Boot with virtual == physical to get closer to native Linux.

[linux-2.6-omap-h63xx.git] / Documentation / lguest / lguest.c

/*P:100 This is the Launcher code, a simple program which lays out the
 * "physical" memory for the new Guest by mapping the kernel image and the
 * virtual devices, then reads repeatedly from /dev/lguest to run the Guest.
:*/
#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 <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 <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 "linux/lguest_launcher.h"
#include "asm-x86/e820.h"
/*:*/

#define PAGE_PRESENT 0x7        /* Present, RW, Execute */
#define NET_PEERNUM 1
#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

/*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 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;

/* 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;

        /* 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);

        /* 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);

        /* Device-specific data. */
        void *priv;
};

/*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 it's
 * "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). */
        addr = mmap(NULL, getpagesize() * num,
                    PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE, fd, 0);
        if (addr == MAP_FAILED)
                err(1, "Mmaping %u pages of /dev/zero", num);

        return addr;
}

/* 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;
}

/* 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(const void *start, const void *end)
{
        const void *p;

        /* The scan gives us the physical starting address.  We boot with
         * pagetables set up with virtual and physical the same, so that's
         * OK. */
        for (p = start; p < end; p++)
                if (memcmp(p, "GenuineLguest", strlen("GenuineLguest")) == 0)
                        return to_guest_phys(p + strlen("GenuineLguest"));

        errx(1, "Is this image a genuine lguest?");
}

/* 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|PROT_EXEC,
                 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)
{
        void *start = (void *)-1, *end = NULL;
        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 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 track the first and last address we mapped, so we can
                 * tell entry_point() where to scan. */
                if (from_guest_phys(phdr[i].p_paddr) < start)
                        start = from_guest_phys(phdr[i].p_paddr);
                if (from_guest_phys(phdr[i].p_paddr) + phdr[i].p_filesz > end)
                        end=from_guest_phys(phdr[i].p_paddr)+phdr[i].p_filesz;

                /* 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);
        }

        return entry_point(start, end);
}

/*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)
{
        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 = from_guest_phys(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);

        return entry_point(img, img + len);
}

/*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 char c;
        int state = 0;

        /* GZIP header is 0x1F 0x8B <method> <flags>... <compressed-by>. */
        while (read(fd, &c, 1) == 1) {
                switch (state) {
                case 0:
                        if (c == 0x1F)
                                state++;
                        break;
                case 1:
                        if (c == 0x8B)
                                state++;
                        else
                                state = 0;
                        break;
                case 2 ... 8:
                        state++;
                        break;
                case 9:
                        /* Seek back to the start of the gzip header. */
                        lseek(fd, -10, SEEK_CUR);
                        /* One final check: "compressed under UNIX". */
                        if (c != 0x03)
                                state = -1;
                        else
                                return unpack_bzimage(fd);
                }
        }
        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)
{
        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 unpack 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;
}

/* Once we know how much memory we have, we can construct simple linear page
 * tables which set virtual == physical which will get the Guest far enough
 * into the boot to create its own.
 *
 * We lay them out of the way, just below the initrd (which is why we need to
 * know its size). */
static unsigned long setup_pagetables(unsigned long mem,
                                      unsigned long initrd_size)
{
        unsigned long *pgdir, *linear;
        unsigned int mapped_pages, i, linear_pages;
        unsigned int ptes_per_page = getpagesize()/sizeof(void *);

        mapped_pages = mem/getpagesize();

        /* 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 put the toplevel page directory page at the top of memory. */
        pgdir = from_guest_phys(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);

        /* The top level points to the linear page table pages above. */
        for (i = 0; i < mapped_pages; i += ptes_per_page) {
                pgdir[i/ptes_per_page]
                        = ((to_guest_phys(linear) + i*sizeof(void *))
                           | PAGE_PRESENT);
        }

        verbose("Linear mapping of %u pages in %u pte pages at %#lx\n",
                mapped_pages, linear_pages, to_guest_phys(linear));

