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    Subject[PATCH 3/7] lguest documentation:Chapter III
    From
    Date
    Documentation: The Drivers

    Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
    ---
    drivers/block/lguest_blk.c | 171 +++++++++++++++++++++++++++---
    drivers/char/hvc_lguest.c | 77 +++++++++++++
    drivers/lguest/lguest_bus.c | 72 ++++++++++++
    drivers/net/lguest_net.c | 222 +++++++++++++++++++++++++++++++++++----
    include/linux/lguest_bus.h | 5
    include/linux/lguest_launcher.h | 60 ++++++++++
    6 files changed, 565 insertions(+), 42 deletions(-)

    ===================================================================
    --- a/drivers/block/lguest_blk.c
    +++ b/drivers/block/lguest_blk.c
    @@ -1,6 +1,12 @@
    -/* A simple block driver for lguest.
    - *
    - * Copyright 2006 Rusty Russell <rusty@rustcorp.com.au> IBM Corporation
    +/*D:400
    + * The Guest block driver
    + *
    + * This is a simple block driver, which appears as /dev/lgba, lgbb, lgbc etc.
    + * The mechanism is simple: we place the information about the request in the
    + * device page, then use SEND_DMA (containing the data for a write, or an empty
    + * "ping" DMA for a read).
    + :*/
    +/* Copyright 2006 Rusty Russell <rusty@rustcorp.com.au> IBM Corporation
    *
    * This program is free software; you can redistribute it and/or modify
    * it under the terms of the GNU General Public License as published by
    @@ -25,27 +31,50 @@

    static char next_block_index = 'a';

    +/*D:420 Here is the structure which holds all the information we need about
    + * each Guest block device.
    + *
    + * I'm sure at this stage, you're wondering "hey, where was the adventure I was
    + * promised?" and thinking "Rusty sucks, I shall say nasty things about him on
    + * my blog". I think Real adventures have boring bits, too, and you're in the
    + * middle of one. But it gets better. Just not quite yet. */
    struct blockdev
    {
    + /* The block queue infrastructure wants a spinlock: it is held while it
    + * calls our block request function. We grab it in our interrupt
    + * handler so the responses don't mess with new requests. */
    spinlock_t lock;

    - /* The disk structure for the kernel. */
    + /* The disk structure registered with kernel. */
    struct gendisk *disk;

    - /* The major number for this disk. */
    + /* The major device number for this disk, and the interrupt. We only
    + * really keep them here for completeness; we'd need them if we
    + * supported device unplugging. */
    int major;
    int irq;

    + /* The physical address of this device's memory page */
    unsigned long phys_addr;
    - /* The mapped block page. */
    + /* The mapped memory page for convenient acces. */
    struct lguest_block_page *lb_page;

    - /* We only have a single request outstanding at a time. */
    + /* We only have a single request outstanding at a time: this is it. */
    struct lguest_dma dma;
    struct request *req;
    };

    -/* Jens gave me this nice helper to end all chunks of a request. */
    +/*D:495 We originally used end_request() throughout the driver, but it turns
    + * out that end_request() is deprecated, and doesn't actually end the request
    + * (which seems like a good reason to deprecate it!). It simply ends the first
    + * bio. So if we had 3 bios in a "struct request" we would do all 3,
    + * end_request(), do 2, end_request(), do 1 and end_request(): twice as much
    + * work as we needed to do.
    + *
    + * This reinforced to me that I do not understand the block layer.
    + *
    + * Nonetheless, Jens Axboe gave me this nice helper to end all chunks of a
    + * request. This improved disk speed by 130%. */
    static void end_entire_request(struct request *req, int uptodate)
    {
    if (end_that_request_first(req, uptodate, req->hard_nr_sectors))
    @@ -55,30 +84,62 @@ static void end_entire_request(struct re
    end_that_request_last(req, uptodate);
    }

    +/* I'm told there are only two stories in the world worth telling: love and
    + * hate. So there used to be a love scene here like this:
    + *
    + * Launcher: We could make beautiful I/O together, you and I.
    + * Guest: My, that's a big disk!
    + *
    + * Unfortunately, it was just too raunchy for our otherwise-gentle tale. */
    +
    +/*D:490 This is the interrupt handler, called when a block read or write has
    + * been completed for us. */
    static irqreturn_t lgb_irq(int irq, void *_bd)
    {
    + /* We handed our "struct blockdev" as the argument to request_irq(), so
    + * it is passed through to us here. This tells us which device we're
    + * dealing with in case we have more than one. */
    struct blockdev *bd = _bd;
    unsigned long flags;

    + /* We weren't doing anything? Strange, but could happen if we shared
    + * interrupts (we don't!). */
    if (!bd->req) {
    pr_debug("No work!\n");
    return IRQ_NONE;
    }

    + /* Not done yet? That's equally strange. */
    if (!bd->lb_page->result) {
    pr_debug("No result!\n");
    return IRQ_NONE;
    }

    + /* We have to grab the lock before ending the request. */
    spin_lock_irqsave(&bd->lock, flags);
    + /* "result" is 1 for success, 2 for failure: end_entire_request() wants
    + * to know whether this succeeded or not. */
    end_entire_request(bd->req, bd->lb_page->result == 1);
    + /* Clear out request, it's done. */
    bd->req = NULL;
    + /* Reset incoming DMA for next time. */
    bd->dma.used_len = 0;
    + /* Ready for more reads or writes */
    blk_start_queue(bd->disk->queue);
    spin_unlock_irqrestore(&bd->lock, flags);
    +
    + /* The interrupt was for us, we dealt with it. */
    return IRQ_HANDLED;
    }

    +/*D:480 The block layer's "struct request" contains a number of "struct bio"s,
    + * each of which contains "struct bio_vec"s, each of which contains a page, an
    + * offset and a length.
    + *
    + * Fortunately there are iterators to help us walk through the "struct
    + * request". Even more fortunately, there were plenty of places to steal the
    + * code from. We pack the "struct request" into our "struct lguest_dma" and
    + * return the total length. */
    static unsigned int req_to_dma(struct request *req, struct lguest_dma *dma)
    {
    unsigned int i = 0, idx, len = 0;
    @@ -87,8 +148,13 @@ static unsigned int req_to_dma(struct re
    rq_for_each_bio(bio, req) {
    struct bio_vec *bvec;
    bio_for_each_segment(bvec, bio, idx) {
    + /* We told the block layer not to give us too many. */
    BUG_ON(i == LGUEST_MAX_DMA_SECTIONS);
    + /* If we had a zero-length segment, it would look like
    + * the end of the data referred to by the "struct
    + * lguest_dma", so make sure that doesn't happen. */
    BUG_ON(!bvec->bv_len);
    + /* Convert page & offset to a physical address */
    dma->addr[i] = page_to_phys(bvec->bv_page)
    + bvec->bv_offset;
    dma->len[i] = bvec->bv_len;
    @@ -96,26 +162,39 @@ static unsigned int req_to_dma(struct re
    i++;
    }
    }
    + /* If the array isn't full, we mark the end with a 0 length */
    if (i < LGUEST_MAX_DMA_SECTIONS)
    dma->len[i] = 0;
    return len;
    }

