[PATCH 1/6] virtio_host: host-side implementation of virtio rings.

* [PATCH 1/6] virtio_host: host-side implementation of virtio rings.
@ 2013-01-17 10:29 Rusty Russell
  2013-01-17 10:29 ` [PATCH 2/6] tools/virtio: fix compile Rusty Russell
                   ` (7 more replies)
  0 siblings, 8 replies; 30+ messages in thread
From: Rusty Russell @ 2013-01-17 10:29 UTC (permalink / raw)
  To: virtualization; +Cc: Michael S . Tsirkin

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

This patch attempts to abstract 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 <rusty@rustcorp.com.au>
---
 drivers/Makefile             |    2 +-
 drivers/vhost/Kconfig        |    8 +
 drivers/vhost/Kconfig.tcm    |    1 +
 drivers/vhost/Makefile       |    2 +
 drivers/vhost/vringh.c       |  818 ++++++++++++++++++++++++++++++++++++++++++
 drivers/virtio/virtio_ring.c |   33 +-
 include/linux/virtio_ring.h  |   57 +++
 include/linux/vringh.h       |  115 ++++++
 8 files changed, 1008 insertions(+), 28 deletions(-)
 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..b28670f
--- /dev/null
+++ b/drivers/vhost/vringh.c
@@ -0,0 +1,818 @@
+/*
+ * 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>
+
+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_iov *iov,
+				      void *ptr, size_t len,
+				      int (*xfer)(void __user *addr, void *ptr,
+						  size_t len))
+{
+	int err, done = 0;
+
+	while (len && iov->i < iov->max) {
+		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->iov[iov->i].iov_base += partlen;
+		iov->iov[iov->i].iov_len -= partlen;
+
+		if (iov->iov[iov->i].iov_len == 0)
+			iov->i++;
+	}
+	return done;
+}
+
+static inline bool check_range(u64 addr, u32 len,
+			       struct vringh_range *range,
+			       bool (*getrange)(u64, struct vringh_range *))
+{
+	if (addr < range->start || addr > range->end_incl) {
+		if (!getrange(addr, range))
+			goto bad;
+	}
+	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 bad;
+	}
+
+	/* Otherwise, don't wrap. */
+	if (unlikely(addr + len < addr))
+		goto bad;
+	if (unlikely(addr + len - 1 > range->end_incl))
+		goto bad;
+	return true;
+
+bad:
+	vringh_bad("Malformed descriptor address %u@0x%llx", len, addr);
+	return false;
+}
+
+/* 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_iov *iov, gfp_t gfp)
+{
+	struct iovec *new;
+	unsigned int new_num = iov->max * 2;
+
+	if (new_num < 8)
+		new_num = 8;
+
+	if (iov->allocated)
+		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->i * sizeof(struct iovec));
+			iov->allocated = true;
+		}
+	}
+	if (!new)
+		return -ENOMEM;
+	iov->iov = new;
+	iov->max = new_num;
+	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 inline int
+__vringh_iov(struct vringh *vrh, u16 i,
+	     struct vringh_iov *riov,
+	     struct vringh_iov *wiov,
+	     bool (*getrange)(u64 addr, struct vringh_range *r),
+	     gfp_t gfp,
+	     int (*getdesc)(struct vring_desc *dst, const struct vring_desc *s))
+{
+	int err, count = 0, up_next, desc_max;
+	struct vring_desc desc, *descs;
+	struct vringh_range range = { -1ULL, 0 };
+
+	/* We start traversing vring's descriptor table. */
+	descs = vrh->vring.desc;
+	desc_max = vrh->vring.num;
+	up_next = -1;
+
+	riov->i = wiov->i = 0;
+	for (;;) {
+		void *addr;
+		struct vringh_iov *iov;
+
+		err = getdesc(&desc, &descs[i]);
+		if (unlikely(err))
+			goto fail;
+
+		/* Make sure it's OK, and get offset. */
+		if (!check_range(desc.addr, desc.len, &range, getrange)) {
+			err = -EINVAL;
+			goto fail;
+		}
+		addr = (void *)(long)desc.addr + range.offset;
+
+		if (unlikely(desc.flags & VRING_DESC_F_INDIRECT)) {
+			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->i)) {
+				vringh_bad("Readable desc %p after writable",
+					   &descs[i]);
+				err = -EINVAL;
+				goto fail;
+			}
+		}
+
+		if (unlikely(iov->i == iov->max)) {
+			err = resize_iovec(iov, gfp);
+			if (err)
+				goto fail;
+		}
+
+		iov->iov[iov->i].iov_base = (__force __user void *)addr;
+		iov->iov[iov->i].iov_len = desc.len;
+		iov->i++;
+
+		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);
+			else
+				break;
+		}
+
+		if (i >= desc_max) {
+			vringh_bad("Chained index %u > %u", i, desc_max);
+			err = -EINVAL;
+			goto fail;
+		}
+	}
+
+	/* Reset for fresh iteration. */
+	riov->max = riov->i;
+	wiov->max = wiov->i;
+	riov->i = wiov->i = 0;
+	return 0;
+
+fail:
+	if (riov->allocated)
+		kfree(riov->iov);
+	if (wiov->allocated)
+		kfree(wiov->iov);
+	return err;
+}
+
+static inline int __vringh_complete(struct vringh *vrh, u16 idx, u32 len,
+				    int (*getu16)(u16 *val, const u16 *p),
+				    int (*putu16)(u16 *p, u16 val),
+				    int (*putused)(struct vring_used_elem *dst,
+						   const struct vring_used_elem
+						   *s),
+				    bool *notify)
+{
+	struct vring_used_elem used;
+	struct vring_used *used_ring;
+	int err;
+	u16 used_idx, old, used_event;
+
+	used.id = idx;
+	used.len = len;
+
+	err = getu16(&used_idx, &vring_used_event(&vrh->vring));
+	if (err) {
+		vringh_bad("Failed to access used event %p",
+			   &vring_used_event(&vrh->vring));
+		return err;
+	}
+
+	used_ring = vrh->vring.used;
+	used_idx = vrh->last_used_idx;
+
+	err = putused(&used_ring->ring[used_idx % vrh->vring.num],
+		      &used);
+	if (err) {
+		vringh_bad("Failed to write used entry %u at %p",
+			   used_idx % vrh->vring.num,
+			   &used_ring->ring[used_idx % vrh->vring.num]);
+		return err;
+	}
+
+	/* Make sure buffer is written before we update index. */
+	virtio_wmb(vrh->weak_barriers);
+
+	old = vrh->last_used_idx;
+	vrh->last_used_idx++;
+
+	err = putu16(&vrh->vring.used->idx, vrh->last_used_idx);
+	if (err) {
+		vringh_bad("Failed to update used index at %p",
+			   &vrh->vring.used->idx);
+		return err;
+	}
+
+	/* If we already know we need to notify, skip re-checking */
