lguest: move the lguest tool to the tools directory

This is a better location instead of having it in Documentation.

Signed-off-by: Davidlohr Bueso <dave@gnu.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (fixed compile)

5 files changed, 2261 insertions, 0 deletions

diff --git a/tools/lguest/.gitignore b/tools/lguest/.gitignore
new file mode 100644
index 000000000000..115587fd5f65
--- /dev/null
+++ b/tools/lguest/.gitignore
@@ -0,0 +1 @@
+lguest
diff --git a/tools/lguest/Makefile b/tools/lguest/Makefile
new file mode 100644
index 000000000000..0ac34206f7a7
--- /dev/null
+++ b/tools/lguest/Makefile
@@ -0,0 +1,8 @@
+# This creates the demonstration utility "lguest" which runs a Linux guest.
+# Missing headers?  Add "-I../../../include -I../../../arch/x86/include"
+CFLAGS:=-m32 -Wall -Wmissing-declarations -Wmissing-prototypes -O3 -U_FORTIFY_SOURCE
+
+all: lguest
+
+clean:
+	rm -f lguest
diff --git a/tools/lguest/extract b/tools/lguest/extract
new file mode 100644
index 000000000000..7730bb6e4b94
--- /dev/null
+++ b/tools/lguest/extract
@@ -0,0 +1,58 @@
+#! /bin/sh
+
+set -e
+
+PREFIX=$1
+shift
+
+trap 'rm -r $TMPDIR' 0
+TMPDIR=`mktemp -d`
+
+exec 3>/dev/null
+for f; do
+    while IFS="
+" read -r LINE; do
+	case "$LINE" in
+	    *$PREFIX:[0-9]*:\**)
+		NUM=`echo "$LINE" | sed "s/.*$PREFIX:\([0-9]*\).*/\1/"`
+		if [ -f $TMPDIR/$NUM ]; then
+		    echo "$TMPDIR/$NUM already exits prior to $f"
+		    exit 1
+		fi
+		exec 3>>$TMPDIR/$NUM
+		echo $f | sed 's,\.\./,,g' > $TMPDIR/.$NUM
+		/bin/echo "$LINE" | sed -e "s/$PREFIX:[0-9]*//" -e "s/:\*/*/" >&3
+		;;
+	    *$PREFIX:[0-9]*)
+		NUM=`echo "$LINE" | sed "s/.*$PREFIX:\([0-9]*\).*/\1/"`
+		if [ -f $TMPDIR/$NUM ]; then
+		    echo "$TMPDIR/$NUM already exits prior to $f"
+		    exit 1
+		fi
+		exec 3>>$TMPDIR/$NUM
+		echo $f | sed 's,\.\./,,g' > $TMPDIR/.$NUM
+		/bin/echo "$LINE" | sed "s/$PREFIX:[0-9]*//" >&3
+		;;
+	    *:\**)
+		/bin/echo "$LINE" | sed -e "s/:\*/*/" -e "s,/\*\*/,," >&3
+		echo >&3
+		exec 3>/dev/null
+		;;
+	    *)
+		/bin/echo "$LINE" >&3
+		;;
+	esac
+    done < $f
+    echo >&3
+    exec 3>/dev/null
+done
+
+LASTFILE=""
+for f in $TMPDIR/*; do
+    if [ "$LASTFILE" != $(cat $TMPDIR/.$(basename $f) ) ]; then
+	LASTFILE=$(cat $TMPDIR/.$(basename $f) )
+	echo "[ $LASTFILE ]"
+    fi
+    cat $f
+done
+
diff --git a/tools/lguest/lguest.c b/tools/lguest/lguest.c
new file mode 100644
index 000000000000..f759f4f097c7
--- /dev/null
+++ b/tools/lguest/lguest.c
@@ -0,0 +1,2065 @@
+/*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>
+#include <string.h>
+#include <unistd.h>
+#include <err.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <elf.h>
+#include <sys/mman.h>
+#include <sys/param.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/wait.h>
+#include <sys/eventfd.h>
+#include <fcntl.h>
+#include <stdbool.h>
+#include <errno.h>
+#include <ctype.h>
+#include <sys/socket.h>
+#include <sys/ioctl.h>
+#include <sys/time.h>
+#include <time.h>
+#include <netinet/in.h>
+#include <net/if.h>
+#include <linux/sockios.h>
+#include <linux/if_tun.h>
+#include <sys/uio.h>
+#include <termios.h>
+#include <getopt.h>
+#include <assert.h>
+#include <sched.h>
+#include <limits.h>
+#include <stddef.h>
+#include <signal.h>
+#include <pwd.h>
+#include <grp.h>
+
+#include <linux/virtio_config.h>
+#include <linux/virtio_net.h>
+#include <linux/virtio_blk.h>
+#include <linux/virtio_console.h>
+#include <linux/virtio_rng.h>
+#include <linux/virtio_ring.h>
+#include <asm/bootparam.h>
+#include "../../include/linux/lguest_launcher.h"
+/*L:110
+ * We can ignore the 43 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.
+ */
+typedef unsigned long long u64;
+typedef uint32_t u32;
+typedef uint16_t u16;
+typedef uint8_t u8;
+/*:*/
+
+#define BRIDGE_PFX "bridge:"
+#ifndef SIOCBRADDIF
+#define SIOCBRADDIF	0x89a2		/* add interface to bridge      */
+#endif
+/* We can have up to 256 pages for devices. */
+#define DEVICE_PAGES 256
+/* 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.
+ */
+static bool verbose;
+#define verbose(args...) \
+	do { if (verbose) printf(args); } while(0)
+/*:*/
+
+/* The pointer to the start of guest memory. */
+static void *guest_base;
+/* The maximum guest physical address allowed, and maximum possible. */
+static unsigned long guest_limit, guest_max;
+/* The /dev/lguest file descriptor. */
+static int lguest_fd;
+
+/* a per-cpu variable indicating whose vcpu is currently running */
+static unsigned int __thread cpu_id;
+
+/* This is our list of devices. */
+struct device_list {
+	/* Counter to assign interrupt numbers. */
+	unsigned int next_irq;
+
+	/* Counter to print out convenient device numbers. */
+	unsigned int device_num;
+
+	/* The descriptor page for the devices. */
+	u8 *descpage;
+
+	/* A single linked list of devices. */
+	struct device *dev;
+	/* And a pointer to the last device for easy append. */
+	struct device *lastdev;
+};
+
+/* The list of Guest devices, based on command line arguments. */
+static struct device_list devices;
+
+/* The device structure describes a single device. */
+struct device {
+	/* The linked-list pointer. */
+	struct device *next;
+
+	/* The device's descriptor, as mapped into the Guest. */
+	struct lguest_device_desc *desc;
+
+	/* We can't trust desc values once Guest has booted: we use these. */
+	unsigned int feature_len;
+	unsigned int num_vq;
+
+	/* The name of this device, for --verbose. */
+	const char *name;
+
+	/* Any queues attached to this device */
+	struct virtqueue *vq;
+
+	/* Is it operational */
+	bool running;
+
+	/* Device-specific data. */
+	void *priv;
+};
+
+/* The virtqueue structure describes a queue attached to a device. */
+struct virtqueue {
+	struct virtqueue *next;
+
+	/* Which device owns me. */
+	struct device *dev;
+
+	/* The configuration for this queue. */
+	struct lguest_vqconfig config;
+
+	/* The actual ring of buffers. */
+	struct vring vring;
+
+	/* Last available index we saw. */
+	u16 last_avail_idx;
+
+	/* How many are used since we sent last irq? */
+	unsigned int pending_used;
+
+	/* Eventfd where Guest notifications arrive. */
+	int eventfd;
+
+	/* Function for the thread which is servicing this virtqueue. */
+	void (*service)(struct virtqueue *vq);
+	pid_t thread;
+};
+
+/* Remember the arguments to the program so we can "reboot" */
+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
+ * threads and so we need to make sure that changes visible to the Guest happen
+ * in precise order.
