drivers/lguest/lguest_device.c - kernel/msm - Gitiles

 /*P:050 Lguest guests use a very simple method to describe devices.  It's a
  * series of device descriptors contained just above the top of normal
  * memory.
  *
  * We use the standard "virtio" device infrastructure, which provides us with a
  * console, a network and a block driver.  Each one expects some configuration
  * information and a "virtqueue" mechanism to send and receive data. :*/
 #include <linux/init.h>
 #include <linux/bootmem.h>
 #include <linux/lguest_launcher.h>
 #include <linux/virtio.h>
 #include <linux/virtio_config.h>
 #include <linux/interrupt.h>
 #include <linux/virtio_ring.h>
 #include <linux/err.h>
 #include <asm/io.h>
 #include <asm/paravirt.h>
 #include <asm/lguest_hcall.h>

 /* The pointer to our (page) of device descriptions. */
 static void *lguest_devices;

 /* Unique numbering for lguest devices. */
 static unsigned int dev_index;

 /* For Guests, device memory can be used as normal memory, so we cast away the
  * __iomem to quieten sparse. */
 static inline void *lguest_map(unsigned long phys_addr, unsigned long pages)
 {
 	return (__force void *)ioremap(phys_addr, PAGE_SIZE*pages);
 }

 static inline void lguest_unmap(void *addr)
 {
 	iounmap((__force void __iomem *)addr);
 }

 /*D:100 Each lguest device is just a virtio device plus a pointer to its entry
  * in the lguest_devices page. */
 struct lguest_device {
 	struct virtio_device vdev;

 	/* The entry in the lguest_devices page for this device. */
 	struct lguest_device_desc *desc;
 };

 /* Since the virtio infrastructure hands us a pointer to the virtio_device all
  * the time, it helps to have a curt macro to get a pointer to the struct
  * lguest_device it's enclosed in.  */
 #define to_lgdev(vdev) container_of(vdev, struct lguest_device, vdev)

 /*D:130
  * Device configurations
  *
  * The configuration information for a device consists of a series of fields.
  * We don't really care what they are: the Launcher set them up, and the driver
  * will look at them during setup.
  *
  * For us these fields come immediately after that device's descriptor in the
  * lguest_devices page.
  *
  * Each field starts with a "type" byte, a "length" byte, then that number of
  * bytes of configuration information.  The device descriptor tells us the
  * total configuration length so we know when we've reached the last field. */

 /* type + length bytes */
 #define FHDR_LEN 2

 /* This finds the first field of a given type for a device's configuration. */
 static void *lg_find(struct virtio_device *vdev, u8 type, unsigned int *len)
 {
 	struct lguest_device_desc *desc = to_lgdev(vdev)->desc;
 	int i;

 	for (i = 0; i < desc->config_len; i += FHDR_LEN + desc->config[i+1]) {
 		if (desc->config[i] == type) {
 			/* Mark it used, so Host can know we looked at it, and
 			 * also so we won't find the same one twice. */
 			desc->config[i] |= 0x80;
 			/* Remember, the second byte is the length. */
 			*len = desc->config[i+1];
 			/* We return a pointer to the field header. */
 			return desc->config + i;
 		}
 	}

 	/* Not found: return NULL for failure. */
 	return NULL;
 }

 /* Once they've found a field, getting a copy of it is easy. */
 static void lg_get(struct virtio_device *vdev, void *token,
 		   void *buf, unsigned len)
 {
 	/* Check they didn't ask for more than the length of the field! */
 	BUG_ON(len > ((u8 *)token)[1]);
 	memcpy(buf, token + FHDR_LEN, len);
 }

 /* Setting the contents is also trivial. */
 static void lg_set(struct virtio_device *vdev, void *token,
 		   const void *buf, unsigned len)
 {
 	BUG_ON(len > ((u8 *)token)[1]);
 	memcpy(token + FHDR_LEN, buf, len);
 }

 /* The operations to get and set the status word just access the status field
  * of the device descriptor. */
 static u8 lg_get_status(struct virtio_device *vdev)
 {
 	return to_lgdev(vdev)->desc->status;
 }

 static void lg_set_status(struct virtio_device *vdev, u8 status)
 {
 	to_lgdev(vdev)->desc->status = status;
 }