+	if (*notify)
+		return 0;
+
+	/* 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;
+		}
+		if (!(flags & VRING_AVAIL_F_NO_INTERRUPT))
+			*notify = true;
+		return 0;
+	}
+
+	/* Modern: we know where other side is up to. */
+	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;
+	}
+	if (vring_need_event(used_event, vrh->last_used_idx, old))
+		*notify = true;
+	return 0;
+}
+
+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. */
+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 getdesc_user(struct vring_desc *dst,
+			       const struct vring_desc *src)
+{
+	return copy_from_user(dst, (__force void *)src, sizeof(*dst)) == 0 ? 0 :
+		-EFAULT;
+}
+
+static inline int putused_user(struct vring_used_elem *dst,
+			       const struct vring_used_elem *s)
+{
+	return copy_to_user((__force void __user *)dst, s, sizeof(*dst)) == 0
+		? 0 : -EFAULT;
+}
+
+static inline int xfer_from_user(void *src, void *dst, size_t len)
+{
+	return copy_from_user(dst, (__force void *)src, len) == 0 ? 0 :
+		-EFAULT;
+}
+
+static inline int xfer_to_user(void *dst, void *src, size_t len)
+{
+	return copy_to_user((__force void *)dst, src, len) == 0 ? 0 :
+		-EFAULT;
+}
+
+/**
+ * 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 %zu", num);
+		return -EINVAL;
+	}
+
+	vrh->event_indices = (features & (1 << VIRTIO_RING_F_EVENT_IDX));
+	vrh->weak_barriers = weak_barriers;
+	vrh->listening = false;
+	vrh->last_avail_idx = 0;
+	vrh->last_used_idx = 0;
+	vrh->vring.num = num;
+	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;
+}
+
+/**
+ * vringh_getdesc_user - get next available descriptor from userspace ring.
+ * @vrh: the userspace vring.
+ * @riov: where to put the readable descriptors.
+ * @wiov: where to put the writable descriptors.
+ * @getrange: function to call to check ranges.
+ * @head: head index we received, for passing to vringh_complete_user().
+ * @gfp: flags for allocating larger riov/wiov.
+ *
+ * Returns 0 if there was no descriptor, 1 if there was, or -errno.
+ *
+ * If it returns 1, riov->allocated and wiov->allocated indicate if you
+ * have to kfree riov->iov and wiov->iov respectively.
+ */
+int vringh_getdesc_user(struct vringh *vrh,
+			struct vringh_iov *riov,
+			struct vringh_iov *wiov,
+			bool (*getrange)(u64 addr, struct vringh_range *r),
+			u16 *head,
+			gfp_t gfp)
+{
+	int err;
+
+	err = __vringh_get_head(vrh, getu16_user, &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, getrange, gfp, getdesc_user);
+	if (err)
+		return err;
+
+	return 1;
+}
+
+/**
+ * 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(riov, dst, len, xfer_from_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(wiov, (void *)src, len, xfer_to_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;
+}
+
+/**
+ * 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.
+ * @notify: set if we should notify the other side, otherwise left alone.
+ */
+int vringh_complete_user(struct vringh *vrh, u16 head, u32 len,
+			 bool *notify)
+{
+	return __vringh_complete(vrh, head, len,
+				 getu16_user, putu16_user, putused_user,
+				 notify);
+}
+
+/**
+ * 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);
+}
+
+/**
+ * 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);
+}
+
+/* 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 getdesc_kern(struct vring_desc *dst,
+			       const struct vring_desc *src)
+{
+	*dst = *src;
+	return 0;
+}
+
+static inline int putused_kern(struct vring_used_elem *dst,
+			       const struct vring_used_elem *s)
+{
+	*dst = *s;
+	return 0;
+}
+
+static inline int xfer_kern(void *src, void *dst, size_t len)
+{
+	memcpy(dst, src, len);
+	return 0;
+}
+
+static inline bool noop_getrange(u64 addr, struct vringh_range *r)
+{
+	r->start = 0;
+	r->end_incl = -1ULL;
+	r->offset = 0;
+	return true;
+}
+
+/**
+ * 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 %zu", num);
+		return -EINVAL;
+	}
+
+	vrh->event_indices = (features & (1 << VIRTIO_RING_F_EVENT_IDX));
+	vrh->weak_barriers = weak_barriers;
+	vrh->listening = false;
+	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;
+}
+
+/**
+ * vringh_getdesc_kern - get next available descriptor from kernelspace ring.
+ * @vrh: the kernelspace vring.
+ * @riov: where to put the readable descriptors.
+ * @wiov: where to put the writable descriptors.
+ * @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.
+ *
+ * If it returns 1, riov->allocated and wiov->allocated indicate if you
+ * have to kfree riov->iov and wiov->iov respectively.
+ */
+int vringh_getdesc_kern(struct vringh *vrh,
+			struct vringh_iov *riov,
+			struct vringh_iov *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, noop_getrange,
+			   gfp, getdesc_kern);
+	if (err)
+		return err;
+
+	return 1;
+}
+
+/**
+ * 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_iov *riov, void *dst, size_t len)
+{
+	return vringh_iov_xfer(riov, dst, len, xfer_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_iov *wiov,
+			     const void *src, size_t len)
+{
+	return vringh_iov_xfer(wiov, (void *)src, len, xfer_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;
+}
+
+/**
+ * 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.
+ * @notify: set if we should notify the other side, otherwise left alone.
+ */
+int vringh_complete_kern(struct vringh *vrh, u16 head, u32 len,
+			 bool *notify)
+{
+	return __vringh_complete(vrh, head, len,
+				 getu16_kern, putu16_kern, putused_kern,
+				 notify);
+}
+
+/**
+ * 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);
+}
+
+/**
+ * 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);
+}
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index ffd7e7d..245177c 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -24,27 +24,6 @@
 #include <linux/module.h>
 #include <linux/hrtimer.h>
 