+ */
+#define wmb() __asm__ __volatile__("" : : : "memory")
+#define mb() __asm__ __volatile__("" : : : "memory")
+
+/*
+ * 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.
+ */
+#define convert(iov, type) \
+	((type *)_convert((iov), sizeof(type), __alignof__(type), #type))
+
+static void *_convert(struct iovec *iov, size_t size, size_t align,
+		      const char *name)
+{
+	if (iov->iov_len != size)
+		errx(1, "Bad iovec size %zu for %s", iov->iov_len, name);
+	if ((unsigned long)iov->iov_base % align != 0)
+		errx(1, "Bad alignment %p for %s", iov->iov_base, name);
+	return iov->iov_base;
+}
+
+/* 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.
+ */
+#define cpu_to_le16(v16) (v16)
+#define cpu_to_le32(v32) (v32)
+#define cpu_to_le64(v64) (v64)
+#define le16_to_cpu(v16) (v16)
+#define le32_to_cpu(v32) (v32)
+#define le64_to_cpu(v64) (v64)
+
+/* Is this iovec empty? */
+static bool iov_empty(const struct iovec iov[], unsigned int num_iov)
+{
+	unsigned int i;
+
+	for (i = 0; i < num_iov; i++)
+		if (iov[i].iov_len)
+			return false;
+	return true;
+}
+
+/* Take len bytes from the front of this iovec. */
+static void iov_consume(struct iovec iov[], unsigned num_iov, unsigned len)
+{
+	unsigned int i;
+
+	for (i = 0; i < num_iov; i++) {
+		unsigned int used;
+
+		used = iov[i].iov_len < len ? iov[i].iov_len : len;
+		iov[i].iov_base += used;
+		iov[i].iov_len -= used;
+		len -= used;
+	}
+	assert(len == 0);
+}
+
+/* The device virtqueue descriptors are followed by feature bitmasks. */
+static u8 *get_feature_bits(struct device *dev)
+{
+	return (u8 *)(dev->desc + 1)
+		+ 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.
+ *
+ * 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 ==
+ * Launcher virtual with an offset.
+ *
+ * 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 its
+ * "physical" addresses:
+ */
+static void *from_guest_phys(unsigned long addr)
+{
+	return guest_base + addr;
+}
+
+static unsigned long to_guest_phys(const void *addr)
+{
+	return (addr - guest_base);
+}
+
+/*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);
+	if (fd < 0)
+		err(1, "Failed to open %s", name);
+	return fd;
+}
+
+/* map_zeroed_pages() takes a number of pages. */
+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 allocate an extra two pages PROT_NONE to act as guard
+	 * pages against read/write attempts that exceed allocated space.
+	 */
+	addr = mmap(NULL, getpagesize() * (num+2),
+		    PROT_NONE, MAP_PRIVATE, fd, 0);
+
+	if (addr == MAP_FAILED)
+		err(1, "Mmapping %u pages of /dev/zero", num);
+
+	if (mprotect(addr + getpagesize(), getpagesize() * num,
+		     PROT_READ|PROT_WRITE) == -1)
+		err(1, "mprotect rw %u pages failed", num);
+
+	/*
+	 * One neat mmap feature is that you can close the fd, and it
+	 * stays mapped.
+	 */
+	close(fd);
+
+	/* Return address after PROT_NONE page */
+	return addr + getpagesize();
+}
+
+/* Get some more pages for a device. */
+static void *get_pages(unsigned int num)
+{
+	void *addr = from_guest_phys(guest_limit);
+
+	guest_limit += num * getpagesize();
+	if (guest_limit > guest_max)
+		errx(1, "Not enough memory for devices");
+	return addr;
+}
+
+/*
+ * 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.
+ */
+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.
+	 * 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.
+	 */
+	if (mmap(addr, len, PROT_READ|PROT_WRITE,
+		 MAP_FIXED|MAP_PRIVATE, fd, offset) != MAP_FAILED)
+		return;
+
+	/* pread does a seek and a read in one shot: saves a few lines. */
+	r = pread(fd, addr, len, offset);
+	if (r != 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
+ * 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.  We use the physical address; the Guest will map itself to the
+ * virtual 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.
+	 */
+	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");
+
+	/*
+	 * 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)
+			continue;
+
+		verbose("Section %i: size %i addr %p\n",
+			i, phdr[i].p_memsz, (void *)phdr[i].p_paddr);
+
+		/* We map this section of the file at its physical address. */
+		map_at(elf_fd, from_guest_phys(phdr[i].p_paddr),
+		       phdr[i].p_offset, phdr[i].p_filesz);
+	}
+
+	/* The entry point is given in the ELF header. */
+	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.
+ *
+ * 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!
+ */
+static unsigned long load_bzimage(int fd)
+{
+	struct boot_params boot;
+	int r;
+	/* 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/boot.txt)
+	 */
+	lseek(fd, 0, SEEK_SET);
+	read(fd, &boot, sizeof(boot));
+
+	/* Inside the setup_hdr, we expect the magic "HdrS" */
+	if (memcmp(&boot.hdr.header, "HdrS", 4) != 0)
+		errx(1, "This doesn't look like a bzImage to me");
+
+	/* Skip over the extra sectors of the header. */
+	lseek(fd, (boot.hdr.setup_sects+1) * 512, SEEK_SET);
+
+	/* Now read everything into memory. in nice big chunks. */
+	while ((r = read(fd, p, 65536)) > 0)
+		p += r;
+
+	/* Finally, code32_start tells us where to enter the kernel. */
+	return boot.hdr.code32_start;
+}
+
+/*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.
