drivers/hv/ring_buffer.c - kernel/msm-5.4 - Gitiles

 // SPDX-License-Identifier: GPL-2.0-only
 /*
  *
  * Copyright (c) 2009, Microsoft Corporation.
  *
  * Authors:
  *   Haiyang Zhang <haiyangz@microsoft.com>
  *   Hank Janssen  <hjanssen@microsoft.com>
  *   K. Y. Srinivasan <kys@microsoft.com>
  */
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

 #include <linux/kernel.h>
 #include <linux/mm.h>
 #include <linux/hyperv.h>
 #include <linux/uio.h>
 #include <linux/vmalloc.h>
 #include <linux/slab.h>
 #include <linux/prefetch.h>

 #include "hyperv_vmbus.h"

 #define VMBUS_PKT_TRAILER	8

 /*
  * When we write to the ring buffer, check if the host needs to
  * be signaled. Here is the details of this protocol:
  *
  *	1. The host guarantees that while it is draining the
  *	   ring buffer, it will set the interrupt_mask to
  *	   indicate it does not need to be interrupted when
  *	   new data is placed.
  *
  *	2. The host guarantees that it will completely drain
  *	   the ring buffer before exiting the read loop. Further,
  *	   once the ring buffer is empty, it will clear the
  *	   interrupt_mask and re-check to see if new data has
  *	   arrived.
  *
  * KYS: Oct. 30, 2016:
  * It looks like Windows hosts have logic to deal with DOS attacks that
  * can be triggered if it receives interrupts when it is not expecting
  * the interrupt. The host expects interrupts only when the ring
  * transitions from empty to non-empty (or full to non full on the guest
  * to host ring).
  * So, base the signaling decision solely on the ring state until the
  * host logic is fixed.
  */

 static void hv_signal_on_write(u32 old_write, struct vmbus_channel *channel)
 {
 	struct hv_ring_buffer_info *rbi = &channel->outbound;

 	virt_mb();
 	if (READ_ONCE(rbi->ring_buffer->interrupt_mask))
 		return;

 	/* check interrupt_mask before read_index */
 	virt_rmb();
 	/*
 	 * This is the only case we need to signal when the
 	 * ring transitions from being empty to non-empty.
 	 */
 	if (old_write == READ_ONCE(rbi->ring_buffer->read_index)) {
 		++channel->intr_out_empty;
 		vmbus_setevent(channel);
 	}
 }

 /* Get the next write location for the specified ring buffer. */
 static inline u32
 hv_get_next_write_location(struct hv_ring_buffer_info *ring_info)
 {
 	u32 next = ring_info->ring_buffer->write_index;

 	return next;
 }

 /* Set the next write location for the specified ring buffer. */
 static inline void
 hv_set_next_write_location(struct hv_ring_buffer_info *ring_info,
 		     u32 next_write_location)
 {
 	ring_info->ring_buffer->write_index = next_write_location;
 }

 /* Set the next read location for the specified ring buffer. */
 static inline void
 hv_set_next_read_location(struct hv_ring_buffer_info *ring_info,
 		    u32 next_read_location)
 {
 	ring_info->ring_buffer->read_index = next_read_location;
 	ring_info->priv_read_index = next_read_location;
 }

 /* Get the size of the ring buffer. */
 static inline u32
 hv_get_ring_buffersize(const struct hv_ring_buffer_info *ring_info)
 {
 	return ring_info->ring_datasize;
 }

 /* Get the read and write indices as u64 of the specified ring buffer. */
 static inline u64
 hv_get_ring_bufferindices(struct hv_ring_buffer_info *ring_info)
 {
 	return (u64)ring_info->ring_buffer->write_index << 32;
 }

 /*
  * Helper routine to copy from source to ring buffer.
  * Assume there is enough room. Handles wrap-around in dest case only!!
  */
 static u32 hv_copyto_ringbuffer(
 	struct hv_ring_buffer_info	*ring_info,
 	u32				start_write_offset,
 	const void			*src,
 	u32				srclen)
 {
 	void *ring_buffer = hv_get_ring_buffer(ring_info);
 	u32 ring_buffer_size = hv_get_ring_buffersize(ring_info);

 	memcpy(ring_buffer + start_write_offset, src, srclen);

 	start_write_offset += srclen;
 	if (start_write_offset >= ring_buffer_size)
 		start_write_offset -= ring_buffer_size;

 	return start_write_offset;
 }

 /*
  *
  * hv_get_ringbuffer_availbytes()
  *
  * Get number of bytes available to read and to write to
  * for the specified ring buffer
  */
 static void
 hv_get_ringbuffer_availbytes(const struct hv_ring_buffer_info *rbi,
 			     u32 *read, u32 *write)
 {
 	u32 read_loc, write_loc, dsize;