    +/* This creates an empty DMA, useful for prodding the Host without sending data
    + * (ie. when we want to do a read) */
    static void empty_dma(struct lguest_dma *dma)
    {
    dma->len[0] = 0;
    }

    +/*D:470 Setting up a request is fairly easy: */
    static void setup_req(struct blockdev *bd,
    int type, struct request *req, struct lguest_dma *dma)
    {
    + /* The type is 1 (write) or 0 (read). */
    bd->lb_page->type = type;
    + /* The sector on disk where the read or write starts. */
    bd->lb_page->sector = req->sector;
    + /* The result is initialized to 0 (unfinished). */
    bd->lb_page->result = 0;
    + /* The current request (so we can end it in the interrupt handler). */
    bd->req = req;
    + /* The number of bytes: returned as a side-effect of req_to_dma(),
    + * which packs the block layer's "struct request" into our "struct
    + * lguest_dma" */
    bd->lb_page->bytes = req_to_dma(req, dma);
    }

    +/*D:450 Write is pretty straightforward: we pack the request into a "struct
    + * lguest_dma", then use SEND_DMA to send the request. */
    static void do_write(struct blockdev *bd, struct request *req)
    {
    struct lguest_dma send;
    @@ -126,6 +205,9 @@ static void do_write(struct blockdev *bd
    lguest_send_dma(bd->phys_addr, &send);
    }

    +/* Read is similar to write, except we pack the request into our receive
    + * "struct lguest_dma" and send through an empty DMA just to tell the Host that
    + * there's a request pending. */
    static void do_read(struct blockdev *bd, struct request *req)
    {
    struct lguest_dma ping;
    @@ -137,21 +219,30 @@ static void do_read(struct blockdev *bd,
    lguest_send_dma(bd->phys_addr, &ping);
    }

    +/*D:440 This where requests come in: we get handed the request queue and are
    + * expected to pull a "struct request" off it until we've finished them or
    + * we're waiting for a reply: */
    static void do_lgb_request(request_queue_t *q)
    {
    struct blockdev *bd;
    struct request *req;

    again:
    + /* This sometimes returns NULL even on the very first time around. I
    + * wonder if it's something to do with letting elves handle the request
    + * queue... */
    req = elv_next_request(q);
    if (!req)
    return;

    + /* We attached the struct blockdev to the disk: get it back */
    bd = req->rq_disk->private_data;
    - /* Sometimes we get repeated requests after blk_stop_queue. */
    + /* Sometimes we get repeated requests after blk_stop_queue(), but we
    + * can only handle one at a time. */
    if (bd->req)
    return;

    + /* We only do reads and writes: no tricky business! */
    if (!blk_fs_request(req)) {
    pr_debug("Got non-command 0x%08x\n", req->cmd_type);
    req->errors++;
    @@ -164,20 +255,31 @@ again:
    else
    do_read(bd, req);

    - /* Wait for interrupt to tell us it's done. */
    + /* We've put out the request, so stop any more coming in until we get
    + * an interrupt, which takes us to lgb_irq() to re-enable the queue. */
    blk_stop_queue(q);
    }

    +/*D:430 This is the "struct block_device_operations" we attach to the disk at
    + * the end of lguestblk_probe(). It doesn't seem to want much. */
    static struct block_device_operations lguestblk_fops = {
    .owner = THIS_MODULE,
    };

    +/*D:425 Setting up a disk device seems to involve a lot of code. I'm not sure
    + * quite why. I do know that the IDE code sent two or three of the maintainers
    + * insane, perhaps this is the fringe of the same disease?
    + *
    + * As in the console code, the probe function gets handed the generic
    + * lguest_device from lguest_bus.c: */
    static int lguestblk_probe(struct lguest_device *lgdev)
    {
    struct blockdev *bd;
    int err;
    int irqflags = IRQF_SHARED;

    + /* First we allocate our own "struct blockdev" and initialize the easy
    + * fields. */
    bd = kmalloc(sizeof(*bd), GFP_KERNEL);
    if (!bd)
    return -ENOMEM;
    @@ -187,59 +289,100 @@ static int lguestblk_probe(struct lguest
    bd->req = NULL;
    bd->dma.used_len = 0;
    bd->dma.len[0] = 0;
    + /* The descriptor in the lguest_devices array provided by the Host
    + * gives the Guest the physical page number of the device's page. */
    bd->phys_addr = (lguest_devices[lgdev->index].pfn << PAGE_SHIFT);

    + /* We use lguest_map() to get a pointer to the device page */
    bd->lb_page = lguest_map(bd->phys_addr, 1);
    if (!bd->lb_page) {
    err = -ENOMEM;
    goto out_free_bd;
    }

    + /* We need a major device number: 0 means "assign one dynamically". */
    bd->major = register_blkdev(0, "lguestblk");
    if (bd->major < 0) {
    err = bd->major;
    goto out_unmap;
    }

    + /* This allocates a "struct gendisk" where we pack all the information
    + * about the disk which the rest of Linux sees. We ask for one minor
    + * number; I do wonder if we should be asking for more. */
    bd->disk = alloc_disk(1);
    if (!bd->disk) {
    err = -ENOMEM;
    goto out_unregister_blkdev;
    }

    + /* Every disk needs a queue for requests to come in: we set up the
    + * queue with a callback function (the core of our driver) and the lock
    + * to use. */
    bd->disk->queue = blk_init_queue(do_lgb_request, &bd->lock);
    if (!bd->disk->queue) {
    err = -ENOMEM;
    goto out_put_disk;
    }

    - /* We can only handle a certain number of sg entries */
    + /* We can only handle a certain number of pointers in our SEND_DMA
    + * call, so we set that with blk_queue_max_hw_segments(). This is not
    + * to be confused with blk_queue_max_phys_segments() of course! I
    + * know, who could possibly confuse the two?
    + *
    + * Well, it's simple to tell them apart: this one seems to work and the
    + * other one didn't. */
    blk_queue_max_hw_segments(bd->disk->queue, LGUEST_MAX_DMA_SECTIONS);
    - /* Buffers must not cross page boundaries */
    +
    + /* Due to technical limitations of our Host (and simple coding) we
    + * can't have a single buffer which crosses a page boundary. Tell it
    + * here. This means that our maximum request size is 16
    + * (LGUEST_MAX_DMA_SECTIONS) pages. */
    blk_queue_segment_boundary(bd->disk->queue, PAGE_SIZE-1);