        /* We return the top level (guest-physical) address: the kernel needs
         * to know where it is. */
        return to_guest_phys(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;

        for (i = 0; args[i]; i++) {
                strcpy(dst+len, args[i]);
                strcat(dst+len, " ");
                len += strlen(args[i]) + 1;
        }
        /* In case it's empty. */
        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 base of guest "physical" memory, the top physical page to allow, the
 * top level pagetable and the entry point for the Guest. */
static int tell_kernel(unsigned long pgdir, unsigned long start)
{
        unsigned long args[] = { LHREQ_INITIALIZE,
                                 (unsigned long)guest_base,
                                 guest_limit / getpagesize(), pgdir, start };
        int fd;

        verbose("Guest: %p - %p (%#lx)\n",
                guest_base, guest_base + guest_limit, guest_limit);
        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)
{
        FD_SET(fd, &devices->infds);
        if (fd > devices->max_infd)
                devices->max_infd = fd;
}

/*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;
                unsigned long 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 /* 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 >= guest_limit || addr + size >= guest_limit)
                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 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__)

/* 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;

                iov[i].iov_base = check_pointer(udma->addr[i], udma->len[i]);
                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)
{
        unsigned long buf[] = { LHREQ_GETDMA, to_guest_phys(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;

        /* 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)
{
        unsigned long buf[] = { LHREQ_IRQ, irq };
        if (write(fd, buf, sizeof(buf)) != 0)
                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];
        *num = 1;
        iov->iov_base = discard_buf;
        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;
};

/* This is the routine which handles console input (ie. stdin). */
static bool handle_console_input(int fd, struct device *dev)
{
        u32 irq = 0, *lenp;
        int len;
        unsigned int num;
        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.
         *
         * 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);
                else if (abort->count == 3) {
                        struct timeval now;
                        gettimeofday(&now, NULL);
                        if (now.tv_sec <= abort->start.tv_sec+1) {
                                unsigned long 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)
{
        /* 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;
        int len;
        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);
        }
        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)
{
        struct lguest_block_page *p = dev->mem;
        u32 irq, *lenp;
        unsigned int len, reply_num;
        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)
                errx(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 */
                        ftruncate64(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)
{
        struct device *i;
        u32 *lenp;
        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));
                                }
                        }
                }
        }
}

/*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)
{
        unsigned int i;

        for (i = 0; i < LGUEST_MAX_DEVICES; i++) {
                if (!descs[i].type) {
                        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. */
                        if (num_pages) {
                                unsigned long pa;
                                pa = to_guest_phys(get_pages(num_pages));
                                descs[i].pfn = pa / getpagesize();
                        }
                        return &descs[i];
                }
        }
        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,
                                 bool (*handle_input)(int, struct device *),
                                 unsigned long watch_off,
                                 u32 (*handle_output)(int,
                                                      const struct iovec *,
                                                      unsigned,
                                                      struct device *))
{
        struct device *dev = malloc(sizeof(*dev));

        /* 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. */
        *devices->lastdev = dev;
        dev->next = NULL;
        devices->lastdev = &dev->next;

        /* Now we populate the fields one at a time. */
        dev->fd = fd;
        /* If we have an input handler for this file descriptor, then we add it
         * to the device_list's fdset and maxfd. */
        if (handle_input)
                set_fd(dev->fd, devices);
        dev->desc = new_dev_desc(devices->descs, type, features, num_pages);
        dev->mem = from_guest_phys(dev->desc->pfn * getpagesize());
        dev->handle_input = handle_input;
        dev->watch_key = to_guest_phys(dev->mem) + watch_off;
        dev->handle_output = handle_output;
        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(struct device_list *devices)
{
        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);
                /* If we exit gracefully, the original settings will be
                 * restored so the user can see what they're typing. */
                atexit(restore_term);
        }

        /* We don't currently require any memory for the console, so we ask for
         * 0 pages. */
        dev = new_device(devices, LGUEST_DEVICE_T_CONSOLE, 0, 0,
                         STDIN_FILENO, handle_console_input,
                         LGUEST_CONSOLE_DMA_KEY, handle_console_output);
        /* 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;
        verbose("device %p: console\n",
                (void *)(dev->desc->pfn * getpagesize()));
}

/* Setting up a block file is also fairly straightforward. */
static void setup_block_file(const char *filename, struct device_list *devices)
{
        int fd;
        struct device *dev;
        off64_t *device_len;
        struct lguest_block_page *p;

        /* We open with O_LARGEFILE because otherwise we get stuck at 2G.  We
         * open with O_DIRECT because otherwise our benchmarks go much too
         * fast. */
        fd = open_or_die(filename, O_RDWR|O_LARGEFILE|O_DIRECT);