 /*
  * Virtqueues
  *
  * The other piece of infrastructure virtio needs is a "virtqueue": a way of
  * the Guest device registering buffers for the other side to read from or
  * write into (ie. send and receive buffers).  Each device can have multiple
  * virtqueues: for example the console driver uses one queue for sending and
  * another for receiving.
  *
  * Fortunately for us, a very fast shared-memory-plus-descriptors virtqueue
  * already exists in virtio_ring.c.  We just need to connect it up.
  *
  * We start with the information we need to keep about each virtqueue.
  */

 /*D:140 This is the information we remember about each virtqueue. */
 struct lguest_vq_info
 {
 	/* A copy of the information contained in the device config. */
 	struct lguest_vqconfig config;

 	/* The address where we mapped the virtio ring, so we can unmap it. */
 	void *pages;
 };

 /* When the virtio_ring code wants to prod the Host, it calls us here and we
  * make a hypercall.  We hand the page number of the virtqueue so the Host
  * knows which virtqueue we're talking about. */
 static void lg_notify(struct virtqueue *vq)
 {
 	/* We store our virtqueue information in the "priv" pointer of the
 	 * virtqueue structure. */
 	struct lguest_vq_info *lvq = vq->priv;

 	hcall(LHCALL_NOTIFY, lvq->config.pfn << PAGE_SHIFT, 0, 0);
 }

 /* This routine finds the first virtqueue described in the configuration of
  * this device and sets it up.
  *
  * This is kind of an ugly duckling.  It'd be nicer to have a standard
  * representation of a virtqueue in the configuration space, but it seems that
  * everyone wants to do it differently.  The KVM coders want the Guest to
  * allocate its own pages and tell the Host where they are, but for lguest it's
  * simpler for the Host to simply tell us where the pages are.
  *
  * So we provide devices with a "find virtqueue and set it up" function. */
 static struct virtqueue *lg_find_vq(struct virtio_device *vdev,
 				    bool (*callback)(struct virtqueue *vq))
 {
 	struct lguest_vq_info *lvq;
 	struct virtqueue *vq;
 	unsigned int len;
 	void *token;
 	int err;

 	/* Look for a field of the correct type to mark a virtqueue.  Note that
 	 * if this succeeds, then the type will be changed so it won't be found
 	 * again, and future lg_find_vq() calls will find the next
 	 * virtqueue (if any). */
 	token = vdev->config->find(vdev, VIRTIO_CONFIG_F_VIRTQUEUE, &len);
 	if (!token)
 		return ERR_PTR(-ENOENT);

 	lvq = kmalloc(sizeof(*lvq), GFP_KERNEL);
 	if (!lvq)
 		return ERR_PTR(-ENOMEM);

 	/* Note: we could use a configuration space inside here, just like we
 	 * do for the device.  This would allow expansion in future, because
 	 * our configuration system is designed to be expansible.  But this is
 	 * way easier. */
 	if (len != sizeof(lvq->config)) {
 		dev_err(&vdev->dev, "Unexpected virtio config len %u\n", len);
 		err = -EIO;
 		goto free_lvq;
 	}
 	/* Make a copy of the "struct lguest_vqconfig" field.  We need a copy
 	 * because the config space might not be aligned correctly. */
 	vdev->config->get(vdev, token, &lvq->config, sizeof(lvq->config));

 	/* Figure out how many pages the ring will take, and map that memory */
 	lvq->pages = lguest_map((unsigned long)lvq->config.pfn << PAGE_SHIFT,
 				DIV_ROUND_UP(vring_size(lvq->config.num),
 					     PAGE_SIZE));
 	if (!lvq->pages) {
 		err = -ENOMEM;
 		goto free_lvq;
 	}

 	/* OK, tell virtio_ring.c to set up a virtqueue now we know its size
 	 * and we've got a pointer to its pages. */
 	vq = vring_new_virtqueue(lvq->config.num, vdev, lvq->pages,
 				 lg_notify, callback);
 	if (!vq) {
 		err = -ENOMEM;
 		goto unmap;
 	}

 	/* Tell the interrupt for this virtqueue to go to the virtio_ring
 	 * interrupt handler. */
 	/* FIXME: We used to have a flag for the Host to tell us we could use
 	 * the interrupt as a source of randomness: it'd be nice to have that
 	 * back.. */
 	err = request_irq(lvq->config.irq, vring_interrupt, IRQF_SHARED,
 			  vdev->dev.bus_id, vq);
 	if (err)
 		goto destroy_vring;

 	/* Last of all we hook up our 'struct lguest_vq_info" to the
 	 * virtqueue's priv pointer. */
 	vq->priv = lvq;
 	return vq;

 destroy_vring:
 	vring_del_virtqueue(vq);
 unmap:
 	lguest_unmap(lvq->pages);
 free_lvq:
 	kfree(lvq);
 	return ERR_PTR(err);
 }
 /*:*/