+ */
+static unsigned long load_kernel(int fd)
+{
+	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);
+
+	/* Otherwise we assume it's a bzImage, and try to load it. */
+	return load_bzimage(fd);
+}
+
+/*
+ * 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));
+}
+
+/*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;
+	struct stat st;
+	unsigned long len;
+
+	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);
+
+	/*
+	 * 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.
+	 */
+	close(ifd);
+	verbose("mapped initrd %s size=%lu @ %p\n", name, len, (void*)mem-len);
+
+	/* We return the initrd size. */
+	return len;
+}
+/*:*/
+
+/*
+ * 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;
+
+	for (i = 0; args[i]; i++) {
+		if (i) {
+			strcat(dst+len, " ");
+			len++;
+		}
+		strcpy(dst+len, args[i]);
+		len += strlen(args[i]);
+	}
+	/* In case it's empty. */
+	dst[len] = '\0';
+}
+
+/*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.
+ */
+static void tell_kernel(unsigned long start)
+{
+	unsigned long args[] = { LHREQ_INITIALIZE,
+				 (unsigned long)guest_base,
+				 guest_limit / getpagesize(), start };
+	verbose("Guest: %p - %p (%#lx)\n",
+		guest_base, guest_base + guest_limit, guest_limit);
+	lguest_fd = open_or_die("/dev/lguest", O_RDWR);
+	if (write(lguest_fd, args, sizeof(args)) < 0)
+		err(1, "Writing to /dev/lguest");
+}
+/*:*/
+
+/*L:200
+ * Device Handling.
+ *
+ * When the Guest gives us a buffer, it sends 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 checks 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)
+{
+	/*
+	 * Check if the requested address and size exceeds the allocated memory,
+	 * or addr + size wraps around.
+	 */
+	if ((addr + size) > guest_limit || (addr + size) < addr)
+		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.
+	 */
+	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
+ * function returns the next descriptor in the chain, or vq->vring.num if we're
+ * at the end.
+ */
+static unsigned next_desc(struct vring_desc *desc,
+			  unsigned int i, unsigned int max)
+{
+	unsigned int next;
+
+	/* If this descriptor says it doesn't chain, we're done. */
+	if (!(desc[i].flags & VRING_DESC_F_NEXT))
+		return max;
+
+	/* Check they're not leading us off end of descriptors. */
+	next = desc[i].next;
+	/* Make sure compiler knows to grab that: we don't want it changing! */
+	wmb();
+
+	if (next >= max)
+		errx(1, "Desc next is %u", next);
+
+	return next;
+}
+
+/*
+ * 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 };
+
+	/* Don't inform them if nothing used. */
+	if (!vq->pending_used)
+		return;
+	vq->pending_used = 0;
+
+	/* If they don't want an interrupt, don't send one... */
+	if (vq->vring.avail->flags & VRING_AVAIL_F_NO_INTERRUPT) {
+		return;
+	}
+
+	/* Send the Guest an interrupt tell them we used something up. */
+	if (write(lguest_fd, buf, sizeof(buf)) != 0)
+		err(1, "Triggering irq %i", vq->config.irq);
+}
+
+/*
+ * 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 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)
+{
+	unsigned int i, head, max;
+	struct vring_desc *desc;
+	u16 last_avail = lg_last_avail(vq);
+
+	/* There's nothing available? */
+	while (last_avail == vq->vring.avail->idx) {
+		u64 event;
+
+		/*
+		 * 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.
+		 */
+		mb();
+		if (last_avail != vq->vring.avail->idx) {
+			vq->vring.used->flags |= VRING_USED_F_NO_NOTIFY;
+			break;
+		}
+
+		/* Nothing new?  Wait for eventfd to tell us they refilled. */
+		if (read(vq->eventfd, &event, sizeof(event)) != sizeof(event))
+			errx(1, "Event read failed?");
+
+		/* We don't need to be notified again. */
+		vq->vring.used->flags |= VRING_USED_F_NO_NOTIFY;
+	}
+
+	/* Check it isn't doing very strange things with descriptor numbers. */
+	if ((u16)(vq->vring.avail->idx - last_avail) > vq->vring.num)
+		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.
+	 */
+	head = vq->vring.avail->ring[last_avail % vq->vring.num];
+	lg_last_avail(vq)++;
+
+	/* If their number is silly, that's a fatal mistake. */
+	if (head >= vq->vring.num)
+		errx(1, "Guest says index %u is available", head);
+
+	/* When we start there are none of either input nor output. */
+	*out_num = *in_num = 0;
+
+	max = vq->vring.num;
+	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 (desc[i].flags & VRING_DESC_F_INDIRECT) {
+		if (desc[i].len % sizeof(struct vring_desc))
+			errx(1, "Invalid size for indirect buffer table");
+
+		max = desc[i].len / sizeof(struct vring_desc);
+		desc = check_pointer(desc[i].addr, desc[i].len);
+		i = 0;
+	}
+
+	do {
+		/* Grab the first descriptor, and check it's OK. */
+		iov[*out_num + *in_num].iov_len = desc[i].len;
+		iov[*out_num + *in_num].iov_base
+			= check_pointer(desc[i].addr, desc[i].len);
+		/* If this is an input descriptor, increment that count. */
+		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 (*in_num)
+				errx(1, "Descriptor has out after in");
+			(*out_num)++;
+		}
+
+		/* If we've got too many, that implies a descriptor loop. */
+		if (*out_num + *in_num > max)
+			errx(1, "Looped descriptor");
+	} while ((i = next_desc(desc, i, max)) != max);
+
+	return head;
+}
+
+/*
+ * 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.
+	 */
+	used = &vq->vring.used->ring[vq->vring.used->idx % vq->vring.num];
+	used->id = head;
+	used->len = len;
+	/* Make sure buffer is written before we update index. */
+	wmb();
+	vq->vring.used->idx++;
+	vq->pending_used++;
+}
+
+/* And here's the combo meal deal.  Supersize me! */
+static void add_used_and_trigger(struct virtqueue *vq, unsigned head, int len)
+{
+	add_used(vq, head, len);
+	trigger_irq(vq);
+}
+
+/*
+ * The Console
+ *
+ * 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;
+};
+
+/* This is the routine which handles console input (ie. stdin). */
+static void console_input(struct virtqueue *vq)
+{
+	int len;
+	unsigned int head, in_num, out_num;
+	struct console_abort *abort = vq->dev->priv;
+	struct iovec iov[vq->vring.num];
+
+	/* 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 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 (;;)
+			pause();
+	}
+
+	/* Tell the Guest we used a buffer. */
+	add_used_and_trigger(vq, head, len);
+
+	/*
+	 * 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) {
+		abort->count = 0;
+		return;
+	}
+
+	abort->count++;
+	if (abort->count == 1)
+		gettimeofday(&abort->start, NULL);
+	else if (abort->count == 3) {
+		struct timeval now;
+		gettimeofday(&now, NULL);
+		/* Kill all Launcher processes with SIGINT, like normal ^C */
+		if (now.tv_sec <= abort->start.tv_sec+1)
+			kill(0, SIGINT);
+		abort->count = 0;
+	}
+}
+
+/* This is the routine which handles console output (ie. stdout). */
+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) {
+			warn("Write to stdout gave %i (%d)", len, errno);
+			break;
+		}
+		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);
+}
+
+/*
+ * The Network
+ *
+ * Handling output for network is also simple: we get all the output buffers
+ * and write them to /dev/net/tun.