    + /* We name our disk: this becomes the device name when udev does its
    + * magic thing and creates the device node, such as /dev/lgba.
    + * next_block_index is a global which starts at 'a'. Unfortunately
    + * this simple increment logic means that the 27th disk will be called
    + * "/dev/lgb{". In that case, I recommend having at least 29 disks, so
    + * your /dev directory will be balanced. */
    sprintf(bd->disk->disk_name, "lgb%c", next_block_index++);
    +
    + /* We look to the device descriptor again to see if this device's
    + * interrupts are expected to be random. If they are, we tell the irq
    + * subsystem. At the moment this bit is always set. */
    if (lguest_devices[lgdev->index].features & LGUEST_DEVICE_F_RANDOMNESS)
    irqflags |= IRQF_SAMPLE_RANDOM;
    +
    + /* Now we have the name and irqflags, we can request the interrupt; we
    + * give it the "struct blockdev" we have set up to pass to lgb_irq()
    + * when there is an interrupt. */
    err = request_irq(bd->irq, lgb_irq, irqflags, bd->disk->disk_name, bd);
    if (err)
    goto out_cleanup_queue;

    + /* We bind our one-entry DMA pool to the key for this block device so
    + * the Host can reply to our requests. The key is equal to the
    + * physical address of the device's page, which is conveniently
    + * unique. */
    err = lguest_bind_dma(bd->phys_addr, &bd->dma, 1, bd->irq);
    if (err)
    goto out_free_irq;

    + /* We finish our disk initialization and add the disk to the system. */
    bd->disk->major = bd->major;
    bd->disk->first_minor = 0;
    bd->disk->private_data = bd;
    bd->disk->fops = &lguestblk_fops;
    - /* This is initialized to the disk size by the other end. */
    + /* This is initialized to the disk size by the Launcher. */
    set_capacity(bd->disk, bd->lb_page->num_sectors);
    add_disk(bd->disk);

    printk(KERN_INFO "%s: device %i at major %d\n",
    bd->disk->disk_name, lgdev->index, bd->major);

    + /* We don't need to keep the "struct blockdev" around, but if we ever
    + * implemented device removal, we'd need this. */
    lgdev->private = bd;
    return 0;

    @@ -258,6 +401,8 @@ out_free_bd:
    return err;
    }

    +/*D:410 The boilerplate code for registering the lguest block driver is just
    + * like the console: */
    static struct lguest_driver lguestblk_drv = {
    .name = "lguestblk",
    .owner = THIS_MODULE,
    ===================================================================
    --- a/drivers/char/hvc_lguest.c
    +++ b/drivers/char/hvc_lguest.c
    @@ -1,6 +1,19 @@
    -/* Simple console for lguest.
    +/*D:300
    + * The Guest console driver
    *
    - * Copyright (C) 2006 Rusty Russell, IBM Corporation
    + * This is a trivial console driver: we use lguest's DMA mechanism to send
    + * bytes out, and register a DMA buffer to receive bytes in. It is assumed to
    + * be present and available from the very beginning of boot.
    + *
    + * Writing console drivers is one of the few remaining Dark Arts in Linux.
    + * Fortunately for us, the path of virtual consoles has been well-trodden by
    + * the PowerPC folks, who wrote "hvc_console.c" to generically support any
    + * virtual console. We use that infrastructure which only requires us to write
    + * the basic put_chars and get_chars functions and call the right register
    + * functions.
    + :*/
    +
    +/* Copyright (C) 2006 Rusty Russell, IBM Corporation
    *
    * This program is free software; you can redistribute it and/or modify
    * it under the terms of the GNU General Public License as published by
    @@ -21,49 +34,81 @@
    #include <linux/lguest_bus.h>
    #include "hvc_console.h"

    +/*D:340 This is our single console input buffer, with associated "struct
    + * lguest_dma" referring to it. Note the 0-terminated length array, and the
    + * use of physical address for the buffer itself. */
    static char inbuf[256];
    static struct lguest_dma cons_input = { .used_len = 0,
    .addr[0] = __pa(inbuf),
    .len[0] = sizeof(inbuf),
    .len[1] = 0 };

    +/*D:310 The put_chars() callback is pretty straightforward.
    + *
    + * First we put the pointer and length in a "struct lguest_dma": we only have
    + * one pointer, so we set the second length to 0. Then we use SEND_DMA to send
    + * the data to (Host) buffers attached to the console key. Usually a device's
    + * key is a physical address within the device's memory, but because the
    + * console device doesn't have any associated physical memory, we use the
    + * LGUEST_CONSOLE_DMA_KEY constant (aka 0). */
    static int put_chars(u32 vtermno, const char *buf, int count)
    {
    struct lguest_dma dma;

    - /* FIXME: what if it's over a page boundary? */
    + /* FIXME: DMA buffers in a "struct lguest_dma" are not allowed
    + * to go over page boundaries. This never seems to happen,
    + * but if it did we'd need to fix this code. */
    dma.len[0] = count;
    dma.len[1] = 0;
    dma.addr[0] = __pa(buf);

    lguest_send_dma(LGUEST_CONSOLE_DMA_KEY, &dma);
    + /* We're expected to return the amount of data we wrote: all of it. */
    return count;
    }

    +/*D:350 get_chars() is the callback from the hvc_console infrastructure when
    + * an interrupt is received.
    + *
    + * Firstly we see if our buffer has been filled: if not, we return. The rest
    + * of the code deals with the fact that the hvc_console() infrastructure only
    + * asks us for 16 bytes at a time. We keep a "cons_offset" variable for
    + * partially-read buffers. */
    static int get_chars(u32 vtermno, char *buf, int count)
    {
    static int cons_offset;

    + /* Nothing left to see here... */
    if (!cons_input.used_len)
    return 0;

    + /* You want more than we have to give? Well, try wanting less! */
    if (cons_input.used_len - cons_offset < count)
    count = cons_input.used_len - cons_offset;

    + /* Copy across to their buffer and increment offset. */
    memcpy(buf, inbuf + cons_offset, count);
    cons_offset += count;
    +
    + /* Finished? Zero offset, and reset cons_input so Host will use it
    + * again. */
    if (cons_offset == cons_input.used_len) {
    cons_offset = 0;
    cons_input.used_len = 0;
    }
    return count;
    }
    +/*:*/

    static struct hv_ops lguest_cons = {
    .get_chars = get_chars,
    .put_chars = put_chars,
    };

    +/*D:320 Console drivers are initialized very early so boot messages can go
    + * out. At this stage, the console is output-only. Our driver checks we're a
    + * Guest, and if so hands hvc_instantiate() the console number (0), priority
    + * (0), and the struct hv_ops containing the put_chars() function. */
    static int __init cons_init(void)
    {
    if (strcmp(paravirt_ops.name, "lguest") != 0)
    @@ -73,21 +118,46 @@ static int __init cons_init(void)
    }
    console_initcall(cons_init);