 	/* Capture the read/write indices before they changed */
 	read_loc = READ_ONCE(rbi->ring_buffer->read_index);
 	write_loc = READ_ONCE(rbi->ring_buffer->write_index);
 	dsize = rbi->ring_datasize;

 	*write = write_loc >= read_loc ? dsize - (write_loc - read_loc) :
 		read_loc - write_loc;
 	*read = dsize - *write;
 }

 /* Get various debug metrics for the specified ring buffer. */
 int hv_ringbuffer_get_debuginfo(struct hv_ring_buffer_info *ring_info,
 				struct hv_ring_buffer_debug_info *debug_info)
 {
 	u32 bytes_avail_towrite;
 	u32 bytes_avail_toread;

 	mutex_lock(&ring_info->ring_buffer_mutex);

 	if (!ring_info->ring_buffer) {
 		mutex_unlock(&ring_info->ring_buffer_mutex);
 		return -EINVAL;
 	}

 	hv_get_ringbuffer_availbytes(ring_info,
 				     &bytes_avail_toread,
 				     &bytes_avail_towrite);
 	debug_info->bytes_avail_toread = bytes_avail_toread;
 	debug_info->bytes_avail_towrite = bytes_avail_towrite;
 	debug_info->current_read_index = ring_info->ring_buffer->read_index;
 	debug_info->current_write_index = ring_info->ring_buffer->write_index;
 	debug_info->current_interrupt_mask
 		= ring_info->ring_buffer->interrupt_mask;
 	mutex_unlock(&ring_info->ring_buffer_mutex);

 	return 0;
 }
 EXPORT_SYMBOL_GPL(hv_ringbuffer_get_debuginfo);

 /* Initialize a channel's ring buffer info mutex locks */
 void hv_ringbuffer_pre_init(struct vmbus_channel *channel)
 {
 	mutex_init(&channel->inbound.ring_buffer_mutex);
 	mutex_init(&channel->outbound.ring_buffer_mutex);
 }

 /* Initialize the ring buffer. */
 int hv_ringbuffer_init(struct hv_ring_buffer_info *ring_info,
 		       struct page *pages, u32 page_cnt)
 {
 	int i;
 	struct page **pages_wraparound;

 	BUILD_BUG_ON((sizeof(struct hv_ring_buffer) != PAGE_SIZE));

 	/*
 	 * First page holds struct hv_ring_buffer, do wraparound mapping for
 	 * the rest.
 	 */
 	pages_wraparound = kcalloc(page_cnt * 2 - 1, sizeof(struct page *),
 				   GFP_KERNEL);
 	if (!pages_wraparound)
 		return -ENOMEM;

 	pages_wraparound[0] = pages;
 	for (i = 0; i < 2 * (page_cnt - 1); i++)
 		pages_wraparound[i + 1] = &pages[i % (page_cnt - 1) + 1];

 	ring_info->ring_buffer = (struct hv_ring_buffer *)
 		vmap(pages_wraparound, page_cnt * 2 - 1, VM_MAP, PAGE_KERNEL);

 	kfree(pages_wraparound);


 	if (!ring_info->ring_buffer)
 		return -ENOMEM;

 	ring_info->ring_buffer->read_index =
 		ring_info->ring_buffer->write_index = 0;

 	/* Set the feature bit for enabling flow control. */
 	ring_info->ring_buffer->feature_bits.value = 1;

 	ring_info->ring_size = page_cnt << PAGE_SHIFT;
 	ring_info->ring_size_div10_reciprocal =
 		reciprocal_value(ring_info->ring_size / 10);
 	ring_info->ring_datasize = ring_info->ring_size -
 		sizeof(struct hv_ring_buffer);
 	ring_info->priv_read_index = 0;

 	spin_lock_init(&ring_info->ring_lock);

 	return 0;
 }

 /* Cleanup the ring buffer. */
 void hv_ringbuffer_cleanup(struct hv_ring_buffer_info *ring_info)
 {
 	mutex_lock(&ring_info->ring_buffer_mutex);
 	vunmap(ring_info->ring_buffer);
 	ring_info->ring_buffer = NULL;
 	mutex_unlock(&ring_info->ring_buffer_mutex);
 }