+ */
+struct net_info {
+	int tunfd;
+};
+
+static void net_output(struct virtqueue *vq)
+{
+	struct net_info *net_info = vq->dev->priv;
+	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)
+		warnx("Write to tun failed (%d)?", errno);
+
+	/*
+	 * Done with that one; wait_for_vq_desc() will send the interrupt if
+	 * all packets are processed.
+	 */
+	add_used(vq, head, 0);
+}
+
+/*
+ * 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;
+	struct timeval zero = { 0, 0 };
+	FD_ZERO(&fdset);
+	FD_SET(fd, &fdset);
+	return select(fd+1, &fdset, NULL, NULL, &zero) != 1;
+}
+
+/*
+ * 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;
+	unsigned int head, out, in;
+	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?");
+
+	/*
+	 * 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)
+		warn("Failed to read from tun (%d).", errno);
+
+	/*
+	 * 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: run the service routine in a loop. */
+static int do_thread(void *_vq)
+{
+	struct virtqueue *vq = _vq;
+
+	for (;;)
+		vq->service(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!
+ */
+static void kill_launcher(int signal)
+{
+	kill(0, SIGTERM);
+}
+
+static void reset_device(struct device *dev)
+{
+	struct virtqueue *vq;
+
+	verbose("Resetting device %s\n", dev->name);
+
+	/* Clear any features they've acked. */
+	memset(get_feature_bits(dev) + dev->feature_len, 0, dev->feature_len);
+
+	/* We're going to be explicitly killing threads, so ignore them. */
+	signal(SIGCHLD, SIG_IGN);
+
+	/* Zero out the virtqueues, get rid of their threads */
+	for (vq = dev->vq; vq; vq = vq->next) {
+		if (vq->thread != (pid_t)-1) {
+			kill(vq->thread, SIGTERM);
+			waitpid(vq->thread, NULL, 0);
+			vq->thread = (pid_t)-1;
+		}
+		memset(vq->vring.desc, 0,
+		       vring_size(vq->config.num, LGUEST_VRING_ALIGN));
+		lg_last_avail(vq) = 0;
+	}
+	dev->running = false;
+
+	/* Now we care if threads die. */
+	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.  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 };
+
+	/* Create a zero-initialized eventfd. */
+	vq->eventfd = eventfd(0, 0);
+	if (vq->eventfd < 0)
+		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.
+	 */
+	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.
+	 */
+	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. */
+	close(vq->eventfd);
+}
+
+static void start_device(struct device *dev)
+{
+	unsigned int i;
+	struct virtqueue *vq;
+
+	verbose("Device %s OK: offered", dev->name);
+	for (i = 0; i < dev->feature_len; i++)
+		verbose(" %02x", get_feature_bits(dev)[i]);
+	verbose(", accepted");
+	for (i = 0; i < dev->feature_len; i++)
+		verbose(" %02x", get_feature_bits(dev)
+			[dev->feature_len+i]);
+
+	for (vq = dev->vq; vq; vq = vq->next) {
+		if (vq->service)
+			create_thread(vq);
+	}
+	dev->running = true;
+}
+
+static void cleanup_devices(void)
+{
+	struct device *dev;
+
+	for (dev = devices.dev; dev; dev = dev->next)
+		reset_device(dev);
+
+	/* If we saved off the original terminal settings, restore them now. */
+	if (orig_term.c_lflag & (ISIG|ICANON|ECHO))
+		tcsetattr(STDIN_FILENO, TCSANOW, &orig_term);
+}
+
+/* When the Guest tells us they updated the status field, we handle it. */
+static void update_device_status(struct device *dev)
+{
+	/* A zero status is a reset, otherwise it's a set of flags. */
+	if (dev->desc->status == 0)
+		reset_device(dev);
+	else if (dev->desc->status & VIRTIO_CONFIG_S_FAILED) {
+		warnx("Device %s configuration FAILED", dev->name);
+		if (dev->running)
+			reset_device(dev);
+	} else {
+		if (dev->running)
+			err(1, "Device %s features finalized twice", dev->name);
+		start_device(dev);
+	}
+}
+
+/*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;
+
+	/* Check each device. */
+	for (i = devices.dev; i; i = i->next) {
+		struct virtqueue *vq;
+
+		/*
+		 * Notifications to device descriptors mean they updated the
+		 * device status.
+		 */
+		if (from_guest_phys(addr) == i->desc) {
+			update_device_status(i);
+			return;
+		}
+
+		/* Devices should not be used before features are finalized. */
+		for (vq = i->vq; vq; vq = vq->next) {
+			if (addr != vq->config.pfn*getpagesize())
+				continue;
+			errx(1, "Notification on %s before setup!", i->name);
+		}
+	}
+
+	/*
+	 * 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);
+
+	write(STDOUT_FILENO, from_guest_phys(addr),
+	      strnlen(from_guest_phys(addr), guest_limit - addr));
+}
+
+/*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 and manage them.
+ */
+
+/*
+ * The layout of the device page is a "struct lguest_device_desc" followed by a
+ * number of virtqueue descriptors, then two sets of feature bits, then an
+ * array of configuration bytes.  This routine returns the configuration
+ * pointer.
+ */
+static u8 *device_config(const struct device *dev)
+{
+	return (void *)(dev->desc + 1)
+		+ dev->num_vq * sizeof(struct lguest_vqconfig)
+		+ dev->feature_len * 2;
+}
+
+/*
+ * 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.
+ */
+static struct lguest_device_desc *new_dev_desc(u16 type)
+{
+	struct lguest_device_desc d = { .type = type };
+	void *p;
+
+	/* Figure out where the next device config is, based on the last one. */
+	if (devices.lastdev)
+		p = device_config(devices.lastdev)
+			+ devices.lastdev->desc->config_len;
+	else
+		p = devices.descpage;
+
+	/* We only have one page for all the descriptors. */
+	if (p + sizeof(d) > (void *)devices.descpage + getpagesize())
+		errx(1, "Too many devices");
+
+	/* p might not be aligned, so we memcpy in. */
+	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.
