lguest: documentation III: Drivers

Documentation: The Drivers

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

4 files changed, 501 insertions, 35 deletions

diff --git a/drivers/block/lguest_blk.c b/drivers/block/lguest_blk.c
index 5b79d0724171..93e3c4001bf5 100644
--- a/drivers/block/lguest_blk.c
+++ b/drivers/block/lguest_blk.c
@@ -1,6 +1,12 @@
-/* A simple block driver for lguest.
+/*D:400
+ * The Guest block driver
  *
- * Copyright 2006 Rusty Russell <rusty@rustcorp.com.au> IBM Corporation
+ * 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 request *req, int uptodate)
         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 request *req, struct lguest_dma *dma)
         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 request *req, struct lguest_dma *dma)
                         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, struct request *req)
         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, struct request *req)
         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(struct request_queue *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_device *lgdev)
         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,
diff --git a/drivers/char/hvc_lguest.c b/drivers/char/hvc_lguest.c
index e7b889e404a7..1de8967cce06 100644
--- 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_drv = {
         .probe = lguestcons_probe,
 };
 
+/* The standard init function */
 static int __init hvc_lguest_init(void)
 {
         return register_lguest_driver(&lguestcons_drv);
diff --git a/drivers/lguest/lguest_bus.c b/drivers/lguest/lguest_bus.c
index 9a22d199502e..55a7940ca732 100644
--- a/drivers/lguest/lguest_bus.c
+++ b/drivers/lguest/lguest_bus.c
@@ -46,6 +46,10 @@ static struct device_attribute lguest_dev_attrs[] = {
         __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 device *_dev, struct device_driver *_drv)
 
         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 device *_dev)
         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_driver *drv)
 
         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 int index)
                 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 int index)
         }
 }
 
+/*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);
diff --git a/drivers/net/lguest_net.c b/drivers/net/lguest_net.c
index 112778652f7d..20df6a848923 100644
--- a/drivers/net/lguest_net.c
+++ b/drivers/net/lguest_net.c
@@ -1,6 +1,13 @@
-/* A simple network driver for lguest.
+/*D:500
+ * The Guest network driver.
  *
- * Copyright 2006 Rusty Russell <rusty@rustcorp.com.au> IBM Corporation
+ * 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
@@ -28,23 +35,28 @@
 #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)
@@ -52,39 +64,82 @@ static unsigned int rest_of_page(void *data)
         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);
@@ -95,11 +150,14 @@ static void lguestnet_set_multicast(struct net_device *dev)
                 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)
 {
@@ -109,6 +167,8 @@ static int mac_eq(const unsigned char mac[ETH_ALEN],
         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)
@@ -116,76 +176,134 @@ static void transfer_packet(struct net_device *dev,
         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;
@@ -194,10 +312,16 @@ static irqreturn_t lguestnet_rcv(int irq, void *dev_id)
                 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);
@@ -205,36 +329,72 @@ static irqreturn_t lguestnet_rcv(int irq, void *dev_id)
                         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;
@@ -245,22 +405,29 @@ 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;
@@ -290,10 +457,16 @@ static int lguestnet_probe(struct lguest_device *lgdev)
         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;
 
@@ -325,6 +498,9 @@ static int lguestnet_probe(struct lguest_device *lgdev)
         }
 
         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;
 
@@ -352,3 +528,11 @@ 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?"
+ */