vringh: host-side implementation of virtio rings.

Getting use of virtio rings correct is tricky, and a recent patch saw
an implementation of in-kernel rings (as separate from userspace).

This abstracts the business of dealing with the virtio ring layout
from the access (userspace or direct); to do this, we use function
pointers, which gcc inlines correctly.

Signed-off-by: Rusty Russell <[e-mail]>
Acked-by: Michael S. Tsirkin <[e-mail]>

drivers/Makefile

···
 obj-$(CONFIG_OF)		+= of/
 obj-$(CONFIG_SSB)		+= ssb/
 obj-$(CONFIG_BCMA)		+= bcma/
-obj-$(CONFIG_VHOST_NET)		+= vhost/
+obj-$(CONFIG_VHOST_RING)	+= vhost/
 obj-$(CONFIG_VLYNQ)		+= vlynq/
 obj-$(CONFIG_STAGING)		+= staging/
 obj-y				+= platform/

drivers/vhost/Kconfig

···
 config VHOST_NET
 	tristate "Host kernel accelerator for virtio net"
 	depends on NET && EVENTFD && (TUN || !TUN) && (MACVTAP || !MACVTAP)
+	select VHOST_RING
 	---help---
 	  This kernel module can be loaded in host kernel to accelerate
 	  guest networking with virtio_net. Not to be confused with virtio_net
···
 if STAGING
 source "drivers/vhost/Kconfig.tcm"
 endif
+
+config VHOST_RING
+	tristate
+	---help---
+	  This option is selected by any driver which needs to access
+	  the host side of a virtio ring.
+

drivers/vhost/Kconfig.tcm

···
 config TCM_VHOST
 	tristate "TCM_VHOST fabric module"
 	depends on TARGET_CORE && EVENTFD && m
+	select VHOST_RING
 	default n
 	---help---
 	Say M here to enable the TCM_VHOST fabric module for use with virtio-scsi guests