+ */
+static void add_virtqueue(struct device *dev, unsigned int num_descs,
+			  void (*service)(struct virtqueue *))
+{
+	unsigned int pages;
+	struct virtqueue **i, *vq = malloc(sizeof(*vq));
+	void *p;
+
+	/* First we need some memory for this virtqueue. */
+	pages = (vring_size(num_descs, LGUEST_VRING_ALIGN) + getpagesize() - 1)
+		/ getpagesize();
+	p = get_pages(pages);
+
+	/* Initialize the virtqueue */
+	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;
+
+	/* Initialize the configuration. */
+	vq->config.num = num_descs;
+	vq->config.irq = devices.next_irq++;
+	vq->config.pfn = to_guest_phys(p) / getpagesize();
+
+	/* Initialize the vring. */
+	vring_init(&vq->vring, num_descs, p, LGUEST_VRING_ALIGN);
+
+	/*
+	 * 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.
+	 */
+	assert(dev->desc->config_len == 0 && dev->desc->feature_len == 0);
+	memcpy(device_config(dev), &vq->config, sizeof(vq->config));
+	dev->num_vq++;
+	dev->desc->num_vq++;
+
+	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.
+	 */
+	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.
+ */
+static void add_feature(struct device *dev, unsigned bit)
+{
+	u8 *features = get_feature_bits(dev);
+
+	/* We can't extend the feature bits once we've added config bytes */
+	if (dev->desc->feature_len <= bit / CHAR_BIT) {
+		assert(dev->desc->config_len == 0);
+		dev->feature_len = dev->desc->feature_len = (bit/CHAR_BIT) + 1;
+	}
+
+	features[bit / CHAR_BIT] |= (1 << (bit % CHAR_BIT));
+}
+
+/*
+ * 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.
+ */
+static void set_config(struct device *dev, unsigned len, const void *conf)
+{
+	/* Check we haven't overflowed our single page. */
+	if (device_config(dev) + len > devices.descpage + getpagesize())
+		errx(1, "Too many devices");
+
+	/* 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.  We
+ * don't actually start the service threads until later.
+ *
+ * See what I mean about userspace being boring?
+ */
+static struct device *new_device(const char *name, u16 type)
+{
+	struct device *dev = malloc(sizeof(*dev));
+
+	/* Now we populate the fields one at a time. */
+	dev->desc = new_dev_desc(type);
+	dev->name = name;
+	dev->vq = NULL;
+	dev->feature_len = 0;
+	dev->num_vq = 0;
+	dev->running = false;
+
+	/*
+	 * 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.
+	 */
+	if (devices.lastdev)
+		devices.lastdev->next = dev;
+	else
+		devices.dev = dev;
+	devices.lastdev = dev;
+
+	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.
+ */
+static void setup_console(void)
+{
+	struct device *dev;
+
+	/* 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.
+		 */
+		term.c_lflag &= ~(ISIG|ICANON|ECHO);
+		tcsetattr(STDIN_FILENO, TCSANOW, &term);
+	}
+
+	dev = new_device("console", VIRTIO_ID_CONSOLE);
+
+	/* We store the console state in dev->priv, and initialize it. */
+	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
+	 * 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.
+	 */
+	add_virtqueue(dev, VIRTQUEUE_NUM, console_input);
+	add_virtqueue(dev, VIRTQUEUE_NUM, console_output);
+
+	verbose("device %u: console\n", ++devices.device_num);
+}
+/*:*/
+
+/*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.
+ *
+ * More sophisticated is to use one of the tools developed for project like UML
+ * to do networking.
+ *
+ * Faster is to do virtio bonding in kernel.  Doing this 1:1 would be
+ * completely generic ("here's my vring, attach to your vring") and would work
+ * for any traffic.  Of course, namespace and permissions issues need to be
+ * dealt with.  A more sophisticated "multi-channel" virtio_net.c could hide
+ * multiple inter-guest channels behind one interface, although it would
+ * require some manner of hotplugging new virtio channels.
+ *
+ * Finally, we could use a virtio network switch in the kernel, ie. vhost.
+:*/
+
+static u32 str2ip(const char *ipaddr)
+{
+	unsigned int b[4];
+
+	if (sscanf(ipaddr, "%u.%u.%u.%u", &b[0], &b[1], &b[2], &b[3]) != 4)
+		errx(1, "Failed to parse IP address '%s'", ipaddr);
+	return (b[0] << 24) | (b[1] << 16) | (b[2] << 8) | b[3];
+}
+
+static void str2mac(const char *macaddr, unsigned char mac[6])
+{
+	unsigned int m[6];
+	if (sscanf(macaddr, "%02x:%02x:%02x:%02x:%02x:%02x",
+		   &m[0], &m[1], &m[2], &m[3], &m[4], &m[5]) != 6)
+		errx(1, "Failed to parse mac address '%s'", macaddr);
+	mac[0] = m[0];
+	mac[1] = m[1];
+	mac[2] = m[2];
+	mac[3] = m[3];
+	mac[4] = m[4];
+	mac[5] = m[5];
+}
+
+/*
+ * 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.
+ */
+static void add_to_bridge(int fd, const char *if_name, const char *br_name)
+{
+	int ifidx;
+	struct ifreq ifr;
+
+	if (!*br_name)
+		errx(1, "must specify bridge name");
+
+	ifidx = if_nametoindex(if_name);
+	if (!ifidx)
+		errx(1, "interface %s does not exist!", if_name);
+
+	strncpy(ifr.ifr_name, br_name, IFNAMSIZ);
+	ifr.ifr_name[IFNAMSIZ-1] = '\0';
+	ifr.ifr_ifindex = ifidx;
+	if (ioctl(fd, SIOCBRADDIF, &ifr) < 0)
+		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
+ * it up so packets will flow, the copies the MAC address into the hwaddr
+ * pointer.
+ */
+static void configure_device(int fd, const char *tapif, u32 ipaddr)
+{
+	struct ifreq ifr;
+	struct sockaddr_in sin;
+
+	memset(&ifr, 0, sizeof(ifr));
+	strcpy(ifr.ifr_name, tapif);
+
+	/* Don't read these incantations.  Just cut & paste them like I did! */
+	sin.sin_family = AF_INET;
+	sin.sin_addr.s_addr = htonl(ipaddr);
+	memcpy(&ifr.ifr_addr, &sin, sizeof(sin));
+	if (ioctl(fd, SIOCSIFADDR, &ifr) != 0)
+		err(1, "Setting %s interface address", tapif);
+	ifr.ifr_flags = IFF_UP;
+	if (ioctl(fd, SIOCSIFFLAGS, &ifr) != 0)
+		err(1, "Bringing interface %s up", tapif);
+}
+
+static int get_tun_device(char tapif[IFNAMSIZ])
+{
+	struct ifreq ifr;
+	int netfd;
+
+	/* 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
+	 * 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!
