lguest_device.c

/*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 one or more
 * virtqueues, a feature bitmaks, and some configuration bytes.  The
 * configuration bytes don't really matter to us: the Launcher set them up, and
 * the driver will look at them during setup.
 *
 * A convenient routine to return the device's virtqueue config array:
 * immediately after the descriptor. */
static struct lguest_vqconfig *lg_vq(const struct lguest_device_desc *desc)
{
	return (void *)(desc + 1);
}

/* The features come immediately after the virtqueues. */
static u8 *lg_features(const struct lguest_device_desc *desc)
{
	return (void *)(lg_vq(desc) + desc->num_vq);
}

/* The config space comes after the two feature bitmasks. */
static u8 *lg_config(const struct lguest_device_desc *desc)
{
	return lg_features(desc) + desc->feature_len * 2;
}

/* The total size of the config page used by this device (incl. desc) */
static unsigned desc_size(const struct lguest_device_desc *desc)
{
	return sizeof(*desc)
		+ desc->num_vq * sizeof(struct lguest_vqconfig)
		+ desc->feature_len * 2
		+ desc->config_len;
}

/* This tests (and acknowleges) a feature bit. */
static bool lg_feature(struct virtio_device *vdev, unsigned fbit)
{
	struct lguest_device_desc *desc = to_lgdev(vdev)->desc;
	u8 *features;

	/* Obviously if they ask for a feature off the end of our feature
	 * bitmap, it's not set. */
	if (fbit / 8 > desc->feature_len)
		return false;

	/* The feature bitmap comes after the virtqueues. */
	features = lg_features(desc);
	if (!(features[fbit / 8] & (1 << (fbit % 8))))
		return false;

	/* We set the matching bit in the other half of the bitmap to tell the
	 * Host we want to use this feature.  We don't use this yet, but we
	 * could in future. */
	features[desc->feature_len + fbit / 8] |= (1 << (fbit % 8));
	return true;
}

/* Once they've found a field, getting a copy of it is easy. */
static void lg_get(struct virtio_device *vdev, unsigned int offset,
		   void *buf, unsigned len)
{
	struct lguest_device_desc *desc = to_lgdev(vdev)->desc;

	/* Check they didn't ask for more than the length of the config! */
	BUG_ON(offset + len > desc->config_len);
	memcpy(buf, lg_config(desc) + offset, len);
}

/* Setting the contents is also trivial. */
static void lg_set(struct virtio_device *vdev, unsigned int offset,
		   const void *buf, unsigned len)
{
	struct lguest_device_desc *desc = to_lgdev(vdev)->desc;

	/* Check they didn't ask for more than the length of the config! */
	BUG_ON(offset + len > desc->config_len);
	memcpy(lg_config(desc) + offset, 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,
				    unsigned index,
				    bool (*callback)(struct virtqueue *vq))
{
	struct lguest_device *ldev = to_lgdev(vdev);
	struct lguest_vq_info *lvq;
	struct virtqueue *vq;
	int err;

	/* We must have this many virtqueues. */
	if (index >= ldev->desc->num_vq)
		return ERR_PTR(-ENOENT);

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

	/* Make a copy of the "struct lguest_vqconfig" entry, which sits after
	 * the descriptor.  We need a copy because the config space might not
	 * be aligned correctly. */
	memcpy(&lvq->config, lg_vq(ldev->desc)+index, sizeof(lvq->config));

	printk("Mapping virtqueue %i addr %lx\n", index,
	       (unsigned long)lvq->config.pfn << PAGE_SHIFT);
	/* 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),
					     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;

	/* Release the interrupt */
	free_irq(lvq->config.irq, vq);
	/* 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 = {
	.feature = lg_feature,
	.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 += desc_size(d)) {
		d = lguest_devices + i;

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

		printk("Device at %i has size %u\n", i, desc_size(d));
		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?". */