    +/*D:370 To set up and manage our virtual console, we call hvc_alloc() and
    + * stash the result in the private pointer of the "struct lguest_device".
    + * Since we never remove the console device we never need this pointer again,
    + * but using ->private is considered good form, and you never know who's going
    + * to copy your driver.
    + *
    + * Once the console is set up, we bind our input buffer ready for input. */
    static int lguestcons_probe(struct lguest_device *lgdev)
    {
    int err;

    + /* The first argument of hvc_alloc() is the virtual console number, so
    + * we use zero. The second argument is the interrupt number.
    + *
    + * The third argument is a "struct hv_ops" containing the put_chars()
    + * and get_chars() pointers. The final argument is the output buffer
    + * size: we use 256 and expect the Host to have room for us to send
    + * that much. */
    lgdev->private = hvc_alloc(0, lgdev_irq(lgdev), &lguest_cons, 256);
    if (IS_ERR(lgdev->private))
    return PTR_ERR(lgdev->private);

    + /* We bind a single DMA buffer at key LGUEST_CONSOLE_DMA_KEY.
    + * "cons_input" is that statically-initialized global DMA buffer we saw
    + * above, and we also give the interrupt we want. */
    err = lguest_bind_dma(LGUEST_CONSOLE_DMA_KEY, &cons_input, 1,
    lgdev_irq(lgdev));
    if (err)
    printk("lguest console: failed to bind buffer.\n");
    return err;
    }
    +/* Note the use of lgdev_irq() for the interrupt number. We tell hvc_alloc()
    + * to expect input when this interrupt is triggered, and then tell
    + * lguest_bind_dma() that is the interrupt to send us when input comes in. */

    +/*D:360 From now on the console driver follows standard Guest driver form:
    + * register_lguest_driver() registers the device type and probe function, and
    + * the probe function sets up the device.
    + *
    + * The standard "struct lguest_driver": */
    static struct lguest_driver lguestcons_drv = {
    .name = "lguestcons",
    .owner = THIS_MODULE,
    @@ -95,6 +165,7 @@ static struct lguest_driver lguestcons_d
    .probe = lguestcons_probe,
    };

    +/* The standard init function */
    static int __init hvc_lguest_init(void)
    {
    return register_lguest_driver(&lguestcons_drv);
    ===================================================================
    --- a/drivers/lguest/lguest_bus.c
    +++ b/drivers/lguest/lguest_bus.c
    @@ -46,6 +46,10 @@ static struct device_attribute lguest_de
    __ATTR_NULL
    };

    +/*D:130 The generic bus infrastructure requires a function which says whether a
    + * device matches a driver. For us, it is simple: "struct lguest_driver"
    + * contains a "device_type" field which indicates what type of device it can
    + * handle, so we just cast the args and compare: */
    static int lguest_dev_match(struct device *_dev, struct device_driver *_drv)
    {
    struct lguest_device *dev = container_of(_dev,struct lguest_device,dev);
    @@ -53,6 +57,7 @@ static int lguest_dev_match(struct devic

    return (drv->device_type == lguest_devices[dev->index].type);
    }
    +/*:*/

    struct lguest_bus {
    struct bus_type bus;
    @@ -71,11 +76,24 @@ static struct lguest_bus lguest_bus = {
    }
    };

    +/*D:140 This is the callback which occurs once the bus infrastructure matches
    + * up a device and driver, ie. in response to add_lguest_device() calling
    + * device_register(), or register_lguest_driver() calling driver_register().
    + *
    + * At the moment it's always the latter: the devices are added first, since
    + * scan_devices() is called from a "core_initcall", and the drivers themselves
    + * called later as a normal "initcall". But it would work the other way too.
    + *
    + * So now we have the happy couple, we add the status bit to indicate that we
    + * found a driver. If the driver truly loves the device, it will return
    + * happiness from its probe function (ok, perhaps this wasn't my greatest
    + * analogy), and we set the final "driver ok" bit so the Host sees it's all
    + * green. */
    static int lguest_dev_probe(struct device *_dev)
    {
    int ret;
    - struct lguest_device *dev = container_of(_dev,struct lguest_device,dev);
    - struct lguest_driver *drv = container_of(dev->dev.driver,
    + struct lguest_device*dev = container_of(_dev,struct lguest_device,dev);
    + struct lguest_driver*drv = container_of(dev->dev.driver,
    struct lguest_driver, drv);

    lguest_devices[dev->index].status |= LGUEST_DEVICE_S_DRIVER;
    @@ -85,6 +103,10 @@ static int lguest_dev_probe(struct devic
    return ret;
    }

    +/* The last part of the bus infrastructure is the function lguest drivers use
    + * to register themselves. Firstly, we do nothing if there's no lguest bus
    + * (ie. this is not a Guest), otherwise we fill in the embedded generic "struct
    + * driver" fields and call the generic driver_register(). */
    int register_lguest_driver(struct lguest_driver *drv)
    {
    if (!lguest_devices)
    @@ -97,12 +119,36 @@ int register_lguest_driver(struct lguest

    return driver_register(&drv->drv);
    }
    +
    +/* At the moment we build all the drivers into the kernel because they're so
    + * simple: 8144 bytes for all three of them as I type this. And as the console
    + * really needs to be built in, it's actually only 3527 bytes for the network
    + * and block drivers.
    + *
    + * If they get complex it will make sense for them to be modularized, so we
    + * need to explicitly export the symbol.
    + *
    + * I don't think non-GPL modules make sense, so it's a GPL-only export.
    + */
    EXPORT_SYMBOL_GPL(register_lguest_driver);

    +/*D:120 This is the core of the lguest bus: actually adding a new device.
    + * It's a separate function because it's neater that way, and because an
    + * earlier version of the code supported hotplug and unplug. They were removed
    + * early on because they were never used.
    + *
    + * As Andrew Tridgell says, "Untested code is buggy code".
    + *
    + * It's worth reading this carefully: we start with an index into the array of
    + * "struct lguest_device_desc"s indicating the device which is new: */
    static void add_lguest_device(unsigned int index)
    {
    struct lguest_device *new;

    + /* Each "struct lguest_device_desc" has a "status" field, which the
    + * Guest updates as the device is probed. In the worst case, the Host
    + * can look at these bits to tell what part of device setup failed,
    + * even if the console isn't available. */
    lguest_devices[index].status |= LGUEST_DEVICE_S_ACKNOWLEDGE;
    new = kmalloc(sizeof(struct lguest_device), GFP_KERNEL);
    if (!new) {
    @@ -111,12 +157,17 @@ static void add_lguest_device(unsigned i
    return;
    }

    + /* The "struct lguest_device" setup is pretty straight-forward example
    + * code. */
    new->index = index;
    new->private = NULL;
    memset(&new->dev, 0, sizeof(new->dev));
    new->dev.parent = &lguest_bus.dev;
    new->dev.bus = &lguest_bus.bus;
    sprintf(new->dev.bus_id, "%u", index);
    +
    + /* device_register() causes the bus infrastructure to look for a
    + * matching driver. */
    if (device_register(&new->dev) != 0) {
    printk(KERN_EMERG "Cannot register lguest device %u\n", index);
    lguest_devices[index].status |= LGUEST_DEVICE_S_FAILED;
    @@ -124,6 +175,9 @@ static void add_lguest_device(unsigned i
    }
    }