 /* Cleaning up a virtqueue is easy */
 static void lg_del_vq(struct virtqueue *vq)
 {
 	struct lguest_vq_info *lvq = vq->priv;

 	/* Tell virtio_ring.c to free the virtqueue. */
 	vring_del_virtqueue(vq);
 	/* Unmap the pages containing the ring. */
 	lguest_unmap(lvq->pages);
 	/* Free our own queue information. */
 	kfree(lvq);
 }

 /* The ops structure which hooks everything together. */
 static struct virtio_config_ops lguest_config_ops = {
 	.find = lg_find,
 	.get = lg_get,
 	.set = lg_set,
 	.get_status = lg_get_status,
 	.set_status = lg_set_status,
 	.find_vq = lg_find_vq,
 	.del_vq = lg_del_vq,
 };

 /* The root device for the lguest virtio devices.  This makes them appear as
  * /sys/devices/lguest/0,1,2 not /sys/devices/0,1,2. */
 static struct device lguest_root = {
 	.parent = NULL,
 	.bus_id = "lguest",
 };

 /*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 a pointer to the new device
  * descriptor in the "lguest_devices" page. */
 static void add_lguest_device(struct lguest_device_desc *d)
 {
 	struct lguest_device *ldev;

 	/* Start with zeroed memory; Linux's device layer seems to count on
 	 * it. */
 	ldev = kzalloc(sizeof(*ldev), GFP_KERNEL);
 	if (!ldev) {
 		printk(KERN_EMERG "Cannot allocate lguest dev %u\n",
 		       dev_index++);
 		return;
 	}

 	/* This devices' parent is the lguest/ dir. */
 	ldev->vdev.dev.parent = &lguest_root;
 	/* We have a unique device index thanks to the dev_index counter. */
 	ldev->vdev.index = dev_index++;
 	/* The device type comes straight from the descriptor.  There's also a
 	 * device vendor field in the virtio_device struct, which we leave as
 	 * 0. */
 	ldev->vdev.id.device = d->type;
 	/* We have a simple set of routines for querying the device's
 	 * configuration information and setting its status. */
 	ldev->vdev.config = &lguest_config_ops;
 	/* And we remember the device's descriptor for lguest_config_ops. */
 	ldev->desc = d;

 	/* register_virtio_device() sets up the generic fields for the struct
 	 * virtio_device and calls device_register().  This makes the bus
 	 * infrastructure look for a matching driver. */
 	if (register_virtio_device(&ldev->vdev) != 0) {
 		printk(KERN_ERR "Failed to register lguest device %u\n",
 		       ldev->vdev.index);
 		kfree(ldev);
 	}
 }

 /*D:110 scan_devices() simply iterates through the device page.  The type 0 is
  * reserved to mean "end of devices". */
 static void scan_devices(void)
 {
 	unsigned int i;
 	struct lguest_device_desc *d;

 	/* We start at the page beginning, and skip over each entry. */
 	for (i = 0; i < PAGE_SIZE; i += sizeof(*d) + d->config_len) {
 		d = lguest_devices + i;

 		/* Once we hit a zero, stop. */
 		if (d->type == 0)
 			break;

 		add_lguest_device(d);
 	}
 }

 /*D:105 Fairly early in boot, lguest_devices_init() is called to set up the
  * lguest device infrastructure.  We check that we are a Guest by checking
  * pv_info.name: there are other ways of checking, but this seems most
  * obvious to me.
  *
  * So we can access the "struct lguest_device_desc"s easily, we map that memory
  * and store the pointer in the global "lguest_devices".  Then we register a
  * root device from which all our devices will hang (this seems to be the
  * correct sysfs incantation).
  *
  * Finally we call scan_devices() which adds all the devices found in the
  * lguest_devices page. */
 static int __init lguest_devices_init(void)
 {
 	if (strcmp(pv_info.name, "lguest") != 0)
 		return 0;

 	if (device_register(&lguest_root) != 0)
 		panic("Could not register lguest root");

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

 	scan_devices();
 	return 0;
 }
 /* We do this after core stuff, but before the drivers. */
 postcore_initcall(lguest_devices_init);