        /* We want one page, and have no input handler (the block file never
         * has anything interesting to say to us).  Our timing will be quite
         * random, so it should be a reasonable randomness source. */
        dev = new_device(devices, LGUEST_DEVICE_T_BLOCK, 1,
                         LGUEST_DEVICE_F_RANDOMNESS,
                         fd, NULL, 0, handle_block_output);

        /* We store the device size in the private area */
        device_len = dev->priv = malloc(sizeof(*device_len));
        /* This is the safe way of establishing the size of our device: it
         * might be a normal file or an actual block device like /dev/hdb. */
        *device_len = lseek64(fd, 0, SEEK_END);

        /* The device memory is a "struct lguest_block_page".  It's zeroed
         * already, we just need to put in the device size.  Block devices
         * think in sectors (ie. 512 byte chunks), so we translate here. */
        p = dev->mem;
        p->num_sectors = *device_len/512;
        verbose("device %p: block %i sectors\n",
                (void *)(dev->desc->pfn * getpagesize()), p->num_sectors);
}

/*
 * Network Devices.
 *
 * Setting up network devices is quite a pain, because we have three types.
 * First, we have the inter-Guest network.  This is a file which is mapped into
 * the address space of the Guests who are on the network.  Because it is a
 * shared mapping, the same page underlies all the devices, and they can send
 * DMA to each other.
 *
 * Remember from our network driver, the Guest is told what slot in the page it
 * is to use.  We use exclusive fnctl locks to reserve a slot.  If another
 * Guest is using a slot, the lock will fail and we try another.  Because fnctl
 * locks are cleaned up automatically when we die, this cleverly means that our
 * reservation on the slot will vanish if we crash. */
static unsigned int find_slot(int netfd, const char *filename)
{
        struct flock fl;

        fl.l_type = F_WRLCK;
        fl.l_whence = SEEK_SET;
        fl.l_len = 1;
        /* Try a 1 byte lock in each possible position number */
        for (fl.l_start = 0;
             fl.l_start < getpagesize()/sizeof(struct lguest_net);
             fl.l_start++) {
                /* If we succeed, return the slot number. */
                if (fcntl(netfd, F_SETLK, &fl) == 0)
                        return fl.l_start;
        }
        errx(1, "No free slots in network file %s", filename);
}

/* This function sets up the network file */
static void setup_net_file(const char *filename,
                           struct device_list *devices)
{
        int netfd;
        struct device *dev;

        /* We don't use open_or_die() here: for friendliness we create the file
         * if it doesn't already exist. */
        netfd = open(filename, O_RDWR, 0);
        if (netfd < 0) {
                if (errno == ENOENT) {
                        netfd = open(filename, O_RDWR|O_CREAT, 0600);
                        if (netfd >= 0) {
                                /* If we succeeded, initialize the file with a
                                 * blank page. */
                                char page[getpagesize()];
                                memset(page, 0, sizeof(page));
                                write(netfd, page, sizeof(page));
                        }
                }
                if (netfd < 0)
                        err(1, "cannot open net file '%s'", filename);
        }

        /* We need 1 page, and the features indicate the slot to use and that
         * no checksum is needed.  We never touch this device again; it's
         * between the Guests on the network, so we don't register input or
         * output handlers. */
        dev = new_device(devices, LGUEST_DEVICE_T_NET, 1,
                         find_slot(netfd, filename)|LGUEST_NET_F_NOCSUM,
                         -1, NULL, 0, NULL);

        /* Map the shared file. */
        if (mmap(dev->mem, getpagesize(), PROT_READ|PROT_WRITE,
                         MAP_FIXED|MAP_SHARED, netfd, 0) != dev->mem)
                        err(1, "could not mmap '%s'", filename);
        verbose("device %p: shared net %s, peer %i\n",
                (void *)(dev->desc->pfn * getpagesize()), filename,
                dev->desc->features & ~LGUEST_NET_F_NOCSUM);
}
/*:*/

static u32 str2ip(const char *ipaddr)
{
        unsigned int byte[4];

        sscanf(ipaddr, "%u.%u.%u.%u", &byte[0], &byte[1], &byte[2], &byte[3]);
        return (byte[0] << 24) | (byte[1] << 16) | (byte[2] << 8) | byte[3];
}

/* 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_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 (in practice, the Host's slot in the network device's memory). */
static void configure_device(int fd, const char *devname, u32 ipaddr,
                             unsigned char hwaddr[6])
{
        struct ifreq ifr;
        struct sockaddr_in *sin = (struct sockaddr_in *)&ifr.ifr_addr;