drivers/vhost/Makefile

···
 vhost_net-y := vhost.o net.o
 
 obj-$(CONFIG_TCM_VHOST) += tcm_vhost.o
+
+obj-$(CONFIG_VHOST_RING) += vringh.o

drivers/vhost/vringh.c

···
+/*
+ * Helpers for the host side of a virtio ring.
+ *
+ * Since these may be in userspace, we use (inline) accessors.
+ */
+#include <linux/vringh.h>
+#include <linux/virtio_ring.h>
+#include <linux/kernel.h>
+#include <linux/ratelimit.h>
+#include <linux/uaccess.h>
+#include <linux/slab.h>
+#include <linux/export.h>
+
+static __printf(1,2) __cold void vringh_bad(const char *fmt, ...)
+{
+	static DEFINE_RATELIMIT_STATE(vringh_rs,
+				      DEFAULT_RATELIMIT_INTERVAL,
+				      DEFAULT_RATELIMIT_BURST);
+	if (__ratelimit(&vringh_rs)) {
+		va_list ap;
+		va_start(ap, fmt);
+		printk(KERN_NOTICE "vringh:");
+		vprintk(fmt, ap);
+		va_end(ap);
+	}
+}
+
+/* Returns vring->num if empty, -ve on error. */
+static inline int __vringh_get_head(const struct vringh *vrh,
+				    int (*getu16)(u16 *val, const u16 *p),
+				    u16 *last_avail_idx)
+{
+	u16 avail_idx, i, head;
+	int err;
+
+	err = getu16(&avail_idx, &vrh->vring.avail->idx);
+	if (err) {
+		vringh_bad("Failed to access avail idx at %p",
+			   &vrh->vring.avail->idx);
+		return err;
+	}
+
+	if (*last_avail_idx == avail_idx)
+		return vrh->vring.num;
+
+	/* Only get avail ring entries after they have been exposed by guest. */
+	virtio_rmb(vrh->weak_barriers);
+
+	i = *last_avail_idx & (vrh->vring.num - 1);
+
+	err = getu16(&head, &vrh->vring.avail->ring[i]);
+	if (err) {
+		vringh_bad("Failed to read head: idx %d address %p",
+			   *last_avail_idx, &vrh->vring.avail->ring[i]);
+		return err;
+	}
+
+	if (head >= vrh->vring.num) {
+		vringh_bad("Guest says index %u > %u is available",
+			   head, vrh->vring.num);
+		return -EINVAL;
+	}
+
+	(*last_avail_idx)++;
+	return head;
+}
+
+/* Copy some bytes to/from the iovec.  Returns num copied. */
+static inline ssize_t vringh_iov_xfer(struct vringh_kiov *iov,
+				      void *ptr, size_t len,
+				      int (*xfer)(void *addr, void *ptr,
+						  size_t len))
+{
+	int err, done = 0;
+
+	while (len && iov->i < iov->used) {
+		size_t partlen;
+
+		partlen = min(iov->iov[iov->i].iov_len, len);
+		err = xfer(iov->iov[iov->i].iov_base, ptr, partlen);
+		if (err)
+			return err;
+		done += partlen;
+		len -= partlen;
+		ptr += partlen;
+		iov->consumed += partlen;
+		iov->iov[iov->i].iov_len -= partlen;
+		iov->iov[iov->i].iov_base += partlen;
+
+		if (!iov->iov[iov->i].iov_len) {
+			/* Fix up old iov element then increment. */
+			iov->iov[iov->i].iov_len = iov->consumed;
+			iov->iov[iov->i].iov_base -= iov->consumed;
+			
+			iov->consumed = 0;
+			iov->i++;
+		}
+	}
+	return done;
+}
+
+/* May reduce *len if range is shorter. */
+static inline bool range_check(struct vringh *vrh, u64 addr, size_t *len,
+			       struct vringh_range *range,
+			       bool (*getrange)(struct vringh *,
+						u64, struct vringh_range *))
+{
+	if (addr < range->start || addr > range->end_incl) {
+		if (!getrange(vrh, addr, range))
+			return false;
+	}
+	BUG_ON(addr < range->start || addr > range->end_incl);
+
+	/* To end of memory? */
+	if (unlikely(addr + *len == 0)) {
+		if (range->end_incl == -1ULL)
+			return true;
+		goto truncate;
+	}
+
+	/* Otherwise, don't wrap. */
+	if (addr + *len < addr) {
+		vringh_bad("Wrapping descriptor %zu@0x%llx",
+			   *len, (unsigned long long)addr);
+		return false;
+	}
+
+	if (unlikely(addr + *len - 1 > range->end_incl))
+		goto truncate;
+	return true;
+
+truncate:
+	*len = range->end_incl + 1 - addr;
+	return true;
+}
+
+static inline bool no_range_check(struct vringh *vrh, u64 addr, size_t *len,
+				  struct vringh_range *range,
+				  bool (*getrange)(struct vringh *,
+						   u64, struct vringh_range *))
+{
+	return true;
+}
+
+/* No reason for this code to be inline. */
+static int move_to_indirect(int *up_next, u16 *i, void *addr,
+			    const struct vring_desc *desc,
+			    struct vring_desc **descs, int *desc_max)
+{
+	/* Indirect tables can't have indirect. */
+	if (*up_next != -1) {
+		vringh_bad("Multilevel indirect %u->%u", *up_next, *i);
+		return -EINVAL;
+	}
+
+	if (unlikely(desc->len % sizeof(struct vring_desc))) {
+		vringh_bad("Strange indirect len %u", desc->len);
+		return -EINVAL;
+	}
+
+	/* We will check this when we follow it! */
+	if (desc->flags & VRING_DESC_F_NEXT)
+		*up_next = desc->next;
+	else
+		*up_next = -2;
+	*descs = addr;
+	*desc_max = desc->len / sizeof(struct vring_desc);
+
+	/* Now, start at the first indirect. */
+	*i = 0;
+	return 0;
+}
+
+static int resize_iovec(struct vringh_kiov *iov, gfp_t gfp)
+{
+	struct kvec *new;
+	unsigned int flag, new_num = (iov->max_num & ~VRINGH_IOV_ALLOCATED) * 2;