 /* Write to the ring buffer. */
 int hv_ringbuffer_write(struct vmbus_channel *channel,
 			const struct kvec *kv_list, u32 kv_count)
 {
 	int i;
 	u32 bytes_avail_towrite;
 	u32 totalbytes_towrite = sizeof(u64);
 	u32 next_write_location;
 	u32 old_write;
 	u64 prev_indices;
 	unsigned long flags;
 	struct hv_ring_buffer_info *outring_info = &channel->outbound;

 	if (channel->rescind)
 		return -ENODEV;

 	for (i = 0; i < kv_count; i++)
 		totalbytes_towrite += kv_list[i].iov_len;

 	spin_lock_irqsave(&outring_info->ring_lock, flags);

 	bytes_avail_towrite = hv_get_bytes_to_write(outring_info);

 	/*
 	 * If there is only room for the packet, assume it is full.
 	 * Otherwise, the next time around, we think the ring buffer
 	 * is empty since the read index == write index.
 	 */
 	if (bytes_avail_towrite <= totalbytes_towrite) {
 		++channel->out_full_total;

 		if (!channel->out_full_flag) {
 			++channel->out_full_first;
 			channel->out_full_flag = true;
 		}

 		spin_unlock_irqrestore(&outring_info->ring_lock, flags);
 		return -EAGAIN;
 	}

 	channel->out_full_flag = false;

 	/* Write to the ring buffer */
 	next_write_location = hv_get_next_write_location(outring_info);

 	old_write = next_write_location;

 	for (i = 0; i < kv_count; i++) {
 		next_write_location = hv_copyto_ringbuffer(outring_info,
 						     next_write_location,
 						     kv_list[i].iov_base,
 						     kv_list[i].iov_len);
 	}

 	/* Set previous packet start */
 	prev_indices = hv_get_ring_bufferindices(outring_info);

 	next_write_location = hv_copyto_ringbuffer(outring_info,
 					     next_write_location,
 					     &prev_indices,
 					     sizeof(u64));

 	/* Issue a full memory barrier before updating the write index */
 	virt_mb();

 	/* Now, update the write location */
 	hv_set_next_write_location(outring_info, next_write_location);


 	spin_unlock_irqrestore(&outring_info->ring_lock, flags);

 	hv_signal_on_write(old_write, channel);

 	if (channel->rescind)
 		return -ENODEV;

 	return 0;
 }

 int hv_ringbuffer_read(struct vmbus_channel *channel,
 		       void *buffer, u32 buflen, u32 *buffer_actual_len,
 		       u64 *requestid, bool raw)
 {
 	struct vmpacket_descriptor *desc;
 	u32 packetlen, offset;

 	if (unlikely(buflen == 0))
 		return -EINVAL;

 	*buffer_actual_len = 0;
 	*requestid = 0;

 	/* Make sure there is something to read */
 	desc = hv_pkt_iter_first(channel);
 	if (desc == NULL) {
 		/*
 		 * No error is set when there is even no header, drivers are
 		 * supposed to analyze buffer_actual_len.
 		 */
 		return 0;
 	}

 	offset = raw ? 0 : (desc->offset8 << 3);
 	packetlen = (desc->len8 << 3) - offset;
 	*buffer_actual_len = packetlen;
 	*requestid = desc->trans_id;

 	if (unlikely(packetlen > buflen))
 		return -ENOBUFS;

 	/* since ring is double mapped, only one copy is necessary */
 	memcpy(buffer, (const char *)desc + offset, packetlen);

 	/* Advance ring index to next packet descriptor */
 	__hv_pkt_iter_next(channel, desc);

 	/* Notify host of update */
 	hv_pkt_iter_close(channel);

 	return 0;
 }

 /*
  * Determine number of bytes available in ring buffer after
  * the current iterator (priv_read_index) location.
  *
  * This is similar to hv_get_bytes_to_read but with private
  * read index instead.
  */
 static u32 hv_pkt_iter_avail(const struct hv_ring_buffer_info *rbi)
 {
 	u32 priv_read_loc = rbi->priv_read_index;
 	u32 write_loc = READ_ONCE(rbi->ring_buffer->write_index);

 	if (write_loc >= priv_read_loc)
 		return write_loc - priv_read_loc;
 	else
 		return (rbi->ring_datasize - priv_read_loc) + write_loc;
 }