    +/*D:110 scan_devices() simply iterates through the device array. The type 0
    + * is reserved to mean "no device", and anything else means we have found a
    + * device: add it. */
    static void scan_devices(void)
    {
    unsigned int i;
    @@ -133,12 +187,23 @@ static void scan_devices(void)
    add_lguest_device(i);
    }

    +/*D:100 Fairly early in boot, lguest_bus_init() is called to set up the lguest
    + * bus. We check that we are a Guest by checking paravirt_ops.name: there are
    + * other ways of checking, but this seems most obvious to me.
    + *
    + * So we can access the array of "struct lguest_device_desc"s easily, we map
    + * that memory and store the pointer in the global "lguest_devices". Then we
    + * register the bus with the core. Doing two registrations seems clunky to me,
    + * but it seems to be the correct sysfs incantation.
    + *
    + * Finally we call scan_devices() which adds all the devices found in the
    + * "struct lguest_device_desc" array. */
    static int __init lguest_bus_init(void)
    {
    if (strcmp(paravirt_ops.name, "lguest") != 0)
    return 0;

    - /* Devices are in page above top of "normal" mem. */
    + /* Devices are in a single page above top of "normal" mem */
    lguest_devices = lguest_map(max_pfn<<PAGE_SHIFT, 1);

    if (bus_register(&lguest_bus.bus) != 0
    @@ -148,4 +213,5 @@ static int __init lguest_bus_init(void)
    scan_devices();
    return 0;
    }
    +/* Do this after core stuff, before devices. */
    postcore_initcall(lguest_bus_init);
    ===================================================================
    --- a/drivers/net/lguest_net.c
    +++ b/drivers/net/lguest_net.c
    @@ -1,6 +1,13 @@
    -/* A simple network driver for lguest.
    - *
    - * Copyright 2006 Rusty Russell <rusty@rustcorp.com.au> IBM Corporation
    +/*D:500
    + * The Guest network driver.
    + *
    + * This is very simple a virtual network driver, and our last Guest driver.
    + * The only trick is that it can talk directly to multiple other recipients
    + * (ie. other Guests on the same network). It can also be used with only the
    + * Host on the network.
    + :*/
    +
    +/* Copyright 2006 Rusty Russell <rusty@rustcorp.com.au> IBM Corporation
    *
    * This program is free software; you can redistribute it and/or modify
    * it under the terms of the GNU General Public License as published by
    @@ -27,23 +34,28 @@
    #define MAX_LANS 4
    #define NUM_SKBS 8

    +/*D:530 The "struct lguestnet_info" contains all the information we need to
    + * know about the network device. */
    struct lguestnet_info
    {
    - /* The shared page(s). */
    + /* The mapped device page(s) (an array of "struct lguest_net"). */
    struct lguest_net *peer;
    + /* The physical address of the device page(s) */
    unsigned long peer_phys;
    + /* The size of the device page(s). */
    unsigned long mapsize;

    /* The lguest_device I come from */
    struct lguest_device *lgdev;

    - /* My peerid. */
    + /* My peerid (ie. my slot in the array). */
    unsigned int me;

    - /* Receive queue. */
    + /* Receive queue: the network packets waiting to be filled. */
    struct sk_buff *skb[NUM_SKBS];
    struct lguest_dma dma[NUM_SKBS];
    };
    +/*:*/

    /* How many bytes left in this page. */
    static unsigned int rest_of_page(void *data)
    @@ -51,39 +63,82 @@ static unsigned int rest_of_page(void *d
    return PAGE_SIZE - ((unsigned long)data % PAGE_SIZE);
    }

    -/* Simple convention: offset 4 * peernum. */
    +/*D:570 Each peer (ie. Guest or Host) on the network binds their receive
    + * buffers to a different key: we simply use the physical address of the
    + * device's memory page plus the peer number. The Host insists that all keys
    + * be a multiple of 4, so we multiply the peer number by 4. */
    static unsigned long peer_key(struct lguestnet_info *info, unsigned peernum)
    {
    return info->peer_phys + 4 * peernum;
    }

    +/* This is the routine which sets up a "struct lguest_dma" to point to a
    + * network packet, similar to req_to_dma() in lguest_blk.c. The structure of a
    + * "struct sk_buff" has grown complex over the years: it consists of a "head"
    + * linear section pointed to by "skb->data", and possibly an array of
    + * "fragments" in the case of a non-linear packet.
    + *
    + * Our receive buffers don't use fragments at all but outgoing skbs might, so
    + * we handle it. */
    static void skb_to_dma(const struct sk_buff *skb, unsigned int headlen,
    struct lguest_dma *dma)
    {
    unsigned int i, seg;

    + /* First, we put the linear region into the "struct lguest_dma". Each
    + * entry can't go over a page boundary, so even though all our packets
    + * are 1514 bytes or less, we might need to use two entries here: */
    for (i = seg = 0; i < headlen; seg++, i += rest_of_page(skb->data+i)) {
    dma->addr[seg] = virt_to_phys(skb->data + i);
    dma->len[seg] = min((unsigned)(headlen - i),
    rest_of_page(skb->data + i));
    }
    +
    + /* Now we handle the fragments: at least they're guaranteed not to go
    + * over a page. skb_shinfo(skb) returns a pointer to the structure
    + * which tells us about the number of fragments and the fragment
    + * array. */
    for (i = 0; i < skb_shinfo(skb)->nr_frags; i++, seg++) {
    const skb_frag_t *f = &skb_shinfo(skb)->frags[i];
    /* Should not happen with MTU less than 64k - 2 * PAGE_SIZE. */
    if (seg == LGUEST_MAX_DMA_SECTIONS) {
    + /* We will end up sending a truncated packet should
    + * this ever happen. Plus, a cool log message! */
    printk("Woah dude! Megapacket!\n");
    break;
    }
    dma->addr[seg] = page_to_phys(f->page) + f->page_offset;
    dma->len[seg] = f->size;
    }
    +
    + /* If after all that we didn't use the entire "struct lguest_dma"
    + * array, we terminate it with a 0 length. */
    if (seg < LGUEST_MAX_DMA_SECTIONS)
    dma->len[seg] = 0;
    }