+
+	if (new_num < 8)
+		new_num = 8;
+
+	flag = (iov->max_num & VRINGH_IOV_ALLOCATED);
+	if (flag)
+		new = krealloc(iov->iov, new_num * sizeof(struct iovec), gfp);
+	else {
+		new = kmalloc(new_num * sizeof(struct iovec), gfp);
+		if (new) {
+			memcpy(new, iov->iov,
+			       iov->max_num * sizeof(struct iovec));
+			flag = VRINGH_IOV_ALLOCATED;
+		}
+	}
+	if (!new)
+		return -ENOMEM;
+	iov->iov = new;
+	iov->max_num = (new_num | flag);
+	return 0;
+}
+
+static u16 __cold return_from_indirect(const struct vringh *vrh, int *up_next,
+				       struct vring_desc **descs, int *desc_max)
+{
+	u16 i = *up_next;
+
+	*up_next = -1;
+	*descs = vrh->vring.desc;
+	*desc_max = vrh->vring.num;
+	return i;
+}
+
+static int slow_copy(struct vringh *vrh, void *dst, const void *src,
+		     bool (*rcheck)(struct vringh *vrh, u64 addr, size_t *len,
+				    struct vringh_range *range,
+				    bool (*getrange)(struct vringh *vrh,
+						     u64,
+						     struct vringh_range *)),
+		     bool (*getrange)(struct vringh *vrh,
+				      u64 addr,
+				      struct vringh_range *r),
+		     struct vringh_range *range,
+		     int (*copy)(void *dst, const void *src, size_t len))
+{
+	size_t part, len = sizeof(struct vring_desc);
+
+	do {
+		u64 addr;
+		int err;
+
+		part = len;
+		addr = (u64)(unsigned long)src - range->offset;
+
+		if (!rcheck(vrh, addr, &part, range, getrange))
+			return -EINVAL;
+
+		err = copy(dst, src, part);
+		if (err)
+			return err;
+
+		dst += part;
+		src += part;
+		len -= part;
+	} while (len);
+	return 0;
+}
+
+static inline int
+__vringh_iov(struct vringh *vrh, u16 i,
+	     struct vringh_kiov *riov,
+	     struct vringh_kiov *wiov,
+	     bool (*rcheck)(struct vringh *vrh, u64 addr, size_t *len,
+			    struct vringh_range *range,
+			    bool (*getrange)(struct vringh *, u64,
+					     struct vringh_range *)),
+	     bool (*getrange)(struct vringh *, u64, struct vringh_range *),
+	     gfp_t gfp,
+	     int (*copy)(void *dst, const void *src, size_t len))
+{
+	int err, count = 0, up_next, desc_max;
+	struct vring_desc desc, *descs;
+	struct vringh_range range = { -1ULL, 0 }, slowrange;
+	bool slow = false;
+
+	/* We start traversing vring's descriptor table. */
+	descs = vrh->vring.desc;
+	desc_max = vrh->vring.num;
+	up_next = -1;
+
+	if (riov)
+		riov->i = riov->used = 0;
+	else if (wiov)
+		wiov->i = wiov->used = 0;
+	else
+		/* You must want something! */
+		BUG();
+
+	for (;;) {
+		void *addr;
+		struct vringh_kiov *iov;
+		size_t len;
+
+		if (unlikely(slow))
+			err = slow_copy(vrh, &desc, &descs[i], rcheck, getrange,
+					&slowrange, copy);
+		else
+			err = copy(&desc, &descs[i], sizeof(desc));
+		if (unlikely(err))
+			goto fail;
+
+		if (unlikely(desc.flags & VRING_DESC_F_INDIRECT)) {
+			/* Make sure it's OK, and get offset. */
+			len = desc.len;
+			if (!rcheck(vrh, desc.addr, &len, &range, getrange)) {
+				err = -EINVAL;
+				goto fail;
+			}
+
+			if (unlikely(len != desc.len)) {
+				slow = true;
+				/* We need to save this range to use offset */
+				slowrange = range;
+			}
+
+			addr = (void *)(long)(desc.addr + range.offset);
+			err = move_to_indirect(&up_next, &i, addr, &desc,
+					       &descs, &desc_max);
+			if (err)
+				goto fail;
+			continue;
+		}
+
+		if (count++ == vrh->vring.num) {
+			vringh_bad("Descriptor loop in %p", descs);
+			err = -ELOOP;
+			goto fail;
+		}
+
+		if (desc.flags & VRING_DESC_F_WRITE)
+			iov = wiov;
+		else {
+			iov = riov;
+			if (unlikely(wiov && wiov->i)) {
+				vringh_bad("Readable desc %p after writable",
+					   &descs[i]);
+				err = -EINVAL;
+				goto fail;
+			}
+		}
+
+		if (!iov) {
+			vringh_bad("Unexpected %s desc",
+				   !wiov ? "writable" : "readable");
+			err = -EPROTO;
+			goto fail;
+		}
+
+	again:
+		/* Make sure it's OK, and get offset. */
+		len = desc.len;
+		if (!rcheck(vrh, desc.addr, &len, &range, getrange)) {
+			err = -EINVAL;
+			goto fail;
+		}
+		addr = (void *)(unsigned long)(desc.addr + range.offset);
+
+		if (unlikely(iov->used == (iov->max_num & ~VRINGH_IOV_ALLOCATED))) {
+			err = resize_iovec(iov, gfp);
+			if (err)
+				goto fail;
+		}
+
+		iov->iov[iov->used].iov_base = addr;
+		iov->iov[iov->used].iov_len = len;
+		iov->used++;
+
+		if (unlikely(len != desc.len)) {
+			desc.len -= len;
+			desc.addr += len;
+			goto again;
+		}
+
+		if (desc.flags & VRING_DESC_F_NEXT) {
+			i = desc.next;
+		} else {
+			/* Just in case we need to finish traversing above. */
+			if (unlikely(up_next > 0)) {
+				i = return_from_indirect(vrh, &up_next,
+							 &descs, &desc_max);
+				slow = false;
+			} else
+				break;
+		}
+
+		if (i >= desc_max) {