 /*D:150 At this point in the journey we used to now wade through the lguest
  * devices themselves: net, block and console.  Since they're all now virtio
  * devices rather than lguest-specific, I've decided to ignore them.  Mostly,
  * they're kind of boring.  But this does mean you'll never experience the
  * thrill of reading the forbidden love scene buried deep in the block driver.
  *
  * "make Launcher" beckons, where we answer questions like "Where do Guests
  * come from?", and "What do you do when someone asks for optimization?". */
	/*P:050 Lguest guests use a very simple method to describe devices. It's a
	* series of device descriptors contained just above the top of normal
	* memory.
	*
	* We use the standard "virtio" device infrastructure, which provides us with a
	* console, a network and a block driver. Each one expects some configuration
	* information and a "virtqueue" mechanism to send and receive data. :*/
	#include <linux/init.h>
	#include <linux/bootmem.h>
	#include <linux/lguest_launcher.h>
	#include <linux/virtio.h>
	#include <linux/virtio_config.h>
	#include <linux/interrupt.h>
	#include <linux/virtio_ring.h>
	#include <linux/err.h>
	#include <asm/io.h>
	#include <asm/paravirt.h>
	#include <asm/lguest_hcall.h>

	/* The pointer to our (page) of device descriptions. */
	static void *lguest_devices;

	/* Unique numbering for lguest devices. */
	static unsigned int dev_index;

	/* For Guests, device memory can be used as normal memory, so we cast away the
	* __iomem to quieten sparse. */
	static inline void *lguest_map(unsigned long phys_addr, unsigned long pages)
	{
	return (__force void )ioremap(phys_addr, PAGE_SIZEpages);
	}

	static inline void lguest_unmap(void *addr)
	{
	iounmap((__force void __iomem *)addr);
	}

	/*D:100 Each lguest device is just a virtio device plus a pointer to its entry
	* in the lguest_devices page. */
	struct lguest_device {
	struct virtio_device vdev;

	/* The entry in the lguest_devices page for this device. */
	struct lguest_device_desc *desc;
	};

	/* Since the virtio infrastructure hands us a pointer to the virtio_device all
	* the time, it helps to have a curt macro to get a pointer to the struct
	* lguest_device it's enclosed in. */
	#define to_lgdev(vdev) container_of(vdev, struct lguest_device, vdev)

	/*D:130
	* Device configurations
	*
	* The configuration information for a device consists of a series of fields.
	* We don't really care what they are: the Launcher set them up, and the driver
	* will look at them during setup.
	*
	* For us these fields come immediately after that device's descriptor in the
	* lguest_devices page.
	*
	* Each field starts with a "type" byte, a "length" byte, then that number of
	* bytes of configuration information. The device descriptor tells us the
	* total configuration length so we know when we've reached the last field. */

	/* type + length bytes */
	#define FHDR_LEN 2

	/* This finds the first field of a given type for a device's configuration. */
	static void lg_find(struct virtio_device vdev, u8 type, unsigned int *len)
	{
	struct lguest_device_desc *desc = to_lgdev(vdev)->desc;
	int i;

	for (i = 0; i < desc->config_len; i += FHDR_LEN + desc->config[i+1]) {
	if (desc->config[i] == type) {
	/* Mark it used, so Host can know we looked at it, and
	* also so we won't find the same one twice. */
	desc->config[i] \|= 0x80;
	/* Remember, the second byte is the length. */
	*len = desc->config[i+1];
	/* We return a pointer to the field header. */
	return desc->config + i;
	}
	}

	/* Not found: return NULL for failure. */
	return NULL;
	}

	/* Once they've found a field, getting a copy of it is easy. */
	static void lg_get(struct virtio_device vdev, void token,
	void *buf, unsigned len)
	{
	/* Check they didn't ask for more than the length of the field! */
	BUG_ON(len > ((u8 *)token)[1]);
	memcpy(buf, token + FHDR_LEN, len);
	}

	/* Setting the contents is also trivial. */
	static void lg_set(struct virtio_device vdev, void token,
	const void *buf, unsigned len)
	{
	BUG_ON(len > ((u8 *)token)[1]);
	memcpy(token + FHDR_LEN, buf, len);
	}

	/* The operations to get and set the status word just access the status field
	* of the device descriptor. */
	static u8 lg_get_status(struct virtio_device *vdev)
	{
	return to_lgdev(vdev)->desc->status;
	}

	static void lg_set_status(struct virtio_device *vdev, u8 status)
	{
	to_lgdev(vdev)->desc->status = status;
	}