-/* virtio guest is communicating with a virtual "device" that actually runs on
- * a host processor.  Memory barriers are used to control SMP effects. */
-#ifdef CONFIG_SMP
-/* Where possible, use SMP barriers which are more lightweight than mandatory
- * barriers, because mandatory barriers control MMIO effects on accesses
- * through relaxed memory I/O windows (which virtio-pci does not use). */
-#define virtio_mb(vq) \
-	do { if ((vq)->weak_barriers) smp_mb(); else mb(); } while(0)
-#define virtio_rmb(vq) \
-	do { if ((vq)->weak_barriers) smp_rmb(); else rmb(); } while(0)
-#define virtio_wmb(vq) \
-	do { if ((vq)->weak_barriers) smp_wmb(); else wmb(); } while(0)
-#else
-/* We must force memory ordering even if guest is UP since host could be
- * running on another CPU, but SMP barriers are defined to barrier() in that
- * configuration. So fall back to mandatory barriers instead. */
-#define virtio_mb(vq) mb()
-#define virtio_rmb(vq) rmb()
-#define virtio_wmb(vq) wmb()
-#endif
-
 #ifdef DEBUG
 /* For development, we want to crash whenever the ring is screwed. */
 #define BAD_RING(_vq, fmt, args...)				\
@@ -276,7 +255,7 @@ add_head:
 