+			vringh_bad("Chained index %u > %u", i, desc_max);
+			err = -EINVAL;
+			goto fail;
+		}
+	}
+
+	return 0;
+
+fail:
+	return err;
+}
+
+static inline int __vringh_complete(struct vringh *vrh,
+				    const struct vring_used_elem *used,
+				    unsigned int num_used,
+				    int (*putu16)(u16 *p, u16 val),
+				    int (*putused)(struct vring_used_elem *dst,
+						   const struct vring_used_elem
+						   *src, unsigned num))
+{
+	struct vring_used *used_ring;
+	int err;
+	u16 used_idx, off;
+
+	used_ring = vrh->vring.used;
+	used_idx = vrh->last_used_idx + vrh->completed;
+
+	off = used_idx % vrh->vring.num;
+
+	/* Compiler knows num_used == 1 sometimes, hence extra check */
+	if (num_used > 1 && unlikely(off + num_used >= vrh->vring.num)) {
+		u16 part = vrh->vring.num - off;
+		err = putused(&used_ring->ring[off], used, part);
+		if (!err)
+			err = putused(&used_ring->ring[0], used + part,
+				      num_used - part);
+	} else
+		err = putused(&used_ring->ring[off], used, num_used);
+
+	if (err) {
+		vringh_bad("Failed to write %u used entries %u at %p",
+			   num_used, off, &used_ring->ring[off]);
+		return err;
+	}
+
+	/* Make sure buffer is written before we update index. */
+	virtio_wmb(vrh->weak_barriers);
+
+	err = putu16(&vrh->vring.used->idx, used_idx + num_used);
+	if (err) {
+		vringh_bad("Failed to update used index at %p",
+			   &vrh->vring.used->idx);
+		return err;
+	}
+
+	vrh->completed += num_used;
+	return 0;
+}
+
+
+static inline int __vringh_need_notify(struct vringh *vrh,
+				       int (*getu16)(u16 *val, const u16 *p))
+{
+	bool notify;
+	u16 used_event;
+	int err;
+
+	/* Flush out used index update. This is paired with the
+	 * barrier that the Guest executes when enabling
+	 * interrupts. */
+	virtio_mb(vrh->weak_barriers);
+
+	/* Old-style, without event indices. */
+	if (!vrh->event_indices) {
+		u16 flags;
+		err = getu16(&flags, &vrh->vring.avail->flags);
+		if (err) {
+			vringh_bad("Failed to get flags at %p",
+				   &vrh->vring.avail->flags);
+			return err;
+		}
+		return (!(flags & VRING_AVAIL_F_NO_INTERRUPT));
+	}
+
+	/* Modern: we know when other side wants to know. */
+	err = getu16(&used_event, &vring_used_event(&vrh->vring));
+	if (err) {
+		vringh_bad("Failed to get used event idx at %p",
+			   &vring_used_event(&vrh->vring));
+		return err;
+	}
+
+	/* Just in case we added so many that we wrap. */
+	if (unlikely(vrh->completed > 0xffff))
+		notify = true;
+	else
+		notify = vring_need_event(used_event,
+					  vrh->last_used_idx + vrh->completed,
+					  vrh->last_used_idx);
+
+	vrh->last_used_idx += vrh->completed;
+	vrh->completed = 0;
+	return notify;
+}
+
+static inline bool __vringh_notify_enable(struct vringh *vrh,
+					  int (*getu16)(u16 *val, const u16 *p),
+					  int (*putu16)(u16 *p, u16 val))
+{
+	u16 avail;
+
+	if (!vrh->event_indices) {
+		/* Old-school; update flags. */
+		if (putu16(&vrh->vring.used->flags, 0) != 0) {
+			vringh_bad("Clearing used flags %p",
+				   &vrh->vring.used->flags);
+			return true;
+		}
+	} else {
+		if (putu16(&vring_avail_event(&vrh->vring),
+			   vrh->last_avail_idx) != 0) {
+			vringh_bad("Updating avail event index %p",
+				   &vring_avail_event(&vrh->vring));
+			return true;
+		}
+	}
+
+	/* They could have slipped one in as we were doing that: make
+	 * sure it's written, then check again. */
+	virtio_mb(vrh->weak_barriers);
+
+	if (getu16(&avail, &vrh->vring.avail->idx) != 0) {
+		vringh_bad("Failed to check avail idx at %p",
+			   &vrh->vring.avail->idx);
+		return true;
+	}
+
+	/* This is unlikely, so we just leave notifications enabled
+	 * (if we're using event_indices, we'll only get one
+	 * notification anyway). */
+	return avail == vrh->last_avail_idx;
+}
+
+static inline void __vringh_notify_disable(struct vringh *vrh,
+					   int (*putu16)(u16 *p, u16 val))
+{
+	if (!vrh->event_indices) {
+		/* Old-school; update flags. */
+		if (putu16(&vrh->vring.used->flags, VRING_USED_F_NO_NOTIFY)) {
+			vringh_bad("Setting used flags %p",
+				   &vrh->vring.used->flags);
+		}
+	}
+}
+
+/* Userspace access helpers: in this case, addresses are really userspace. */
+static inline int getu16_user(u16 *val, const u16 *p)
+{
+	return get_user(*val, (__force u16 __user *)p);
+}
+
+static inline int putu16_user(u16 *p, u16 val)
+{
+	return put_user(val, (__force u16 __user *)p);
+}
+
+static inline int copydesc_user(void *dst, const void *src, size_t len)
+{
+	return copy_from_user(dst, (__force void __user *)src, len) ?
+		-EFAULT : 0;
+}
+
+static inline int putused_user(struct vring_used_elem *dst,
+			       const struct vring_used_elem *src,
+			       unsigned int num)
+{