+	 */
+	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");
+	if (ioctl(netfd, TUNSETIFF, &ifr) != 0)
+		err(1, "configuring /dev/net/tun");
+
+	if (ioctl(netfd, TUNSETOFFLOAD,
+		  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!
+	 */
+	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
+ * 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.
+ */
+static void setup_tun_net(char *arg)
+{
+	struct device *dev;
+	struct net_info *net_info = malloc(sizeof(*net_info));
+	int ipfd;
+	u32 ip = INADDR_ANY;
+	bool bridging = false;
+	char tapif[IFNAMSIZ], *p;
+	struct virtio_net_config conf;
+
+	net_info->tunfd = get_tun_device(tapif);
+
+	/* First we create a new network device. */
+	dev = new_device("net", VIRTIO_ID_NET);
+	dev->priv = net_info;
+
+	/* 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!
+	 */
+	ipfd = socket(PF_INET, SOCK_DGRAM, IPPROTO_IP);
+	if (ipfd < 0)
+		err(1, "opening IP socket");
+
+	/* If the command line was --tunnet=bridge:<name> do bridging. */
+	if (!strncmp(BRIDGE_PFX, arg, strlen(BRIDGE_PFX))) {
+		arg += strlen(BRIDGE_PFX);
+		bridging = true;
+	}
+
+	/* A mac address may follow the bridge name or IP address */
+	p = strchr(arg, ':');
+	if (p) {
+		str2mac(p+1, conf.mac);
+		add_feature(dev, VIRTIO_NET_F_MAC);
+		*p = '\0';
+	}
+
+	/* arg is now either an IP address or a bridge name */
+	if (bridging)
+		add_to_bridge(ipfd, tapif, arg);
+	else
+		ip = str2ip(arg);
+
+	/* Set up the tun device. */
+	configure_device(ipfd, tapif, ip);
+
+	/* Expect Guest to handle everything except UFO */
+	add_feature(dev, VIRTIO_NET_F_CSUM);
+	add_feature(dev, VIRTIO_NET_F_GUEST_CSUM);
+	add_feature(dev, VIRTIO_NET_F_GUEST_TSO4);
+	add_feature(dev, VIRTIO_NET_F_GUEST_TSO6);
+	add_feature(dev, VIRTIO_NET_F_GUEST_ECN);
+	add_feature(dev, VIRTIO_NET_F_HOST_TSO4);
+	add_feature(dev, VIRTIO_NET_F_HOST_TSO6);
+	add_feature(dev, VIRTIO_NET_F_HOST_ECN);
+	/* We handle indirect ring entries */
+	add_feature(dev, VIRTIO_RING_F_INDIRECT_DESC);
+	set_config(dev, sizeof(conf), &conf);
+
+	/* We don't need the socket any more; setup is done. */
+	close(ipfd);
+
+	devices.device_num++;
+
+	if (bridging)
+		verbose("device %u: tun %s attached to bridge: %s\n",
+			devices.device_num, tapif, arg);
+	else
+		verbose("device %u: tun %s: %s\n",
+			devices.device_num, tapif, arg);
+}
+/*:*/
+
+/* This hangs off device->priv. */
+struct vblk_info {
+	/* The size of the file. */
+	off64_t len;
+
+	/* The file descriptor for the file. */
+	int fd;
+
+};
+
+/*L:210
+ * The Disk
+ *
+ * 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)
+{
+	struct vblk_info *vblk = vq->dev->priv;
+	unsigned int head, out_num, in_num, wlen;
+	int ret;
+	u8 *in;
+	struct virtio_blk_outhdr *out;
+	struct iovec iov[vq->vring.num];
+	off64_t off;
+
+	/*
+	 * 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
+	 * (detailing the location on disk and the type of request) and one
+	 * 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;
+
+	/*
+	 * 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.
+		 */
+		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
+		 * make sure they didn't try to write over the end of the block
+		 * file (possibly extending it).
+		 */
+		if (ret > 0 && off + ret > vblk->len) {
+			/* Trim it back to the correct length */
+			ftruncate64(vblk->fd, vblk->len);
+			/* Die, bad Guest, die. */
+			errx(1, "Write past end %llu+%u", off, ret);
+		}
+
+		wlen = sizeof(*in);
+		*in = (ret >= 0 ? VIRTIO_BLK_S_OK : VIRTIO_BLK_S_IOERR);
+	} else if (out->type & VIRTIO_BLK_T_FLUSH) {
+		/* Flush */
+		ret = fdatasync(vblk->fd);
+		verbose("FLUSH fdatasync: %i\n", ret);
+		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.
+		 */
+		if (lseek64(vblk->fd, off, SEEK_SET) != off)
+			err(1, "Bad seek to sector %llu", out->sector);
+
+		ret = readv(vblk->fd, iov+1, in_num-1);
+		verbose("READ from sector %llu: %i\n", out->sector, ret);
+		if (ret >= 0) {
+			wlen = sizeof(*in) + ret;
+			*in = VIRTIO_BLK_S_OK;
+		} else {
+			wlen = sizeof(*in);
+			*in = VIRTIO_BLK_S_IOERR;
+		}
+	}
+
+	/* Finished that request. */
+	add_used(vq, head, wlen);
+}
+
+/*L:198 This actually sets up a virtual block device. */
+static void setup_block_file(const char *filename)
+{
+	struct device *dev;
+	struct vblk_info *vblk;
+	struct virtio_blk_config conf;
+
+	/* Creat the device. */
+	dev = new_device("block", VIRTIO_ID_BLOCK);
+
+	/* The device has one virtqueue, where the Guest places requests. */
+	add_virtqueue(dev, VIRTQUEUE_NUM, blk_request);
+
+	/* Allocate the room for our own bookkeeping */
+	vblk = dev->priv = malloc(sizeof(*vblk));
+
+	/* First we open the file and store the length. */
+	vblk->fd = open_or_die(filename, O_RDWR|O_LARGEFILE);
+	vblk->len = lseek64(vblk->fd, 0, SEEK_END);
+
+	/* We support FLUSH. */
+	add_feature(dev, VIRTIO_BLK_F_FLUSH);
+
+	/* 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.
+	 */
+	add_feature(dev, VIRTIO_BLK_F_SEG_MAX);
+	conf.seg_max = cpu_to_le32(VIRTQUEUE_NUM - 2);
+
+	/* 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));
+}
+
+/*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.
