[PATCH 2/5] vringh: host-side implementation of virtio rings.

[Rusty Russell]

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.

There are patches pending to make vhost use this interface, but it's
being merged now for the CAIF virtio patches, which want it
immediately.

Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
---
 drivers/Makefile          |    2 +-
 drivers/vhost/Kconfig     |    8 +
 drivers/vhost/Kconfig.tcm |    1 +
 drivers/vhost/Makefile    |    2 +
 drivers/vhost/vringh.c    | 1012 +++++++++++++++++++++++++++++++++++++++++++++
 include/linux/vringh.h    |  185 +++++++++
 6 files changed, 1209 insertions(+), 1 deletion(-)
 create mode 100644 drivers/vhost/vringh.c
 create mode 100644 include/linux/vringh.h

diff --git a/drivers/Makefile b/drivers/Makefile
index 7863b9f..351a34f 100644
--- a/drivers/Makefile
+++ b/drivers/Makefile
@@ -123,7 +123,7 @@ obj-$(CONFIG_PPC_PS3)		+= ps3/
 obj-$(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/
diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
index 202bba6..613b074 100644
--- a/drivers/vhost/Kconfig
+++ b/drivers/vhost/Kconfig
@@ -1,6 +1,7 @@
 config VHOST_NET
 	tristate "Host kernel accelerator for virtio net (EXPERIMENTAL)"
 	depends on NET && EVENTFD && (TUN || !TUN) && (MACVTAP || !MACVTAP) && EXPERIMENTAL
+	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
@@ -12,3 +13,10 @@ config VHOST_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.
+
diff --git a/drivers/vhost/Kconfig.tcm b/drivers/vhost/Kconfig.tcm
index a9c6f76..0218f77 100644
--- a/drivers/vhost/Kconfig.tcm
+++ b/drivers/vhost/Kconfig.tcm
@@ -1,6 +1,7 @@
 config TCM_VHOST
 	tristate "TCM_VHOST fabric module (EXPERIMENTAL)"
 	depends on TARGET_CORE && EVENTFD && EXPERIMENTAL && m
+	select VHOST_RING
 	default n
 	---help---
 	Say M here to enable the TCM_VHOST fabric module for use with virtio-scsi guests
diff --git a/drivers/vhost/Makefile b/drivers/vhost/Makefile
index a27b053..1d37f5e 100644
--- a/drivers/vhost/Makefile
+++ b/drivers/vhost/Makefile
@@ -2,3 +2,5 @@ obj-$(CONFIG_VHOST_NET) += vhost_net.o
 vhost_net-y := vhost.o net.o
 
 obj-$(CONFIG_TCM_VHOST) += tcm_vhost.o
+
+obj-$(CONFIG_VHOST_RING) += vringh.o
diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
new file mode 100644
index 0000000..c05d918
--- /dev/null
+++ b/drivers/vhost/vringh.c
@@ -0,0 +1,1012 @@
+/*
+ * 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 - iov->off, len);
+		err = xfer(iov->iov[iov->i].iov_base + iov->off, ptr, partlen);
+		if (err)
+			return err;
+		done += partlen;
+		len -= partlen;
+		ptr += partlen;
+		iov->off += partlen;
+
+		if (iov->off == iov->iov[iov->i].iov_len) {
+			iov->off = 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;
+
+	/* Already enabled? */
+	if (vrh->listening)
+		return false;
+
+	vrh->listening = true;
+
+	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 false;
+		}
+	} 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 false;
+		}
+	}
+
+	/* 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 false;
+	}
+
+	/* This is so unlikely, we just leave notifications enabled. */
+	return avail != vrh->last_avail_idx;
+}
+
+static inline void __vringh_notify_disable(struct vringh *vrh,
+					   int (*putu16)(u16 *p, u16 val))
+{
+	/* Already disabled? */
+	if (!vrh->listening)
+		return;
+
+	vrh->listening = false;
+	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->listening = false;
+	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 indentical 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 true 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 = *p;
+	return 0;
+}
+
+static inline int putu16_kern(u16 *p, u16 val)
+{
+	*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->listening = false;
+	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 true 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);
diff --git a/include/linux/vringh.h b/include/linux/vringh.h
new file mode 100644
index 0000000..ab41185
--- /dev/null
+++ b/include/linux/vringh.h
@@ -0,0 +1,185 @@
+/*
+ * 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 <rusty@xxxxxxxxxxxxxxx>
+ */
+#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;
+
+	/* Have we told the other end we want to be notified? */
+	bool listening;
+
+	/* 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;
+};
+
+/* All the information about an iovec. */
+struct vringh_iov {
+	struct iovec *iov;
+	size_t off; /* Within iov[i] */
+	unsigned i, used, max_num;
+};
+
+/* All the information about a kvec. */
+struct vringh_kiov {
+	struct kvec *iov;
+	size_t off; /* 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->off = 0;
+	iov->max_num = num;
+	iov->iov = iovec;
+}
+
+static inline void vringh_iov_reset(struct vringh_iov *iov)
+{
+	iov->off = 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->off = 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->off = 0;
+	kiov->max_num = num;
+	kiov->iov = kvec;
+}
+
+static inline void vringh_kiov_reset(struct vringh_kiov *kiov)
+{
+	kiov->off = 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->off = 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 */
-- 
1.7.10.4

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