+	return copy_to_user((__force void __user *)dst, src,
+			    sizeof(*dst) * num) ? -EFAULT : 0;
+}
+
+static inline int xfer_from_user(void *src, void *dst, size_t len)
+{
+	return copy_from_user(dst, (__force void __user *)src, len) ?
+		-EFAULT : 0;
+}
+
+static inline int xfer_to_user(void *dst, void *src, size_t len)
+{
+	return copy_to_user((__force void __user *)dst, src, len) ?
+		-EFAULT : 0;
+}
+
+/**
+ * vringh_init_user - initialize a vringh for a userspace vring.
+ * @vrh: the vringh to initialize.
+ * @features: the feature bits for this ring.
+ * @num: the number of elements.
+ * @weak_barriers: true if we only need memory barriers, not I/O.
+ * @desc: the userpace descriptor pointer.
+ * @avail: the userpace avail pointer.
+ * @used: the userpace used pointer.
+ *
+ * Returns an error if num is invalid: you should check pointers
+ * yourself!
+ */
+int vringh_init_user(struct vringh *vrh, u32 features,
+		     unsigned int num, bool weak_barriers,
+		     struct vring_desc __user *desc,
+		     struct vring_avail __user *avail,
+		     struct vring_used __user *used)
+{
+	/* Sane power of 2 please! */
+	if (!num || num > 0xffff || (num & (num - 1))) {
+		vringh_bad("Bad ring size %u", num);
+		return -EINVAL;
+	}
+
+	vrh->event_indices = (features & (1 << VIRTIO_RING_F_EVENT_IDX));
+	vrh->weak_barriers = weak_barriers;
+	vrh->completed = 0;
+	vrh->last_avail_idx = 0;
+	vrh->last_used_idx = 0;
+	vrh->vring.num = num;
+	/* vring expects kernel addresses, but only used via accessors. */
+	vrh->vring.desc = (__force struct vring_desc *)desc;
+	vrh->vring.avail = (__force struct vring_avail *)avail;
+	vrh->vring.used = (__force struct vring_used *)used;
+	return 0;
+}
+EXPORT_SYMBOL(vringh_init_user);
+
+/**
+ * vringh_getdesc_user - get next available descriptor from userspace ring.
+ * @vrh: the userspace vring.
+ * @riov: where to put the readable descriptors (or NULL)
+ * @wiov: where to put the writable descriptors (or NULL)
+ * @getrange: function to call to check ranges.
+ * @head: head index we received, for passing to vringh_complete_user().
+ *
+ * Returns 0 if there was no descriptor, 1 if there was, or -errno.
+ *
+ * Note that on error return, you can tell the difference between an
+ * invalid ring and a single invalid descriptor: in the former case,
+ * *head will be vrh->vring.num.  You may be able to ignore an invalid
+ * descriptor, but there's not much you can do with an invalid ring.
+ *
+ * Note that you may need to clean up riov and wiov, even on error!
+ */
+int vringh_getdesc_user(struct vringh *vrh,
+			struct vringh_iov *riov,
+			struct vringh_iov *wiov,
+			bool (*getrange)(struct vringh *vrh,
+					 u64 addr, struct vringh_range *r),
+			u16 *head)
+{
+	int err;
+
+	*head = vrh->vring.num;
+	err = __vringh_get_head(vrh, getu16_user, &vrh->last_avail_idx);
+	if (err < 0)
+		return err;
+
+	/* Empty... */
+	if (err == vrh->vring.num)
+		return 0;
+
+	/* We need the layouts to be the identical for this to work */
+	BUILD_BUG_ON(sizeof(struct vringh_kiov) != sizeof(struct vringh_iov));
+	BUILD_BUG_ON(offsetof(struct vringh_kiov, iov) !=
+		     offsetof(struct vringh_iov, iov));
+	BUILD_BUG_ON(offsetof(struct vringh_kiov, i) !=
+		     offsetof(struct vringh_iov, i));
+	BUILD_BUG_ON(offsetof(struct vringh_kiov, used) !=
+		     offsetof(struct vringh_iov, used));
+	BUILD_BUG_ON(offsetof(struct vringh_kiov, max_num) !=
+		     offsetof(struct vringh_iov, max_num));
+	BUILD_BUG_ON(sizeof(struct iovec) != sizeof(struct kvec));
+	BUILD_BUG_ON(offsetof(struct iovec, iov_base) !=
+		     offsetof(struct kvec, iov_base));
+	BUILD_BUG_ON(offsetof(struct iovec, iov_len) !=
+		     offsetof(struct kvec, iov_len));
+	BUILD_BUG_ON(sizeof(((struct iovec *)NULL)->iov_base)
+		     != sizeof(((struct kvec *)NULL)->iov_base));
+	BUILD_BUG_ON(sizeof(((struct iovec *)NULL)->iov_len)
+		     != sizeof(((struct kvec *)NULL)->iov_len));
+
+	*head = err;
+	err = __vringh_iov(vrh, *head, (struct vringh_kiov *)riov,
+			   (struct vringh_kiov *)wiov,
+			   range_check, getrange, GFP_KERNEL, copydesc_user);
+	if (err)
+		return err;
+
+	return 1;
+}
+EXPORT_SYMBOL(vringh_getdesc_user);
+
+/**
+ * vringh_iov_pull_user - copy bytes from vring_iov.
+ * @riov: the riov as passed to vringh_getdesc_user() (updated as we consume)
+ * @dst: the place to copy.
+ * @len: the maximum length to copy.
+ *
+ * Returns the bytes copied <= len or a negative errno.
+ */
+ssize_t vringh_iov_pull_user(struct vringh_iov *riov, void *dst, size_t len)
+{
+	return vringh_iov_xfer((struct vringh_kiov *)riov,
+			       dst, len, xfer_from_user);
+}
+EXPORT_SYMBOL(vringh_iov_pull_user);
+
+/**
+ * vringh_iov_push_user - copy bytes into vring_iov.