+ */
+struct rng_info {
+	int rfd;
+};
+
+static void rng_input(struct virtqueue *vq)
+{
+	int len;
+	unsigned int head, in_num, out_num, totlen = 0;
+	struct rng_info *rng_info = vq->dev->priv;
+	struct iovec iov[vq->vring.num];
+
+	/* First we need a buffer from the Guests's virtqueue. */
+	head = wait_for_vq_desc(vq, iov, &out_num, &in_num);
+	if (out_num)
+		errx(1, "Output buffers in rng?");
+
+	/*
+	 * 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)
+			err(1, "Read from /dev/random gave %i", len);
+		iov_consume(iov, in_num, len);
+		totlen += len;
+	}
+
+	/* Tell the Guest about the new input. */
+	add_used(vq, head, totlen);
+}
+
+/*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);
+
+	/* Create the new device. */
+	dev = new_device("rng", VIRTIO_ID_RNG);
+	dev->priv = rng_info;
+
+	/* The device has one virtqueue, where the Guest places inbufs. */
+	add_virtqueue(dev, VIRTQUEUE_NUM, rng_input);
+
+	verbose("device %u: rng\n", devices.device_num++);
+}
+/* That's the end of device setup. */
+
+/*L:230 Reboot is pretty easy: clean up and exec() the Launcher afresh. */
+static void __attribute__((noreturn)) restart_guest(void)
+{
+	unsigned int i;
+
+	/*
+	 * Since we don't track all open fds, we simply close everything beyond
+	 * stderr.
+	 */
+	for (i = 3; i < FD_SETSIZE; i++)
+		close(i);
+
+	/* Reset all the devices (kills all threads). */
+	cleanup_devices();
+
+	execv(main_args[0], main_args);
+	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.
+ */
+static void __attribute__((noreturn)) run_guest(void)
+{
+	for (;;) {
+		unsigned long notify_addr;
+		int readval;
+
+		/* We read from the /dev/lguest device to run the Guest. */
+		readval = pread(lguest_fd, &notify_addr,
+				sizeof(notify_addr), cpu_id);
+
+		/* One unsigned long means the Guest did HCALL_NOTIFY */
+		if (readval == sizeof(notify_addr)) {
+			verbose("Notify on address %#lx\n", notify_addr);
+			handle_output(notify_addr);
+		/* ENOENT means the Guest died.  Reading tells us why. */
+		} else if (errno == ENOENT) {
+			char reason[1024] = { 0 };
+			pread(lguest_fd, reason, sizeof(reason)-1, cpu_id);
+			errx(1, "%s", reason);
+		/* ERESTART means that we need to reboot the guest */
+		} else if (errno == ERESTART) {
+			restart_guest();
+		/* Anything else means a bug or incompatible change. */
+		} else
+			err(1, "Running guest failed");
+	}
+}
+/*L:240
+ * This is the end of the Launcher.  The good news: we are over halfway
+ * through!  The bad news: the most fiendish part of the code still lies ahead
+ * of us.
+ *
+ * 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' },
+	{ "tunnet", 1, NULL, 't' },
+	{ "block", 1, NULL, 'b' },
+	{ "rng", 0, NULL, 'r' },
+	{ "initrd", 1, NULL, 'i' },
+	{ "username", 1, NULL, 'u' },
+	{ "chroot", 1, NULL, 'c' },
+	{ NULL },
+};
+static void usage(void)
+{
+	errx(1, "Usage: lguest [--verbose] "
+	     "[--tunnet=(<ipaddr>:<macaddr>|bridge:<bridgename>:<macaddr>)\n"
+	     "|--block=<filename>|--initrd=<filename>]...\n"
+	     "<mem-in-mb> vmlinux [args...]");
+}
+
+/*L:105 The main routine is where the real work begins: */
+int main(int argc, char *argv[])
+{
+	/* Memory, code startpoint and size of the (optional) initrd. */
+	unsigned long mem = 0, start, initrd_size = 0;
+	/* Two temporaries. */
+	int i, c;
+	/* The boot information for the Guest. */
+	struct boot_params *boot;
+	/* If they specify an initrd file to load. */
+	const char *initrd_name = NULL;
+
+	/* Password structure for initgroups/setres[gu]id */
+	struct passwd *user_details = NULL;
+
+	/* Directory to chroot to */
+	char *chroot_path = NULL;
+
+	/* 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
+	 * device, and the next interrupt number to use for devices (1:
+	 * 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
+	 * 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.
+	 */
+	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
+			 * guest-physical memory range.  This fills it with 0,
+			 * and ensures that the Guest won't be killed when it
+			 * tries to access it.
+			 */
+			guest_base = map_zeroed_pages(mem / getpagesize()
+						      + DEVICE_PAGES);
+			guest_limit = mem;
+			guest_max = mem + DEVICE_PAGES*getpagesize();
+			devices.descpage = get_pages(1);
+			break;
+		}
+	}
+
+	/* The options are fairly straight-forward */
+	while ((c = getopt_long(argc, argv, "v", opts, NULL)) != EOF) {
+		switch (c) {
+		case 'v':
+			verbose = true;
+			break;
+		case 't':
+			setup_tun_net(optarg);
+			break;
+		case 'b':
+			setup_block_file(optarg);
+			break;
+		case 'r':
+			setup_rng();
+			break;
+		case 'i':
+			initrd_name = optarg;
+			break;
+		case 'u':
+			user_details = getpwnam(optarg);
+			if (!user_details)
+				err(1, "getpwnam failed, incorrect username?");
+			break;
+		case 'c':
+			chroot_path = optarg;
+			break;
+		default:
+			warnx("Unknown argument %s", argv[optind]);
+			usage();
+		}
+	}
+	/*
+	 * After the other arguments we expect memory and kernel image name,
+	 * followed by command line arguments for the kernel.
+	 */
+	if (optind + 2 > argc)
+		usage();
+
+	verbose("Guest base is at %p\n", guest_base);
+
+	/* We always have a console device */
+	setup_console();
+
+	/* Now we load the kernel */
+	start = load_kernel(open_or_die(argv[optind+1], O_RDONLY));
+
+	/* Boot information is stashed at physical address 0 */
+	boot = from_guest_phys(0);
+
+	/* 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.
+		 */
+		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.
+	 */
+	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.