 /*
  * Get first vmbus packet from ring buffer after read_index
  *
  * If ring buffer is empty, returns NULL and no other action needed.
  */
 struct vmpacket_descriptor *hv_pkt_iter_first(struct vmbus_channel *channel)
 {
 	struct hv_ring_buffer_info *rbi = &channel->inbound;
 	struct vmpacket_descriptor *desc;

 	if (hv_pkt_iter_avail(rbi) < sizeof(struct vmpacket_descriptor))
 		return NULL;

 	desc = hv_get_ring_buffer(rbi) + rbi->priv_read_index;
 	if (desc)
 		prefetch((char *)desc + (desc->len8 << 3));

 	return desc;
 }
 EXPORT_SYMBOL_GPL(hv_pkt_iter_first);

 /*
  * Get next vmbus packet from ring buffer.
  *
  * Advances the current location (priv_read_index) and checks for more
  * data. If the end of the ring buffer is reached, then return NULL.
  */
 struct vmpacket_descriptor *
 __hv_pkt_iter_next(struct vmbus_channel *channel,
 		   const struct vmpacket_descriptor *desc)
 {
 	struct hv_ring_buffer_info *rbi = &channel->inbound;
 	u32 packetlen = desc->len8 << 3;
 	u32 dsize = rbi->ring_datasize;

 	/* bump offset to next potential packet */
 	rbi->priv_read_index += packetlen + VMBUS_PKT_TRAILER;
 	if (rbi->priv_read_index >= dsize)
 		rbi->priv_read_index -= dsize;

 	/* more data? */
 	return hv_pkt_iter_first(channel);
 }
 EXPORT_SYMBOL_GPL(__hv_pkt_iter_next);

 /* How many bytes were read in this iterator cycle */
 static u32 hv_pkt_iter_bytes_read(const struct hv_ring_buffer_info *rbi,
 					u32 start_read_index)
 {
 	if (rbi->priv_read_index >= start_read_index)
 		return rbi->priv_read_index - start_read_index;
 	else
 		return rbi->ring_datasize - start_read_index +
 			rbi->priv_read_index;
 }

 /*
  * Update host ring buffer after iterating over packets. If the host has
  * stopped queuing new entries because it found the ring buffer full, and
  * sufficient space is being freed up, signal the host. But be careful to
  * only signal the host when necessary, both for performance reasons and
  * because Hyper-V protects itself by throttling guests that signal
  * inappropriately.
  *
  * Determining when to signal is tricky. There are three key data inputs
  * that must be handled in this order to avoid race conditions:
  *
  * 1. Update the read_index
  * 2. Read the pending_send_sz
  * 3. Read the current write_index
  *
  * The interrupt_mask is not used to determine when to signal. The
  * interrupt_mask is used only on the guest->host ring buffer when
  * sending requests to the host. The host does not use it on the host->
  * guest ring buffer to indicate whether it should be signaled.
  */
 void hv_pkt_iter_close(struct vmbus_channel *channel)
 {
 	struct hv_ring_buffer_info *rbi = &channel->inbound;
 	u32 curr_write_sz, pending_sz, bytes_read, start_read_index;

 	/*
 	 * Make sure all reads are done before we update the read index since
 	 * the writer may start writing to the read area once the read index
 	 * is updated.
 	 */
 	virt_rmb();
 	start_read_index = rbi->ring_buffer->read_index;
 	rbi->ring_buffer->read_index = rbi->priv_read_index;

 	/*
 	 * Older versions of Hyper-V (before WS2102 and Win8) do not
 	 * implement pending_send_sz and simply poll if the host->guest
 	 * ring buffer is full.  No signaling is needed or expected.
 	 */
 	if (!rbi->ring_buffer->feature_bits.feat_pending_send_sz)
 		return;

 	/*
 	 * Issue a full memory barrier before making the signaling decision.
 	 * If reading pending_send_sz were to be reordered and happen
 	 * before we commit the new read_index, a race could occur.  If the
 	 * host were to set the pending_send_sz after we have sampled
 	 * pending_send_sz, and the ring buffer blocks before we commit the
 	 * read index, we could miss sending the interrupt. Issue a full
 	 * memory barrier to address this.
 	 */
 	virt_mb();

 	/*
 	 * If the pending_send_sz is zero, then the ring buffer is not
 	 * blocked and there is no need to signal.  This is far by the
 	 * most common case, so exit quickly for best performance.
 	 */
 	pending_sz = READ_ONCE(rbi->ring_buffer->pending_send_sz);
 	if (!pending_sz)
 		return;