        /* Don't read these incantations.  Just cut & paste them like I did! */
        memset(&ifr, 0, sizeof(ifr));
        strcpy(ifr.ifr_name, devname);
        sin->sin_family = AF_INET;
        sin->sin_addr.s_addr = htonl(ipaddr);
        if (ioctl(fd, SIOCSIFADDR, &ifr) != 0)
                err(1, "Setting %s interface address", devname);
        ifr.ifr_flags = IFF_UP;
        if (ioctl(fd, SIOCSIFFLAGS, &ifr) != 0)
                err(1, "Bringing interface %s up", devname);

        /* SIOC stands for Socket I/O Control.  G means Get (vs S for Set
         * above).  IF means Interface, and HWADDR is hardware address.
         * Simple! */
        if (ioctl(fd, SIOCGIFHWADDR, &ifr) != 0)
                err(1, "getting hw address for %s", devname);
        memcpy(hwaddr, ifr.ifr_hwaddr.sa_data, 6);
}

/*L:195 The other kind of network is a Host<->Guest network.  This can either
 * use briding 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(const char *arg, struct device_list *devices)
{
        struct device *dev;
        struct ifreq ifr;
        int netfd, ipfd;
        u32 ip;
        const char *br_name = NULL;

        /* 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);
        memset(&ifr, 0, sizeof(ifr));
        ifr.ifr_flags = IFF_TAP | IFF_NO_PI;
        strcpy(ifr.ifr_name, "tap%d");
        if (ioctl(netfd, TUNSETIFF, &ifr) != 0)
                err(1, "configuring /dev/net/tun");
        /* We don't need checksums calculated for packets coming in this
         * device: trust us! */
        ioctl(netfd, TUNSETNOCSUM, 1);

        /* We create the net device with 1 page, using the features field of
         * the descriptor to tell the Guest it is in slot 1 (NET_PEERNUM), and
         * that the device has fairly random timing.  We do *not* specify
         * LGUEST_NET_F_NOCSUM: these packets can reach the real world.
         *
         * We will put our MAC address is slot 0 for the Guest to see, so
         * it will send packets to us using the key "peer_offset(0)": */
        dev = new_device(devices, LGUEST_DEVICE_T_NET, 1,
                         NET_PEERNUM|LGUEST_DEVICE_F_RANDOMNESS, netfd,
                         handle_tun_input, peer_offset(0), handle_tun_output);

        /* We keep a flag which says whether we've seen packets come out from
         * this network device. */
        dev->priv = malloc(sizeof(bool));
        *(bool *)dev->priv = false;

        /* 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))) {
                ip = INADDR_ANY;
                br_name = arg + strlen(BRIDGE_PFX);
                add_to_bridge(ipfd, ifr.ifr_name, br_name);
        } else /* It is an IP address to set up the device with */
                ip = str2ip(arg);

        /* We are peer 0, ie. first slot, so we hand dev->mem to this routine
         * to write the MAC address at the start of the device memory.  */
        configure_device(ipfd, ifr.ifr_name, ip, dev->mem);

        /* Set "promisc" bit: we want every single packet if we're going to
         * bridge to other machines (and otherwise it doesn't matter). */
        *((u8 *)dev->mem) |= 0x1;

        close(ipfd);

        verbose("device %p: tun net %u.%u.%u.%u\n",
                (void *)(dev->desc->pfn * getpagesize()),
                (u8)(ip>>24), (u8)(ip>>16), (u8)(ip>>8), (u8)ip);
        if (br_name)
                verbose("attached to bridge: %s\n", br_name);
}
/* That's the end of device setup. */

/*L:220 Finally we reach the core of the Launcher, which runs the Guest, serves
 * its input and output, and finally, lays it to rest. */
static void __attribute__((noreturn))
run_guest(int lguest_fd, struct device_list *device_list)
{
        for (;;) {
                unsigned long args[] = { LHREQ_BREAK, 0 };
                unsigned long arr[2];
                int readval;

                /* We read from the /dev/lguest device to run the Guest. */
                readval = read(lguest_fd, arr, sizeof(arr));

                /* The read can only really return sizeof(arr) (the Guest did a
                 * SEND_DMA to us), or an error. */