+	 */
+	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);
+
+	/* Set kernel alignment to 16M (CONFIG_PHYSICAL_ALIGN) */
+	boot->hdr.kernel_alignment = 0x1000000;
+
+	/* Boot protocol version: 2.07 supports the fields for lguest. */
+	boot->hdr.version = 0x207;
+
+	/* The hardware_subarch value of "1" tells the Guest it's an lguest. */
+	boot->hdr.hardware_subarch = 1;
+
+	/* Tell the entry path not to try to reload segment registers. */
+	boot->hdr.loadflags |= KEEP_SEGMENTS;
+
+	/* We tell the kernel to initialize the Guest. */
+	tell_kernel(start);
+
+	/* 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. */
+	atexit(cleanup_devices);
+
+	/* If requested, chroot to a directory */
+	if (chroot_path) {
+		if (chroot(chroot_path) != 0)
+			err(1, "chroot(\"%s\") failed", chroot_path);
+
+		if (chdir("/") != 0)
+			err(1, "chdir(\"/\") failed");
+
+		verbose("chroot done\n");
+	}
+
+	/* If requested, drop privileges */
+	if (user_details) {
+		uid_t u;
+		gid_t g;
+
+		u = user_details->pw_uid;
+		g = user_details->pw_gid;
+
+		if (initgroups(user_details->pw_name, g) != 0)
+			err(1, "initgroups failed");
+
+		if (setresgid(g, g, g) != 0)
+			err(1, "setresgid failed");
+
+		if (setresuid(u, u, u) != 0)
+			err(1, "setresuid failed");
+
+		verbose("Dropping privileges completed\n");
+	}
+
+	/* Finally, run the Guest.  This doesn't return. */
+	run_guest();
+}
+/*:*/
+
+/*M:999
+ * Mastery is done: you now know everything I do.
+ *
+ * But surely you have seen code, features and bugs in your wanderings which
+ * you now yearn to attack?  That is the real game, and I look forward to you
+ * patching and forking lguest into the Your-Name-Here-visor.
+ *
+ * Farewell, and good coding!
+ * Rusty Russell.
+ */
diff --git a/tools/lguest/lguest.txt b/tools/lguest/lguest.txt
new file mode 100644
index 000000000000..bff0c554485d
--- /dev/null
+++ b/tools/lguest/lguest.txt
@@ -0,0 +1,129 @@
+      __
+ (___()'`;  Rusty's Remarkably Unreliable Guide to Lguest
+ /,    /`      - or, A Young Coder's Illustrated Hypervisor
+ \\"--\\    http://lguest.ozlabs.org
+
+Lguest is designed to be a minimal 32-bit x86 hypervisor for the Linux kernel,
+for Linux developers and users to experiment with virtualization with the
+minimum of complexity.  Nonetheless, it should have sufficient features to
+make it useful for specific tasks, and, of course, you are encouraged to fork
+and enhance it (see drivers/lguest/README).
+
+Features:
+
+- Kernel module which runs in a normal kernel.
+- Simple I/O model for communication.
+- Simple program to create new guests.
+- Logo contains cute puppies: http://lguest.ozlabs.org
+
+Developer features:
+
+- Fun to hack on.
+- No ABI: being tied to a specific kernel anyway, you can change anything.
+- Many opportunities for improvement or feature implementation.
+
+Running Lguest:
+
+- The easiest way to run lguest is to use same kernel as guest and host.
+  You can configure them differently, but usually it's easiest not to.
+
+  You will need to configure your kernel with the following options:
+
+  "General setup":
+     "Prompt for development and/or incomplete code/drivers" = Y
+        (CONFIG_EXPERIMENTAL=y)
+
+  "Processor type and features":
+     "Paravirtualized guest support" = Y
+        "Lguest guest support" = Y
+     "High Memory Support" = off/4GB
+     "Alignment value to which kernel should be aligned" = 0x100000
+        (CONFIG_PARAVIRT=y, CONFIG_LGUEST_GUEST=y, CONFIG_HIGHMEM64G=n and
+         CONFIG_PHYSICAL_ALIGN=0x100000)
+
+  "Device Drivers":
+     "Block devices"
+        "Virtio block driver (EXPERIMENTAL)" = M/Y
+     "Network device support"
+        "Universal TUN/TAP device driver support" = M/Y
+        "Virtio network driver (EXPERIMENTAL)" = M/Y
+           (CONFIG_VIRTIO_BLK=m, CONFIG_VIRTIO_NET=m and CONFIG_TUN=m)
+
+  "Virtualization"
+     "Linux hypervisor example code" = M/Y
+        (CONFIG_LGUEST=m)
+
+- A tool called "lguest" is available in this directory: type "make"
+  to build it.  If you didn't build your kernel in-tree, use "make
+  O=<builddir>".
+
+- Create or find a root disk image.  There are several useful ones
+  around, such as the xm-test tiny root image at
+	  http://xm-test.xensource.com/ramdisks/initrd-1.1-i386.img
+
+  For more serious work, I usually use a distribution ISO image and
+  install it under qemu, then make multiple copies:
+
+	  dd if=/dev/zero of=rootfile bs=1M count=2048
+	  qemu -cdrom image.iso -hda rootfile -net user -net nic -boot d
+
+  Make sure that you install a getty on /dev/hvc0 if you want to log in on the
+  console!
+
+- "modprobe lg" if you built it as a module.
+
+- Run an lguest as root:
+
+      Documentation/virtual/lguest/lguest 64 vmlinux --tunnet=192.168.19.1 \
+        --block=rootfile root=/dev/vda
+
+   Explanation:
+    64: the amount of memory to use, in MB.
+
+    vmlinux: the kernel image found in the top of your build directory.  You
+       can also use a standard bzImage.
+
+    --tunnet=192.168.19.1: configures a "tap" device for networking with this
+       IP address.
+
+    --block=rootfile: a file or block device which becomes /dev/vda
+       inside the guest.
+
+    root=/dev/vda: this (and anything else on the command line) are
+       kernel boot parameters.
+
+- Configuring networking.  I usually have the host masquerade, using
+  "iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE" and "echo 1 >
+  /proc/sys/net/ipv4/ip_forward".  In this example, I would configure
+  eth0 inside the guest at 192.168.19.2.
+
+  Another method is to bridge the tap device to an external interface
+  using --tunnet=bridge:<bridgename>, and perhaps run dhcp on the guest
+  to obtain an IP address.  The bridge needs to be configured first:
+  this option simply adds the tap interface to it.
+
+  A simple example on my system:
+
+    ifconfig eth0 0.0.0.0
+    brctl addbr lg0
+    ifconfig lg0 up
+    brctl addif lg0 eth0
+    dhclient lg0
+
+  Then use --tunnet=bridge:lg0 when launching the guest.
+
+  See:
+  
+    http://www.linuxfoundation.org/collaborate/workgroups/networking/bridge
+    
+  for general information on how to get bridging to work.
+
+- Random number generation. Using the --rng option will provide a
+  /dev/hwrng in the guest that will read from the host's /dev/random.
+  Use this option in conjunction with rng-tools (see ../hw_random.txt)
+  to provide entropy to the guest kernel's /dev/random.
+
+There is a helpful mailing list at http://ozlabs.org/mailman/listinfo/lguest
+
+Good luck!
+Rusty Russell rusty@rustcorp.com.au.