    -/* We overload multicast bit to show promiscuous mode. */
    +/*
    + * Packet transmission.
    + *
    + * Our packet transmission is a little unusual. A real network card would just
    + * send out the packet and leave the receivers to decide if they're interested.
    + * Instead, we look through the network device memory page and see if any of
    + * the ethernet addresses match the packet destination, and if so we send it to
    + * that Guest.
    + *
    + * This is made a little more complicated in two cases. The first case is
    + * broadcast packets: for that we send the packet to all Guests on the network,
    + * one at a time. The second case is "promiscuous" mode, where a Guest wants
    + * to see all the packets on the network. We need a way for the Guest to tell
    + * us it wants to see all packets, so it sets the "multicast" bit on its
    + * published MAC address, which is never valid in a real ethernet address.
    + */
    #define PROMISC_BIT 0x01

    +/* This is the callback which is summoned whenever the network device's
    + * multicast or promiscuous state changes. If the card is in promiscuous mode,
    + * we advertise that in our ethernet address in the device's memory. We do the
    + * same if Linux wants any or all multicast traffic. */
    static void lguestnet_set_multicast(struct net_device *dev)
    {
    struct lguestnet_info *info = netdev_priv(dev);
    @@ -94,11 +149,14 @@ static void lguestnet_set_multicast(stru
    info->peer[info->me].mac[0] &= ~PROMISC_BIT;
    }

    +/* A simple test function to see if a peer wants to see all packets.*/
    static int promisc(struct lguestnet_info *info, unsigned int peer)
    {
    return info->peer[peer].mac[0] & PROMISC_BIT;
    }

    +/* Another simple function to see if a peer's advertised ethernet address
    + * matches a packet's destination ethernet address. */
    static int mac_eq(const unsigned char mac[ETH_ALEN],
    struct lguestnet_info *info, unsigned int peer)
    {
    @@ -108,6 +166,8 @@ static int mac_eq(const unsigned char ma
    return memcmp(mac+1, info->peer[peer].mac+1, ETH_ALEN-1) == 0;
    }

    +/* This is the function which actually sends a packet once we've decided a
    + * peer wants it: */
    static void transfer_packet(struct net_device *dev,
    struct sk_buff *skb,
    unsigned int peernum)
    @@ -115,76 +175,134 @@ static void transfer_packet(struct net_d
    struct lguestnet_info *info = netdev_priv(dev);
    struct lguest_dma dma;

    + /* We use our handy "struct lguest_dma" packing function to prepare
    + * the skb for sending. */
    skb_to_dma(skb, skb_headlen(skb), &dma);
    pr_debug("xfer length %04x (%u)\n", htons(skb->len), skb->len);

    + /* This is the actual send call which copies the packet. */
    lguest_send_dma(peer_key(info, peernum), &dma);
    +
    + /* Check that the entire packet was transmitted. If not, it could mean
    + * that the other Guest registered a short receive buffer, but this
    + * driver should never do that. More likely, the peer is dead. */
    if (dma.used_len != skb->len) {
    dev->stats.tx_carrier_errors++;
    pr_debug("Bad xfer to peer %i: %i of %i (dma %p/%i)\n",
    peernum, dma.used_len, skb->len,
    (void *)dma.addr[0], dma.len[0]);
    } else {
    + /* On success we update the stats. */
    dev->stats.tx_bytes += skb->len;
    dev->stats.tx_packets++;
    }
    }

    +/* Another helper function to tell is if a slot in the device memory is unused.
    + * Since we always set the Local Assignment bit in the ethernet address, the
    + * first byte can never be 0. */
    static int unused_peer(const struct lguest_net peer[], unsigned int num)
    {
    return peer[num].mac[0] == 0;
    }

    +/* Finally, here is the routine which handles an outgoing packet. It's called
    + * "start_xmit" for traditional reasons. */
    static int lguestnet_start_xmit(struct sk_buff *skb, struct net_device *dev)
    {
    unsigned int i;
    int broadcast;
    struct lguestnet_info *info = netdev_priv(dev);
    + /* Extract the destination ethernet address from the packet. */
    const unsigned char *dest = ((struct ethhdr *)skb->data)->h_dest;

    pr_debug("%s: xmit %02x:%02x:%02x:%02x:%02x:%02x\n",
    dev->name, dest[0],dest[1],dest[2],dest[3],dest[4],dest[5]);

    + /* If it's a multicast packet, we broadcast to everyone. That's not
    + * very efficient, but there are very few applications which actually
    + * use multicast, which is a shame really.
    + *
    + * As etherdevice.h points out: "By definition the broadcast address is
    + * also a multicast address." So we don't have to test for broadcast
    + * packets separately. */
    broadcast = is_multicast_ether_addr(dest);
    +
    + /* Look through all the published ethernet addresses to see if we
    + * should send this packet. */
    for (i = 0; i < info->mapsize/sizeof(struct lguest_net); i++) {
    + /* We don't send to ourselves (we actually can't SEND_DMA to
    + * ourselves anyway), and don't send to unused slots.*/
    if (i == info->me || unused_peer(info->peer, i))
    continue;

    + /* If it's broadcast we send it. If they want every packet we
    + * send it. If the destination matches their address we send
    + * it. Otherwise we go to the next peer. */
    if (!broadcast && !promisc(info, i) && !mac_eq(dest, info, i))
    continue;

    pr_debug("lguestnet %s: sending from %i to %i\n",
    dev->name, info->me, i);
    + /* Our routine which actually does the transfer. */
    transfer_packet(dev, skb, i);
    }
    +
    + /* An xmit routine is expected to dispose of the packet, so we do. */
    dev_kfree_skb(skb);
    +
    + /* As per kernel convention, 0 means success. This is why I love
    + * networking: even if we never sent to anyone, that's still
    + * success! */
    return 0;
    }

    -/* Find a new skb to put in this slot in shared mem. */
    +/*D:560
    + * Packet receiving.
    + *
    + * First, here's a helper routine which fills one of our array of receive
    + * buffers: */
    static int fill_slot(struct net_device *dev, unsigned int slot)
    {
    struct lguestnet_info *info = netdev_priv(dev);
    - /* Try to create and register a new one. */
    +
    + /* We can receive ETH_DATA_LEN (1500) byte packets, plus a standard
    + * ethernet header of ETH_HLEN (14) bytes. */
    info->skb[slot] = netdev_alloc_skb(dev, ETH_HLEN + ETH_DATA_LEN);
    if (!info->skb[slot]) {
    printk("%s: could not fill slot %i\n", dev->name, slot);
    return -ENOMEM;
    }

    + /* skb_to_dma() is a helper which sets up the "struct lguest_dma" to
    + * point to the data in the skb: we also use it for sending out a
    + * packet. */
    skb_to_dma(info->skb[slot], ETH_HLEN + ETH_DATA_LEN, &info->dma[slot]);
    +
    + /* This is a Write Memory Barrier: it ensures that the entry in the
    + * receive buffer array is written *before* we set the "used_len" entry
    + * to 0. If the Host were looking at the receive buffer array from a
    + * different CPU, it could potentially see "used_len = 0" and not see
    + * the updated receive buffer information. This would be a horribly
    + * nasty bug, so make sure the compiler and CPU know this has to happen
    + * first. */
    wmb();
    - /* Now we tell hypervisor it can use the slot. */
    + /* Writing 0 to "used_len" tells the Host it can use this receive
    + * buffer now. */
    info->dma[slot].used_len = 0;
    return 0;
    }