                /* For a successful read, arr[0] is the address of the "struct
                 * lguest_dma", and arr[1] is the key the Guest sent to. */
                if (readval == sizeof(arr)) {
                        handle_output(lguest_fd, arr[0], arr[1], device_list);
                        continue;
                /* ENOENT means the Guest died.  Reading tells us why. */
                } else if (errno == ENOENT) {
                        char reason[1024] = { 0 };
                        read(lguest_fd, reason, sizeof(reason)-1);
                        errx(1, "%s", reason);
                /* EAGAIN means the waker wanted us to look at some input.
                 * Anything else means a bug or incompatible change. */
                } else if (errno != EAGAIN)
                        err(1, "Running guest failed");

                /* Service input, then unset the BREAK which releases
                 * the Waker. */
                handle_input(lguest_fd, device_list);
                if (write(lguest_fd, args, sizeof(args)) < 0)
                        err(1, "Resetting break");
        }
}
/*
 * This is the end of the Launcher.
 *
 * But wait!  We've seen I/O from the Launcher, and we've seen I/O from the
 * Drivers.  If we were to see the Host kernel I/O code, our understanding
 * would be complete... :*/

static struct option opts[] = {
        { "verbose", 0, NULL, 'v' },
        { "sharenet", 1, NULL, 's' },
        { "tunnet", 1, NULL, 't' },
        { "block", 1, NULL, 'b' },
        { "initrd", 1, NULL, 'i' },
        { NULL },
};
static void usage(void)
{
        errx(1, "Usage: lguest [--verbose] "
             "[--sharenet=<filename>|--tunnet=(<ipaddr>|bridge:<bridgename>)\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, top-level pagetable, code startpoint and size of the
         * (optional) initrd. */
        unsigned long mem = 0, pgdir, start, initrd_size = 0;
        /* A temporary and the /dev/lguest file descriptor. */
        int i, c, lguest_fd;
        /* The list of Guest devices, based on command line arguments. */
        struct device_list device_list;
        /* The boot information for the Guest. */
        void *boot;
        /* If they specify an initrd file to load. */
        const char *initrd_name = NULL;

        /* First we initialize the device list.  Since console and network
         * device receive input from a file descriptor, we keep an fdset
         * (infds) and the maximum fd number (max_infd) with the head of the
         * list.  We also keep a pointer to the last device, for easy appending
         * to the list. */
        device_list.max_infd = -1;
        device_list.dev = NULL;
        device_list.lastdev = &device_list.dev;
        FD_ZERO(&device_list.infds);

        /* 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();
                        device_list.descs = 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 's':
                        setup_net_file(optarg, &device_list);
                        break;
                case 't':
                        setup_tun_net(optarg, &device_list);
                        break;
                case 'b':
                        setup_block_file(optarg, &device_list);
                        break;
                case 'i':
                        initrd_name = 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(&device_list);

        /* 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. */
                *(unsigned long *)(boot+0x218) = mem - initrd_size;
                *(unsigned long *)(boot+0x21c) = initrd_size;
                /* The bootloader type 0xFF means "unknown"; that's OK. */
                *(unsigned char *)(boot+0x210) = 0xFF;
        }

        /* Set up the initial linear pagetables, starting below the initrd. */
        pgdir = setup_pagetables(mem, initrd_size);

        /* The Linux boot header contains an "E820" memory map: ours is a
         * simple, single region. */
        *(char*)(boot+E820NR) = 1;
        *((struct e820entry *)(boot+E820MAP))
                = ((struct e820entry) { 0, mem, E820_RAM });
        /* The boot header contains a command line pointer: we put the command
         * line after the boot header (at address 4096) */
        *(u32 *)(boot + 0x228) = 4096;
        concat(boot + 4096, argv+optind+2);

        /* The guest type value of "1" tells the Guest it's under lguest. */
        *(int *)(boot + 0x23c) = 1;

        /* We tell the kernel to initialize the Guest: this returns the open
         * /dev/lguest file descriptor. */
        lguest_fd = tell_kernel(pgdir, start);

        /* We fork off a child process, which wakes the Launcher whenever one
         * of the input file descriptors needs attention.  Otherwise we would
         * run the Guest until it tries to output something. */
        waker_fd = setup_waker(lguest_fd, &device_list);

        /* Finally, run the Guest.  This doesn't return. */
        run_guest(lguest_fd, &device_list);
}
/*:*/

/*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.
 */