Lguest support for Virtio

This makes lguest able to use the virtio devices.

We change the device descriptor page from a simple array to a variable
length "type, config_len, status, config data..." format, and
implement virtio_config_ops to read from that config data.

We use the virtio ring implementation for an efficient Guest <-> Host
virtqueue mechanism, and the new LHCALL_NOTIFY hypercall to kick the
host when it changes.

We also use LHCALL_NOTIFY on kernel addresses for very very early
console output.  We could have another hypercall, but this hack works
quite well.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

arch/x86/lguest/Kconfig

@@ -2,8 +2,13 @@ config LGUEST_GUEST
 	bool "Lguest guest support"
 	select PARAVIRT
 	depends on !X86_PAE
+	select VIRTIO
 	select VIRTIO_RING
+	select VIRTIO_CONSOLE
 	help
 	  Lguest is a tiny in-kernel hypervisor.  Selecting this will
 	  allow your kernel to boot under lguest.  This option will increase
 	  your kernel size by about 6k.  If in doubt, say N.
+
+	  If you say Y here, make sure you say Y (or M) to the virtio block
+	  and net drivers which lguest needs.

arch/x86/lguest/boot.c

@@ -55,6 +55,7 @@
 #include <linux/clockchips.h>
 #include <linux/lguest.h>
 #include <linux/lguest_launcher.h>
+#include <linux/virtio_console.h>
 #include <asm/paravirt.h>
 #include <asm/param.h>
 #include <asm/page.h>
@@ -849,6 +850,23 @@ static __init char *lguest_memory_setup(void)
 	return "LGUEST";
 }
 
+/* Before virtqueues are set up, we use LHCALL_NOTIFY on normal memory to
+ * produce console output. */
+static __init int early_put_chars(u32 vtermno, const char *buf, int count)
+{
+	char scratch[17];
+	unsigned int len = count;
+
+	if (len > sizeof(scratch) - 1)
+		len = sizeof(scratch) - 1;
+	scratch[len] = '\0';
+	memcpy(scratch, buf, len);
+	hcall(LHCALL_NOTIFY, __pa(scratch), 0, 0);
+
+	/* This routine returns the number of bytes actually written. */
+	return len;
+}
+
 /*G:050
  * Patching (Powerfully Placating Performance Pedants)
  *
@@ -1048,6 +1066,9 @@ __init void lguest_init(void *boot)
 	 * adapted for lguest's use. */
 	add_preferred_console("hvc", 0, NULL);
 
+	/* Register our very early console. */
+	virtio_cons_early_init(early_put_chars);
+
 	/* Last of all, we set the power management poweroff hook to point to
 	 * the Guest routine to power off. */
 	pm_power_off = lguest_power_off;

drivers/lguest/Makefile

+# Guest requires the device configuration and probing code.
+obj-$(CONFIG_LGUEST_GUEST) += lguest_device.o
+
 # Host requires the other files, which can be a module.
 obj-$(CONFIG_LGUEST)	+= lg.o
 lg-y = core.o hypercalls.o page_tables.o interrupts_and_traps.o \

drivers/lguest/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 a series of fields.
+ * The device will look for these fields 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 has one queue for sending and one 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 guys 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;
+
+	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?". */

include/linux/lguest_launcher.h

@@ -22,37 +22,28 @@
  * 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:
+ * Devices are described by a simplified ID, a status byte, and some "config"
+ * bytes which describe this device's configuration.  This is 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. */
-#define LGUEST_DEVICE_S_DRIVER		2 /* We have found a driver */
-#define LGUEST_DEVICE_S_DRIVER_OK	4 /* Driver says OK! */
-#define LGUEST_DEVICE_S_REMOVED		8 /* Device has gone away. */
-#define LGUEST_DEVICE_S_REMOVED_ACK	16 /* Driver has been told. */
-#define LGUEST_DEVICE_S_FAILED		128 /* Something actually failed */
+	/* The device type: console, network, disk etc.  Type 0 terminates. */
+	__u8 type;
+	/* The number of bytes of the config array. */
+	__u8 config_len;
+	/* A status byte, written by the Guest. */
+	__u8 status;
+	__u8 config[0];
+};
 
-	/* Each device exists somewhere in Guest physical memory, over some
-	 * number of pages. */
-	__u16 num_pages;
+/*D:135 This is how we expect the device configuration field for a virtqueue
+ * (type VIRTIO_CONFIG_F_VIRTQUEUE) to be laid out: */
+struct lguest_vqconfig {
+	/* The number of entries in the virtio_ring */
+	__u16 num;
+	/* The interrupt we get when something happens. */
+	__u16 irq;
+	/* The page number of the virtio ring for this device. */
 	__u32 pfn;
 };
 /*:*/