+ * @wiov: the wiov as passed to vringh_getdesc_user() (updated as we consume)
+ * @dst: the place to copy.
+ * @len: the maximum length to copy.
+ *
+ * Returns the bytes copied <= len or a negative errno.
+ */
+ssize_t vringh_iov_push_user(struct vringh_iov *wiov,
+			     const void *src, size_t len)
+{
+	return vringh_iov_xfer((struct vringh_kiov *)wiov,
+			       (void *)src, len, xfer_to_user);
+}
+EXPORT_SYMBOL(vringh_iov_push_user);
+
+/**
+ * vringh_abandon_user - we've decided not to handle the descriptor(s).
+ * @vrh: the vring.
+ * @num: the number of descriptors to put back (ie. num
+ *	 vringh_get_user() to undo).
+ *
+ * The next vringh_get_user() will return the old descriptor(s) again.
+ */
+void vringh_abandon_user(struct vringh *vrh, unsigned int num)
+{
+	/* We only update vring_avail_event(vr) when we want to be notified,
+	 * so we haven't changed that yet. */
+	vrh->last_avail_idx -= num;
+}
+EXPORT_SYMBOL(vringh_abandon_user);
+
+/**
+ * vringh_complete_user - we've finished with descriptor, publish it.
+ * @vrh: the vring.
+ * @head: the head as filled in by vringh_getdesc_user.
+ * @len: the length of data we have written.
+ *
+ * You should check vringh_need_notify_user() after one or more calls
+ * to this function.
+ */
+int vringh_complete_user(struct vringh *vrh, u16 head, u32 len)
+{
+	struct vring_used_elem used;
+
+	used.id = head;
+	used.len = len;
+	return __vringh_complete(vrh, &used, 1, putu16_user, putused_user);
+}
+EXPORT_SYMBOL(vringh_complete_user);
+
+/**
+ * vringh_complete_multi_user - we've finished with many descriptors.
+ * @vrh: the vring.
+ * @used: the head, length pairs.
+ * @num_used: the number of used elements.
+ *
+ * You should check vringh_need_notify_user() after one or more calls
+ * to this function.
+ */
+int vringh_complete_multi_user(struct vringh *vrh,
+			       const struct vring_used_elem used[],
+			       unsigned num_used)
+{
+	return __vringh_complete(vrh, used, num_used,
+				 putu16_user, putused_user);
+}
+EXPORT_SYMBOL(vringh_complete_multi_user);
+
+/**
+ * vringh_notify_enable_user - we want to know if something changes.
+ * @vrh: the vring.
+ *
+ * This always enables notifications, but returns false if there are
+ * now more buffers available in the vring.
+ */
+bool vringh_notify_enable_user(struct vringh *vrh)
+{
+	return __vringh_notify_enable(vrh, getu16_user, putu16_user);
+}
+EXPORT_SYMBOL(vringh_notify_enable_user);
+
+/**
+ * vringh_notify_disable_user - don't tell us if something changes.
+ * @vrh: the vring.
+ *
+ * This is our normal running state: we disable and then only enable when
+ * we're going to sleep.
+ */
+void vringh_notify_disable_user(struct vringh *vrh)
+{
+	__vringh_notify_disable(vrh, putu16_user);
+}
+EXPORT_SYMBOL(vringh_notify_disable_user);
+
+/**
+ * vringh_need_notify_user - must we tell the other side about used buffers?
+ * @vrh: the vring we've called vringh_complete_user() on.
+ *
+ * Returns -errno or 0 if we don't need to tell the other side, 1 if we do.
+ */
+int vringh_need_notify_user(struct vringh *vrh)
+{
+	return __vringh_need_notify(vrh, getu16_user);
+}
+EXPORT_SYMBOL(vringh_need_notify_user);
+
+/* Kernelspace access helpers. */
+static inline int getu16_kern(u16 *val, const u16 *p)
+{
+	*val = ACCESS_ONCE(*p);
+	return 0;
+}
+
+static inline int putu16_kern(u16 *p, u16 val)
+{
+	ACCESS_ONCE(*p) = val;
+	return 0;
+}
+
+static inline int copydesc_kern(void *dst, const void *src, size_t len)
+{
+	memcpy(dst, src, len);
+	return 0;
+}
+
+static inline int putused_kern(struct vring_used_elem *dst,
+			       const struct vring_used_elem *src,
+			       unsigned int num)
+{
+	memcpy(dst, src, num * sizeof(*dst));
+	return 0;
+}
+
+static inline int xfer_kern(void *src, void *dst, size_t len)
+{
+	memcpy(dst, src, len);
+	return 0;
+}
+
+/**
+ * vringh_init_kern - initialize a vringh for a kernelspace vring.
+ * @vrh: the vringh to initialize.
+ * @features: the feature bits for this ring.
+ * @num: the number of elements.
+ * @weak_barriers: true if we only need memory barriers, not I/O.
+ * @desc: the userpace descriptor pointer.
+ * @avail: the userpace avail pointer.
+ * @used: the userpace used pointer.
+ *
+ * Returns an error if num is invalid.
+ */
+int vringh_init_kern(struct vringh *vrh, u32 features,
+		     unsigned int num, bool weak_barriers,
+		     struct vring_desc *desc,
+		     struct vring_avail *avail,
+		     struct vring_used *used)
+{
+	/* Sane power of 2 please! */
+	if (!num || num > 0xffff || (num & (num - 1))) {
+		vringh_bad("Bad ring size %u", num);
+		return -EINVAL;
+	}
+
+	vrh->event_indices = (features & (1 << VIRTIO_RING_F_EVENT_IDX));