 	/* Descriptors and available array need to be set before we expose the
 	 * new available array entries. */
-	virtio_wmb(vq);
+	virtio_wmb(vq->weak_barriers);
 	vq->vring.avail->idx++;
 	vq->num_added++;
 
@@ -312,7 +291,7 @@ bool virtqueue_kick_prepare(struct virtqueue *_vq)
 	START_USE(vq);
 	/* We need to expose available array entries before checking avail
 	 * event. */
-	virtio_mb(vq);
+	virtio_mb(vq->weak_barriers);
 
 	old = vq->vring.avail->idx - vq->num_added;
 	new = vq->vring.avail->idx;
@@ -436,7 +415,7 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int *len)
 	}
 
 	/* Only get used array entries after they have been exposed by host. */
-	virtio_rmb(vq);
+	virtio_rmb(vq->weak_barriers);
 
 	last_used = (vq->last_used_idx & (vq->vring.num - 1));
 	i = vq->vring.used->ring[last_used].id;
@@ -460,7 +439,7 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int *len)
 	 * the read in the next get_buf call. */
 	if (!(vq->vring.avail->flags & VRING_AVAIL_F_NO_INTERRUPT)) {
 		vring_used_event(&vq->vring) = vq->last_used_idx;
-		virtio_mb(vq);
+		virtio_mb(vq->weak_barriers);
 	}
 
 #ifdef DEBUG
@@ -513,7 +492,7 @@ bool virtqueue_enable_cb(struct virtqueue *_vq)
 	 * entry. Always do both to keep code simple. */
 	vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
 	vring_used_event(&vq->vring) = vq->last_used_idx;
-	virtio_mb(vq);
+	virtio_mb(vq->weak_barriers);
 	if (unlikely(more_used(vq))) {
 		END_USE(vq);
 		return false;
@@ -553,7 +532,7 @@ bool virtqueue_enable_cb_delayed(struct virtqueue *_vq)
 	/* TODO: tune this threshold */
 	bufs = (u16)(vq->vring.avail->idx - vq->last_used_idx) * 3 / 4;
 	vring_used_event(&vq->vring) = vq->last_used_idx + bufs;
-	virtio_mb(vq);
+	virtio_mb(vq->weak_barriers);
 	if (unlikely((u16)(vq->vring.used->idx - vq->last_used_idx) > bufs)) {
 		END_USE(vq);
 		return false;
diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
index 63c6ea1..ca3ad41 100644
--- a/include/linux/virtio_ring.h
+++ b/include/linux/virtio_ring.h
@@ -4,6 +4,63 @@
 #include <linux/irqreturn.h>
 #include <uapi/linux/virtio_ring.h>
 