    +/* This is the actual receive routine. When we receive an interrupt from the
    + * Host to tell us a packet has been delivered, we arrive here: */
    static irqreturn_t lguestnet_rcv(int irq, void *dev_id)
    {
    struct net_device *dev = dev_id;
    struct lguestnet_info *info = netdev_priv(dev);
    unsigned int i, done = 0;

    + /* Look through our entire receive array for an entry which has data
    + * in it. */
    for (i = 0; i < ARRAY_SIZE(info->dma); i++) {
    unsigned int length;
    struct sk_buff *skb;
    @@ -193,10 +311,16 @@ static irqreturn_t lguestnet_rcv(int irq
    if (length == 0)
    continue;

    + /* We've found one! Remember the skb (we grabbed the length
    + * above), and immediately refill the slot we've taken it
    + * from. */
    done++;
    skb = info->skb[i];
    fill_slot(dev, i);

    + /* This shouldn't happen: micropackets could be sent by a
    + * badly-behaved Guest on the network, but the Host will never
    + * stuff more data in the buffer than the buffer length. */
    if (length < ETH_HLEN || length > ETH_HLEN + ETH_DATA_LEN) {
    pr_debug(KERN_WARNING "%s: unbelievable skb len: %i\n",
    dev->name, length);
    @@ -204,36 +328,72 @@ static irqreturn_t lguestnet_rcv(int irq
    continue;
    }

    + /* skb_put(), what a great function! I've ranted about this
    + * function before (http://lkml.org/lkml/1999/9/26/24). You
    + * call it after you've added data to the end of an skb (in
    + * this case, it was the Host which wrote the data). */
    skb_put(skb, length);
    +
    + /* The ethernet header contains a protocol field: we use the
    + * standard helper to extract it, and place the result in
    + * skb->protocol. The helper also sets up skb->pkt_type and
    + * eats up the ethernet header from the front of the packet. */
    skb->protocol = eth_type_trans(skb, dev);
    - /* This is a reliable transport. */
    +
    + /* If this device doesn't need checksums for sending, we also
    + * don't need to check the packets when they come in. */
    if (dev->features & NETIF_F_NO_CSUM)
    skb->ip_summed = CHECKSUM_UNNECESSARY;
    +
    + /* As a last resort for debugging the driver or the lguest I/O
    + * subsystem, you can uncomment the "#define DEBUG" at the top
    + * of this file, which turns all the pr_debug() into printk()
    + * and floods the logs. */
    pr_debug("Receiving skb proto 0x%04x len %i type %i\n",
    ntohs(skb->protocol), skb->len, skb->pkt_type);

    + /* Update the packet and byte counts (visible from ifconfig,
    + * and good for debugging). */
    dev->stats.rx_bytes += skb->len;
    dev->stats.rx_packets++;
    +
    + /* Hand our fresh network packet into the stack's "network
    + * interface receive" routine. That will free the packet
    + * itself when it's finished. */
    netif_rx(skb);
    }
    +
    + /* If we found any packets, we assume the interrupt was for us. */
    return done ? IRQ_HANDLED : IRQ_NONE;
    }

    +/*D:550 This is where we start: when the device is brought up by dhcpd or
    + * ifconfig. At this point we advertise our MAC address to the rest of the
    + * network, and register receive buffers ready for incoming packets. */
    static int lguestnet_open(struct net_device *dev)
    {
    int i;
    struct lguestnet_info *info = netdev_priv(dev);

    - /* Set up our MAC address */
    + /* Copy our MAC address into the device page, so others on the network
    + * can find us. */
    memcpy(info->peer[info->me].mac, dev->dev_addr, ETH_ALEN);

    - /* Turn on promisc mode if needed */
    + /* We might already be in promisc mode (dev->flags & IFF_PROMISC). Our
    + * set_multicast callback handles this already, so we call it now. */
    lguestnet_set_multicast(dev);

    + /* Allocate packets and put them into our "struct lguest_dma" array.
    + * If we fail to allocate all the packets we could still limp along,
    + * but it's a sign of real stress so we should probably give up now. */
    for (i = 0; i < ARRAY_SIZE(info->dma); i++) {
    if (fill_slot(dev, i) != 0)
    goto cleanup;
    }
    +
    + /* Finally we tell the Host where our array of "struct lguest_dma"
    + * receive buffers is, binding it to the key corresponding to the
    + * device's physical memory plus our peerid. */
    if (lguest_bind_dma(peer_key(info,info->me), info->dma,
    NUM_SKBS, lgdev_irq(info->lgdev)) != 0)
    goto cleanup;
    @@ -244,22 +404,29 @@ cleanup:
    dev_kfree_skb(info->skb[i]);
    return -ENOMEM;
    }
    -
    +/*:*/
    +
    +/* The close routine is called when the device is no longer in use: we clean up
    + * elegantly. */
    static int lguestnet_close(struct net_device *dev)
    {
    unsigned int i;
    struct lguestnet_info *info = netdev_priv(dev);

    - /* Clear all trace: others might deliver packets, we'll ignore it. */
    + /* Clear all trace of our existence out of the device memory by setting
    + * the slot which held our MAC address to 0 (unused). */
    memset(&info->peer[info->me], 0, sizeof(info->peer[info->me]));

    - /* Deregister sg lists. */
    + /* Unregister our array of receive buffers */
    lguest_unbind_dma(peer_key(info, info->me), info->dma);
    for (i = 0; i < ARRAY_SIZE(info->dma); i++)
    dev_kfree_skb(info->skb[i]);
    return 0;
    }

    +/*D:510 The network device probe function is basically a standard ethernet
    + * device setup. It reads the "struct lguest_device_desc" and sets the "struct
    + * net_device". Oh, the line-by-line excitement! Let's skip over it. :*/
    static int lguestnet_probe(struct lguest_device *lgdev)
    {
    int err, irqf = IRQF_SHARED;
    @@ -289,10 +456,16 @@ static int lguestnet_probe(struct lguest
    dev->stop = lguestnet_close;
    dev->hard_start_xmit = lguestnet_start_xmit;

    - /* Turning on/off promisc will call dev->set_multicast_list.
    - * We don't actually support multicast yet */
    + /* We don't actually support multicast yet, but turning on/off
    + * promisc also calls dev->set_multicast_list. */
    dev->set_multicast_list = lguestnet_set_multicast;
    SET_NETDEV_DEV(dev, &lgdev->dev);
    +
    + /* The network code complains if you have "scatter-gather" capability
    + * if you don't also handle checksums (it seem that would be
    + * "illogical"). So we use a lie of omission and don't tell it that we
    + * can handle scattered packets unless we also don't want checksums,
    + * even though to us they're completely independent. */
    if (desc->features & LGUEST_NET_F_NOCSUM)
    dev->features = NETIF_F_SG|NETIF_F_NO_CSUM;