+	vrh->weak_barriers = weak_barriers;
+	vrh->completed = 0;
+	vrh->last_avail_idx = 0;
+	vrh->last_used_idx = 0;
+	vrh->vring.num = num;
+	vrh->vring.desc = desc;
+	vrh->vring.avail = avail;
+	vrh->vring.used = used;
+	return 0;
+}
+EXPORT_SYMBOL(vringh_init_kern);
+
+/**
+ * vringh_getdesc_kern - get next available descriptor from kernelspace ring.
+ * @vrh: the kernelspace vring.
+ * @riov: where to put the readable descriptors (or NULL)
+ * @wiov: where to put the writable descriptors (or NULL)
+ * @head: head index we received, for passing to vringh_complete_kern().
+ * @gfp: flags for allocating larger riov/wiov.
+ *
+ * Returns 0 if there was no descriptor, 1 if there was, or -errno.
+ *
+ * Note that on error return, you can tell the difference between an
+ * invalid ring and a single invalid descriptor: in the former case,
+ * *head will be vrh->vring.num.  You may be able to ignore an invalid
+ * descriptor, but there's not much you can do with an invalid ring.
+ *
+ * Note that you may need to clean up riov and wiov, even on error!
+ */
+int vringh_getdesc_kern(struct vringh *vrh,
+			struct vringh_kiov *riov,
+			struct vringh_kiov *wiov,
+			u16 *head,
+			gfp_t gfp)
+{
+	int err;
+
+	err = __vringh_get_head(vrh, getu16_kern, &vrh->last_avail_idx);
+	if (err < 0)
+		return err;
+
+	/* Empty... */
+	if (err == vrh->vring.num)
+		return 0;
+
+	*head = err;
+	err = __vringh_iov(vrh, *head, riov, wiov, no_range_check, NULL,
+			   gfp, copydesc_kern);
+	if (err)
+		return err;
+
+	return 1;
+}
+EXPORT_SYMBOL(vringh_getdesc_kern);
+
+/**
+ * vringh_iov_pull_kern - copy bytes from vring_iov.
+ * @riov: the riov as passed to vringh_getdesc_kern() (updated as we consume)
+ * @dst: the place to copy.
+ * @len: the maximum length to copy.
+ *
+ * Returns the bytes copied <= len or a negative errno.
+ */
+ssize_t vringh_iov_pull_kern(struct vringh_kiov *riov, void *dst, size_t len)
+{
+	return vringh_iov_xfer(riov, dst, len, xfer_kern);
+}
+EXPORT_SYMBOL(vringh_iov_pull_kern);
+
+/**
+ * vringh_iov_push_kern - copy bytes into vring_iov.
+ * @wiov: the wiov as passed to vringh_getdesc_kern() (updated as we consume)
+ * @dst: the place to copy.
+ * @len: the maximum length to copy.
+ *
+ * Returns the bytes copied <= len or a negative errno.
+ */
+ssize_t vringh_iov_push_kern(struct vringh_kiov *wiov,
+			     const void *src, size_t len)
+{
+	return vringh_iov_xfer(wiov, (void *)src, len, xfer_kern);
+}
+EXPORT_SYMBOL(vringh_iov_push_kern);
+
+/**
+ * vringh_abandon_kern - we've decided not to handle the descriptor(s).
+ * @vrh: the vring.
+ * @num: the number of descriptors to put back (ie. num
+ *	 vringh_get_kern() to undo).
+ *
+ * The next vringh_get_kern() will return the old descriptor(s) again.
+ */
+void vringh_abandon_kern(struct vringh *vrh, unsigned int num)
+{
+	/* We only update vring_avail_event(vr) when we want to be notified,
+	 * so we haven't changed that yet. */
+	vrh->last_avail_idx -= num;
+}
+EXPORT_SYMBOL(vringh_abandon_kern);
+
+/**
+ * vringh_complete_kern - we've finished with descriptor, publish it.
+ * @vrh: the vring.
+ * @head: the head as filled in by vringh_getdesc_kern.
+ * @len: the length of data we have written.
+ *
+ * You should check vringh_need_notify_kern() after one or more calls
+ * to this function.
+ */
+int vringh_complete_kern(struct vringh *vrh, u16 head, u32 len)
+{
+	struct vring_used_elem used;
+
+	used.id = head;
+	used.len = len;
+
+	return __vringh_complete(vrh, &used, 1, putu16_kern, putused_kern);
+}
+EXPORT_SYMBOL(vringh_complete_kern);
+
+/**
+ * vringh_notify_enable_kern - we want to know if something changes.
+ * @vrh: the vring.
+ *
+ * This always enables notifications, but returns false if there are
+ * now more buffers available in the vring.
+ */
+bool vringh_notify_enable_kern(struct vringh *vrh)
+{
+	return __vringh_notify_enable(vrh, getu16_kern, putu16_kern);
+}
+EXPORT_SYMBOL(vringh_notify_enable_kern);
+
+/**
+ * vringh_notify_disable_kern - don't tell us if something changes.
+ * @vrh: the vring.
+ *
+ * This is our normal running state: we disable and then only enable when
+ * we're going to sleep.
+ */
+void vringh_notify_disable_kern(struct vringh *vrh)
+{
+	__vringh_notify_disable(vrh, putu16_kern);
+}
+EXPORT_SYMBOL(vringh_notify_disable_kern);
+
+/**
+ * vringh_need_notify_kern - must we tell the other side about used buffers?
+ * @vrh: the vring we've called vringh_complete_kern() on.
+ *
+ * Returns -errno or 0 if we don't need to tell the other side, 1 if we do.
+ */
+int vringh_need_notify_kern(struct vringh *vrh)
+{
+	return __vringh_need_notify(vrh, getu16_kern);
+}
+EXPORT_SYMBOL(vringh_need_notify_kern);