	/*
	* Virtqueues
	*
	* The other piece of infrastructure virtio needs is a "virtqueue": a way of
	* the Guest device registering buffers for the other side to read from or
	* write into (ie. send and receive buffers). Each device can have multiple
	* virtqueues: for example the console driver uses one queue for sending and
	* another for receiving.
	*
	* Fortunately for us, a very fast shared-memory-plus-descriptors virtqueue
	* already exists in virtio_ring.c. We just need to connect it up.
	*
	* We start with the information we need to keep about each virtqueue.
	*/

	/D:140 This is the information we remember about each virtqueue. /
	struct lguest_vq_info
	{
	/* A copy of the information contained in the device config. */
	struct lguest_vqconfig config;

	/* The address where we mapped the virtio ring, so we can unmap it. */
	void *pages;
	};

	/* When the virtio_ring code wants to prod the Host, it calls us here and we
	* make a hypercall. We hand the page number of the virtqueue so the Host
	* knows which virtqueue we're talking about. */
	static void lg_notify(struct virtqueue *vq)
	{
	/* We store our virtqueue information in the "priv" pointer of the
	* virtqueue structure. */
	struct lguest_vq_info *lvq = vq->priv;

	hcall(LHCALL_NOTIFY, lvq->config.pfn << PAGE_SHIFT, 0, 0);
	}

	/* This routine finds the first virtqueue described in the configuration of
	* this device and sets it up.
	*
	* This is kind of an ugly duckling. It'd be nicer to have a standard
	* representation of a virtqueue in the configuration space, but it seems that
	* everyone wants to do it differently. The KVM coders want the Guest to
	* allocate its own pages and tell the Host where they are, but for lguest it's
	* simpler for the Host to simply tell us where the pages are.
	*
	* So we provide devices with a "find virtqueue and set it up" function. */
	static struct virtqueue lg_find_vq(struct virtio_device vdev,
	bool (callback)(struct virtqueue vq))
	{
	struct lguest_vq_info *lvq;
	struct virtqueue *vq;
	unsigned int len;
	void *token;
	int err;

	/* Look for a field of the correct type to mark a virtqueue. Note that
	* if this succeeds, then the type will be changed so it won't be found
	* again, and future lg_find_vq() calls will find the next
	* virtqueue (if any). */
	token = vdev->config->find(vdev, VIRTIO_CONFIG_F_VIRTQUEUE, &len);
	if (!token)
	return ERR_PTR(-ENOENT);

	lvq = kmalloc(sizeof(*lvq), GFP_KERNEL);
	if (!lvq)
	return ERR_PTR(-ENOMEM);

	/* Note: we could use a configuration space inside here, just like we
	* do for the device. This would allow expansion in future, because
	* our configuration system is designed to be expansible. But this is
	* way easier. */
	if (len != sizeof(lvq->config)) {
	dev_err(&vdev->dev, "Unexpected virtio config len %u\n", len);
	err = -EIO;
	goto free_lvq;
	}
	/* Make a copy of the "struct lguest_vqconfig" field. We need a copy
	* because the config space might not be aligned correctly. */
	vdev->config->get(vdev, token, &lvq->config, sizeof(lvq->config));

	/* Figure out how many pages the ring will take, and map that memory */
	lvq->pages = lguest_map((unsigned long)lvq->config.pfn << PAGE_SHIFT,
	DIV_ROUND_UP(vring_size(lvq->config.num),
	PAGE_SIZE));
	if (!lvq->pages) {
	err = -ENOMEM;
	goto free_lvq;
	}

	/* OK, tell virtio_ring.c to set up a virtqueue now we know its size
	* and we've got a pointer to its pages. */
	vq = vring_new_virtqueue(lvq->config.num, vdev, lvq->pages,
	lg_notify, callback);
	if (!vq) {
	err = -ENOMEM;
	goto unmap;
	}

	/* Tell the interrupt for this virtqueue to go to the virtio_ring
	* interrupt handler. */
	/* FIXME: We used to have a flag for the Host to tell us we could use
	* the interrupt as a source of randomness: it'd be nice to have that
	* back.. */
	err = request_irq(lvq->config.irq, vring_interrupt, IRQF_SHARED,
	vdev->dev.bus_id, vq);
	if (err)
	goto destroy_vring;

	/* Last of all we hook up our 'struct lguest_vq_info" to the
	* virtqueue's priv pointer. */
	vq->priv = lvq;
	return vq;

	destroy_vring:
	vring_del_virtqueue(vq);
	unmap:
	lguest_unmap(lvq->pages);
	free_lvq:
	kfree(lvq);
	return ERR_PTR(err);
	}
	/:/

