 	} else {
+		/* A read request.  They sent an empty DMA to start the
+		 * request, and we put the read contents into the reply
+		 * buffer. */
 		len = readv(dev->fd, reply, reply_num);
 		*lenp = len;
 	}
 
+	/* The result is 1 (done), 2 if there was an error (short read or
+	 * write). */
 	p->result = 1 + (p->bytes != len);
+	/* Now tell them we've used their reply buffer. */
 	trigger_irq(fd, irq);
+
+	/* We're supposed to return the number of bytes of the output buffer we
+	 * used.  But the block device uses the "result" field instead, so we
+	 * don't bother. */
 	return 0;
 }
 
+/* This is the generic routine we call when the Guest sends some DMA out. */
 static void handle_output(int fd, unsigned long dma, unsigned long key,
 			  struct device_list *devices)
 {
@@ -602,30 +918,53 @@ static void handle_output(int fd, unsigned long dma, unsigned long key,
 	struct iovec iov[LGUEST_MAX_DMA_SECTIONS];
 	unsigned num = 0;
 
+	/* Convert the "struct lguest_dma" they're sending to a "struct
+	 * iovec". */
 	lenp = dma2iov(dma, iov, &num);
+
+	/* Check each device: if they expect output to this key, tell them to
+	 * handle it. */
 	for (i = devices->dev; i; i = i->next) {
 		if (i->handle_output && key == i->watch_key) {
+			/* We write the result straight into the used_len field
+			 * for them. */
 			*lenp = i->handle_output(fd, iov, num, i);
 			return;
 		}
 	}
+
+	/* This can happen: the kernel sends any SEND_DMA which doesn't match
+	 * another Guest to us.  It could be that another Guest just left a
+	 * network, for example.  But it's unusual. */
 	warnx("Pending dma %p, key %p", (void *)dma, (void *)key);
 }
 
+/* This is called when the waker wakes us up: check for incoming file
+ * descriptors. */
 static void handle_input(int fd, struct device_list *devices)
 {
+	/* select() wants a zeroed timeval to mean "don't wait". */
 	struct timeval poll = { .tv_sec = 0, .tv_usec = 0 };
 
 	for (;;) {
 		struct device *i;
 		fd_set fds = devices->infds;
 
+		/* If nothing is ready, we're done. */
 		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.  */
 		for (i = devices->dev; i; i = i->next) {
 			if (i->handle_input && FD_ISSET(i->fd, &fds)) {
+				/* If handle_input() returns false, it means we
+				 * should no longer service it.
+				 * handle_console_input() does this. */
 				if (!i->handle_input(fd, i)) {
+					/* Clear it from the set of input file
+					 * descriptors kept at the head of the
+					 * device list. */
 					FD_CLR(i->fd, &devices->infds);
 					/* Tell waker to ignore it too... */
 					write(waker_fd, &i->fd, sizeof(i->fd));
@@ -635,6 +974,15 @@ static void handle_input(int fd, struct device_list *devices)
 	}
 }
 
+/*L:190
+ * Device Setup
+ *
+ * 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 them.
+ *
+ * This routine allocates a new "struct lguest_device_desc" from descriptor
+ * table in the devices array just above the Guest's normal memory. */
 static struct lguest_device_desc *
 new_dev_desc(struct lguest_device_desc *descs,
 	     u16 type, u16 features, u16 num_pages)
@@ -646,6 +994,8 @@ new_dev_desc(struct lguest_device_desc *descs,
 			descs[i].type = type;
 			descs[i].features = features;
 			descs[i].num_pages = num_pages;
+			/* If they said the device needs memory, we allocate
+			 * that now, bumping up the top of Guest memory. */
 			if (num_pages) {
 				map_zeroed_pages(top, num_pages);
 				descs[i].pfn = top/getpagesize();
@@ -657,6 +1007,9 @@ new_dev_desc(struct lguest_device_desc *descs,
 	errx(1, "too many devices");
 }
 
+/* This monster routine does all the creation and setup of a new device,
+ * including caling new_dev_desc() to allocate the descriptor and device
+ * memory. */
 static struct device *new_device(struct device_list *devices,
 				 u16 type, u16 num_pages, u16 features,
 				 int fd,
@@ -669,12 +1022,18 @@ static struct device *new_device(struct device_list *devices,
 {
 	struct device *dev = malloc(sizeof(*dev));
 
-	/* Append to device list. */
+	/* 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/lgba, etc. */