    @@ -324,6 +497,9 @@ static int lguestnet_probe(struct lguest
    }

    pr_debug("lguestnet: registered device %s\n", dev->name);
    + /* Finally, we put the "struct net_device" in the generic "struct
    + * lguest_device"s private pointer. Again, it's not necessary, but
    + * makes sure the cool kernel kids don't tease us. */
    lgdev->private = dev;
    return 0;

    @@ -351,3 +527,11 @@ module_init(lguestnet_init);

    MODULE_DESCRIPTION("Lguest network driver");
    MODULE_LICENSE("GPL");
    +
    +/*D:580
    + * This is the last of the Drivers, and with this we have covered the many and
    + * wonderous and fine (and boring) details of the Guest.
    + *
    + * "make Launcher" beckons, where we answer questions like "Where do Guests
    + * come from?", and "What do you do when someone asks for optimization?"
    + */
    ===================================================================
    --- a/include/linux/lguest_bus.h
    +++ b/include/linux/lguest_bus.h
    @@ -15,11 +15,14 @@ struct lguest_device {
    void *private;
    };

    -/* By convention, each device can use irq index+1 if it wants to. */
    +/*D:380 Since interrupt numbers are arbitrary, we use a convention: each device
    + * can use the interrupt number corresponding to its index. The +1 is because
    + * interrupt 0 is not usable (it's actually the timer interrupt). */
    static inline int lgdev_irq(const struct lguest_device *dev)
    {
    return dev->index + 1;
    }
    +/*:*/

    /* dma args must not be vmalloced! */
    void lguest_send_dma(unsigned long key, struct lguest_dma *dma);
    ===================================================================
    --- a/include/linux/lguest_launcher.h
    +++ b/include/linux/lguest_launcher.h
    @@ -9,14 +9,45 @@
    /* How many devices? Assume each one wants up to two dma arrays per device. */
    #define LGUEST_MAX_DEVICES (LGUEST_MAX_DMA/2)

    +/*D:200
    + * Lguest I/O
    + *
    + * The lguest I/O mechanism is the only way Guests can talk to devices. There
    + * are two hypercalls involved: SEND_DMA for output and BIND_DMA for input. In
    + * each case, "struct lguest_dma" describes the buffer: this contains 16
    + * addr/len pairs, and if there are fewer buffer elements the len array is
    + * terminated with a 0.
    + *
    + * I/O is organized by keys: BIND_DMA attaches buffers to a particular key, and
    + * SEND_DMA transfers to buffers bound to particular key. By convention, keys
    + * correspond to a physical address within the device's page. This means that
    + * devices will never accidentally end up with the same keys, and allows the
    + * Host use The Futex Trick (as we'll see later in our journey).
    + *
    + * SEND_DMA simply indicates a key to send to, and the physical address of the
    + * "struct lguest_dma" to send. The Host will write the number of bytes
    + * transferred into the "struct lguest_dma"'s used_len member.
    + *
    + * BIND_DMA indicates a key to bind to, a pointer to an array of "struct
    + * lguest_dma"s ready for receiving, the size of that array, and an interrupt
    + * to trigger when data is received. The Host will only allow transfers into
    + * buffers with a used_len of zero: it then sets used_len to the number of
    + * bytes transferred and triggers the interrupt for the Guest to process the
    + * new input. */
    struct lguest_dma
    {
    - /* 0 if free to be used, filled by hypervisor. */
    + /* 0 if free to be used, filled by the Host. */
    u32 used_len;
    unsigned long addr[LGUEST_MAX_DMA_SECTIONS];
    u16 len[LGUEST_MAX_DMA_SECTIONS];
    };
    +/*:*/

    +/*D:460 This is the layout of a block device memory page. The Launcher sets up
    + * the num_sectors initially to tell the Guest the size of the disk. The Guest
    + * puts the type, sector and length of the request in the first three fields,
    + * then DMAs to the Host. The Host processes the request, sets up the result,
    + * then DMAs back to the Guest. */
    struct lguest_block_page
    {
    /* 0 is a read, 1 is a write. */
    @@ -28,27 +59,47 @@ struct lguest_block_page
    u32 num_sectors; /* Disk length = num_sectors * 512 */
    };

    -/* There is a shared page of these. */
    +/*D:520 The network device is basically a memory page where all the Guests on
    + * the network publish their MAC (ethernet) addresses: it's an array of "struct
    + * lguest_net": */
    struct lguest_net
    {
    /* Simply the mac address (with multicast bit meaning promisc). */
    unsigned char mac[6];
    };
    +/*:*/

    /* Where the Host expects the Guest to SEND_DMA console output to. */
    #define LGUEST_CONSOLE_DMA_KEY 0

    -/* We have a page of these descriptors in the lguest_device page. */
    +/*D:010
    + * Drivers
    + *
    + * The Guest needs devices to do anything useful. Since we don't let it touch
    + * real devices (think of the damage it could do!) we provide virtual devices.
    + * We could emulate a PCI bus with various devices on it, but that is a fairly
    + * complex burden for the Host and suboptimal for the Guest, so we have our own
    + * "lguest" bus and simple drivers.
    + *
    + * Devices are described by an array of LGUEST_MAX_DEVICES of these structs,
    + * placed by the Launcher just above the top of physical memory:
    + */
    struct lguest_device_desc {
    + /* The device type: console, network, disk etc. */
    u16 type;
    #define LGUEST_DEVICE_T_CONSOLE 1
    #define LGUEST_DEVICE_T_NET 2
    #define LGUEST_DEVICE_T_BLOCK 3

    + /* The specific features of this device: these depends on device type
    + * except for LGUEST_DEVICE_F_RANDOMNESS. */
    u16 features;
    #define LGUEST_NET_F_NOCSUM 0x4000 /* Don't bother checksumming */
    #define LGUEST_DEVICE_F_RANDOMNESS 0x8000 /* IRQ is fairly random */

    + /* This is how the Guest reports status of the device: the Host can set
    + * LGUEST_DEVICE_S_REMOVED to indicate removal, but the rest are only
    + * ever manipulated by the Guest, and only ever set. */
    u16 status;
    /* 256 and above are device specific. */
    #define LGUEST_DEVICE_S_ACKNOWLEDGE 1 /* We have seen device. */
    @@ -58,9 +109,12 @@ struct lguest_device_desc {
    #define LGUEST_DEVICE_S_REMOVED_ACK 16 /* Driver has been told. */
    #define LGUEST_DEVICE_S_FAILED 128 /* Something actually failed */

    + /* Each device exists somewhere in Guest physical memory, over some
    + * number of pages. */
    u16 num_pages;
    u32 pfn;
    };
    +/*:*/

    /* Write command first word is a request. */
    enum lguest_req

    -
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