	/* Cleaning up a virtqueue is easy */
	static void lg_del_vq(struct virtqueue *vq)
	{
	struct lguest_vq_info *lvq = vq->priv;

	/* Tell virtio_ring.c to free the virtqueue. */
	vring_del_virtqueue(vq);
	/* Unmap the pages containing the ring. */
	lguest_unmap(lvq->pages);
	/* Free our own queue information. */
	kfree(lvq);
	}

	/* The ops structure which hooks everything together. */
	static struct virtio_config_ops lguest_config_ops = {
	.find = lg_find,
	.get = lg_get,
	.set = lg_set,
	.get_status = lg_get_status,
	.set_status = lg_set_status,
	.find_vq = lg_find_vq,
	.del_vq = lg_del_vq,
	};

	/* The root device for the lguest virtio devices. This makes them appear as
	* /sys/devices/lguest/0,1,2 not /sys/devices/0,1,2. */
	static struct device lguest_root = {
	.parent = NULL,
	.bus_id = "lguest",
	};

	/*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 a pointer to the new device
	* descriptor in the "lguest_devices" page. */
	static void add_lguest_device(struct lguest_device_desc *d)
	{
	struct lguest_device *ldev;

	/* Start with zeroed memory; Linux's device layer seems to count on
	* it. */
	ldev = kzalloc(sizeof(*ldev), GFP_KERNEL);
	if (!ldev) {
	printk(KERN_EMERG "Cannot allocate lguest dev %u\n",
	dev_index++);
	return;
	}

	/* This devices' parent is the lguest/ dir. */
	ldev->vdev.dev.parent = &lguest_root;
	/* We have a unique device index thanks to the dev_index counter. */
	ldev->vdev.index = dev_index++;
	/* The device type comes straight from the descriptor. There's also a
	* device vendor field in the virtio_device struct, which we leave as
	* 0. */
	ldev->vdev.id.device = d->type;
	/* We have a simple set of routines for querying the device's
	* configuration information and setting its status. */
	ldev->vdev.config = &lguest_config_ops;
	/* And we remember the device's descriptor for lguest_config_ops. */
	ldev->desc = d;

	/* register_virtio_device() sets up the generic fields for the struct
	* virtio_device and calls device_register(). This makes the bus
	* infrastructure look for a matching driver. */
	if (register_virtio_device(&ldev->vdev) != 0) {
	printk(KERN_ERR "Failed to register lguest device %u\n",
	ldev->vdev.index);
	kfree(ldev);
	}
	}

	/*D:110 scan_devices() simply iterates through the device page. The type 0 is
	* reserved to mean "end of devices". */
	static void scan_devices(void)
	{
	unsigned int i;
	struct lguest_device_desc *d;

	/* We start at the page beginning, and skip over each entry. */
	for (i = 0; i < PAGE_SIZE; i += sizeof(*d) + d->config_len) {
	d = lguest_devices + i;

	/* Once we hit a zero, stop. */
	if (d->type == 0)
	break;

	add_lguest_device(d);
	}
	}

	/*D:105 Fairly early in boot, lguest_devices_init() is called to set up the
	* lguest device infrastructure. We check that we are a Guest by checking
	* pv_info.name: there are other ways of checking, but this seems most
	* obvious to me.
	*
	* So we can access the "struct lguest_device_desc"s easily, we map that memory
	* and store the pointer in the global "lguest_devices". Then we register a
	* root device from which all our devices will hang (this seems to be the
	* correct sysfs incantation).
	*
	* Finally we call scan_devices() which adds all the devices found in the
	* lguest_devices page. */
	static int __init lguest_devices_init(void)
	{
	if (strcmp(pv_info.name, "lguest") != 0)
	return 0;

	if (device_register(&lguest_root) != 0)
	panic("Could not register lguest root");

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

	scan_devices();
	return 0;
	}
	/* We do this after core stuff, but before the drivers. */
	postcore_initcall(lguest_devices_init);

	/*D:150 At this point in the journey we used to now wade through the lguest
	* devices themselves: net, block and console. Since they're all now virtio
	* devices rather than lguest-specific, I've decided to ignore them. Mostly,
	* they're kind of boring. But this does mean you'll never experience the
	* thrill of reading the forbidden love scene buried deep in the block driver.
	*
	* "make Launcher" beckons, where we answer questions like "Where do Guests
	* come from?", and "What do you do when someone asks for optimization?". */