include/linux/vringh.h

···
+/*
+ * Linux host-side vring helpers; for when the kernel needs to access
+ * someone else's vring.
+ *
+ * Copyright IBM Corporation, 2013.
+ * Parts taken from drivers/vhost/vhost.c Copyright 2009 Red Hat, Inc.
+ *
+ * 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
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+ *
+ * Written by: Rusty Russell <[e-mail]>
+ */
+#ifndef _LINUX_VRINGH_H
+#define _LINUX_VRINGH_H
+#include <uapi/linux/virtio_ring.h>
+#include <linux/uio.h>
+#include <linux/slab.h>
+#include <asm/barrier.h>
+
+/* virtio_ring with information needed for host access. */
+struct vringh {
+	/* Guest publishes used event idx (note: we always do). */
+	bool event_indices;
+
+	/* Can we get away with weak barriers? */
+	bool weak_barriers;
+
+	/* Last available index we saw (ie. where we're up to). */
+	u16 last_avail_idx;
+
+	/* Last index we used. */
+	u16 last_used_idx;
+
+	/* How many descriptors we've completed since last need_notify(). */
+	u32 completed;
+
+	/* The vring (note: it may contain user pointers!) */
+	struct vring vring;
+};
+
+/* The memory the vring can access, and what offset to apply. */
+struct vringh_range {
+	u64 start, end_incl;
+	u64 offset;
+};
+
+/**
+ * struct vringh_iov - iovec mangler.
+ *
+ * Mangles iovec in place, and restores it.
+ * Remaining data is iov + i, of used - i elements.
+ */
+struct vringh_iov {
+	struct iovec *iov;
+	size_t consumed; /* Within iov[i] */
+	unsigned i, used, max_num;
+};
+
+/**
+ * struct vringh_iov - kvec mangler.
+ *
+ * Mangles kvec in place, and restores it.
+ * Remaining data is iov + i, of used - i elements.
+ */
+struct vringh_kiov {
+	struct kvec *iov;
+	size_t consumed; /* Within iov[i] */
+	unsigned i, used, max_num;
+};
+
+/* Flag on max_num to indicate we're kmalloced. */
+#define VRINGH_IOV_ALLOCATED 0x8000000
+
+/* Helpers for userspace vrings. */
+int vringh_init_user(struct vringh *vrh, u32 features,
+		     unsigned int num, bool weak_barriers,
+		     struct vring_desc __user *desc,
+		     struct vring_avail __user *avail,
+		     struct vring_used __user *used);
+
+static inline void vringh_iov_init(struct vringh_iov *iov,
+				   struct iovec *iovec, unsigned num)
+{
+	iov->used = iov->i = 0;
+	iov->consumed = 0;
+	iov->max_num = num;
+	iov->iov = iovec;
+}
+
+static inline void vringh_iov_reset(struct vringh_iov *iov)
+{
+	iov->iov[iov->i].iov_len += iov->consumed;
+	iov->iov[iov->i].iov_base -= iov->consumed;
+	iov->consumed = 0;
+	iov->i = 0;
+}
+
+static inline void vringh_iov_cleanup(struct vringh_iov *iov)
+{
+	if (iov->max_num & VRINGH_IOV_ALLOCATED)
+		kfree(iov->iov);
+	iov->max_num = iov->used = iov->i = iov->consumed = 0;
+	iov->iov = NULL;
+}
+
+/* Convert a descriptor into iovecs. */
+int vringh_getdesc_user(struct vringh *vrh,
+			struct vringh_iov *riov,
+			struct vringh_iov *wiov,
+			bool (*getrange)(struct vringh *vrh,
+					 u64 addr, struct vringh_range *r),
+			u16 *head);
+
+/* Copy bytes from readable vsg, consuming it (and incrementing wiov->i). */
+ssize_t vringh_iov_pull_user(struct vringh_iov *riov, void *dst, size_t len);
+
+/* Copy bytes into writable vsg, consuming it (and incrementing wiov->i). */
+ssize_t vringh_iov_push_user(struct vringh_iov *wiov,
+			     const void *src, size_t len);
+
+/* Mark a descriptor as used. */
+int vringh_complete_user(struct vringh *vrh, u16 head, u32 len);
+int vringh_complete_multi_user(struct vringh *vrh,
+			       const struct vring_used_elem used[],
+			       unsigned num_used);
+
+/* Pretend we've never seen descriptor (for easy error handling). */
+void vringh_abandon_user(struct vringh *vrh, unsigned int num);
+
+/* Do we need to fire the eventfd to notify the other side? */
+int vringh_need_notify_user(struct vringh *vrh);
+
+bool vringh_notify_enable_user(struct vringh *vrh);
+void vringh_notify_disable_user(struct vringh *vrh);
+
+/* Helpers for kernelspace vrings. */
+int vringh_init_kern(struct vringh *vrh, u32 features,
+		     unsigned int num, bool weak_barriers,
+		     struct vring_desc *desc,
+		     struct vring_avail *avail,
+		     struct vring_used *used);
+
+static inline void vringh_kiov_init(struct vringh_kiov *kiov,
+				    struct kvec *kvec, unsigned num)
+{
+	kiov->used = kiov->i = 0;
+	kiov->consumed = 0;
+	kiov->max_num = num;
+	kiov->iov = kvec;
+}
+
+static inline void vringh_kiov_reset(struct vringh_kiov *kiov)
+{
+	kiov->iov[kiov->i].iov_len += kiov->consumed;
+	kiov->iov[kiov->i].iov_base -= kiov->consumed;
+	kiov->consumed = 0;
+	kiov->i = 0;
+}
+
+static inline void vringh_kiov_cleanup(struct vringh_kiov *kiov)
+{
+	if (kiov->max_num & VRINGH_IOV_ALLOCATED)
+		kfree(kiov->iov);
+	kiov->max_num = kiov->used = kiov->i = kiov->consumed = 0;
+	kiov->iov = NULL;
+}
+
+int vringh_getdesc_kern(struct vringh *vrh,
+			struct vringh_kiov *riov,
+			struct vringh_kiov *wiov,
+			u16 *head,
+			gfp_t gfp);
+
+ssize_t vringh_iov_pull_kern(struct vringh_kiov *riov, void *dst, size_t len);
+ssize_t vringh_iov_push_kern(struct vringh_kiov *wiov,
+			     const void *src, size_t len);
+void vringh_abandon_kern(struct vringh *vrh, unsigned int num);
+int vringh_complete_kern(struct vringh *vrh, u16 head, u32 len);
+
+bool vringh_notify_enable_kern(struct vringh *vrh);
+void vringh_notify_disable_kern(struct vringh *vrh);
+
+int vringh_need_notify_kern(struct vringh *vrh);
+
+#endif /* _LINUX_VRINGH_H */