+/*
+ * Barriers in virtio are tricky.  Non-SMP virtio guests can't assume
+ * they're not on an SMP host system, so they need to assume real
+ * barriers.  Non-SMP virtio hosts could skip the barriers, but does
+ * anyone care?
+ *
+ * For virtio_pci on SMP, we don't need to order with respect to MMIO
+ * accesses through relaxed memory I/O windows, so smp_mb() et al are
+ * sufficient.
+ *
+ * For using virtio to talk to real devices (eg. other heterogeneous
+ * CPUs) we do need real barriers.  In theory, we could be using both
+ * kinds of virtio, so it's a runtime decision, and the branch is
+ * actually quite cheap.
+ */
+
+#ifdef CONFIG_SMP
+static inline void virtio_mb(bool weak_barriers)
+{
+	if (weak_barriers)
+		smp_mb();
+	else
+		mb();
+}
+
+static inline void virtio_rmb(bool weak_barriers)
+{
+	if (weak_barriers)
+		smp_rmb();
+	else
+		rmb();
+}
+
+static inline void virtio_wmb(bool weak_barriers)
+{
+	if (weak_barriers)
+		smp_wmb();
+	else
+		wmb();
+}
+#else
+static inline void virtio_mb(bool weak_barriers)
+{
+	mb();
+}
+
+static inline void virtio_rmb(bool weak_barriers)
+{
+	rmb();
+}
+
+static inline void virtio_wmb(bool weak_barriers)
+{
+	wmb();
+}
+#endif
+
 struct virtio_device;
 struct virtqueue;
 
diff --git a/include/linux/vringh.h b/include/linux/vringh.h
new file mode 100644
index 0000000..508b5e5
--- /dev/null
+++ b/include/linux/vringh.h
@@ -0,0 +1,115 @@
+/*
+ * 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@rustcorp.com.au>
+ */
+#ifndef _LINUX_VRINGH_H
+#define _LINUX_VRINGH_H
+#include <uapi/linux/virtio_ring.h>
+#include <uapi/linux/uio.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;
+
+	/* 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;
+	unsigned i, max;
+	bool allocated;
+};
+
+/* 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);
+
+/* Convert a descriptor into iovecs. */
+int vringh_getdesc_user(struct vringh *vrh,
+			struct vringh_iov *riov,
+			struct vringh_iov *wiov,
+			bool (*getrange)(u64 addr, struct vringh_range *r),
+			u16 *head,
+			gfp_t gfp);
+
+/* 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.  Sets notify if you should fire eventfd. */
+int vringh_complete_user(struct vringh *vrh, u16 head, u32 len,
+			 bool *notify);
+
+/* Pretend we've never seen descriptor (for easy error handling). */
+void vringh_abandon_user(struct vringh *vrh, unsigned int num);
+
+/* 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);
+
+int vringh_getdesc_kern(struct vringh *vrh,
+			struct vringh_iov *riov,
+			struct vringh_iov *wiov,
+			u16 *head,
+			gfp_t gfp);
+
+ssize_t vringh_iov_pull_kern(struct vringh_iov *riov, void *dst, size_t len);
+ssize_t vringh_iov_push_user(struct vringh_iov *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 *notify);
+
+bool vringh_notify_enable_kern(struct vringh *vrh);
+void vringh_notify_disable_kern(struct vringh *vrh);
+
+#endif /* _LINUX_VRINGH_H */
-- 
1.7.10.4

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