 	/*
 	 * Ensure the read of write_index in hv_get_bytes_to_write()
 	 * happens after the read of pending_send_sz.
 	 */
 	virt_rmb();
 	curr_write_sz = hv_get_bytes_to_write(rbi);
 	bytes_read = hv_pkt_iter_bytes_read(rbi, start_read_index);

 	/*
 	 * We want to signal the host only if we're transitioning
 	 * from a "not enough free space" state to a "enough free
 	 * space" state.  For example, it's possible that this function
 	 * could run and free up enough space to signal the host, and then
 	 * run again and free up additional space before the host has a
 	 * chance to clear the pending_send_sz.  The 2nd invocation would
 	 * be a null transition from "enough free space" to "enough free
 	 * space", which doesn't warrant a signal.
 	 *
 	 * Exactly filling the ring buffer is treated as "not enough
 	 * space". The ring buffer always must have at least one byte
 	 * empty so the empty and full conditions are distinguishable.
 	 * hv_get_bytes_to_write() doesn't fully tell the truth in
 	 * this regard.
 	 *
 	 * So first check if we were in the "enough free space" state
 	 * before we began the iteration. If so, the host was not
 	 * blocked, and there's no need to signal.
 	 */
 	if (curr_write_sz - bytes_read > pending_sz)
 		return;

 	/*
 	 * Similarly, if the new state is "not enough space", then
 	 * there's no need to signal.
 	 */
 	if (curr_write_sz <= pending_sz)
 		return;

 	++channel->intr_in_full;
 	vmbus_setevent(channel);
 }
 EXPORT_SYMBOL_GPL(hv_pkt_iter_close);
	// SPDX-License-Identifier: GPL-2.0-only
	/*
	*
	* Copyright (c) 2009, Microsoft Corporation.
	*
	* Authors:
	* Haiyang Zhang <haiyangz@microsoft.com>
	* Hank Janssen <hjanssen@microsoft.com>
	* K. Y. Srinivasan <kys@microsoft.com>
	*/
	#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

	#include <linux/kernel.h>
	#include <linux/mm.h>
	#include <linux/hyperv.h>
	#include <linux/uio.h>
	#include <linux/vmalloc.h>
	#include <linux/slab.h>
	#include <linux/prefetch.h>

	#include "hyperv_vmbus.h"

	#define VMBUS_PKT_TRAILER 8

	/*
	* When we write to the ring buffer, check if the host needs to
	* be signaled. Here is the details of this protocol:
	*
	* 1. The host guarantees that while it is draining the
	* ring buffer, it will set the interrupt_mask to
	* indicate it does not need to be interrupted when
	* new data is placed.
	*
	* 2. The host guarantees that it will completely drain
	* the ring buffer before exiting the read loop. Further,
	* once the ring buffer is empty, it will clear the
	* interrupt_mask and re-check to see if new data has
	* arrived.
	*
	* KYS: Oct. 30, 2016:
	* It looks like Windows hosts have logic to deal with DOS attacks that
	* can be triggered if it receives interrupts when it is not expecting
	* the interrupt. The host expects interrupts only when the ring
	* transitions from empty to non-empty (or full to non full on the guest
	* to host ring).
	* So, base the signaling decision solely on the ring state until the
	* host logic is fixed.
	*/

	static void hv_signal_on_write(u32 old_write, struct vmbus_channel *channel)
	{
	struct hv_ring_buffer_info *rbi = &channel->outbound;

	virt_mb();
	if (READ_ONCE(rbi->ring_buffer->interrupt_mask))
	return;

	/* check interrupt_mask before read_index */
	virt_rmb();
	/*
	* This is the only case we need to signal when the
	* ring transitions from being empty to non-empty.
	*/
	if (old_write == READ_ONCE(rbi->ring_buffer->read_index)) {
	++channel->intr_out_empty;
	vmbus_setevent(channel);
	}
	}

	/* Get the next write location for the specified ring buffer. */
	static inline u32
	hv_get_next_write_location(struct hv_ring_buffer_info *ring_info)
	{
	u32 next = ring_info->ring_buffer->write_index;

	return next;
	}

	/* Set the next write location for the specified ring buffer. */
	static inline void
	hv_set_next_write_location(struct hv_ring_buffer_info *ring_info,
	u32 next_write_location)
	{
	ring_info->ring_buffer->write_index = next_write_location;
	}

	/* Set the next read location for the specified ring buffer. */
	static inline void
	hv_set_next_read_location(struct hv_ring_buffer_info *ring_info,
	u32 next_read_location)
	{
	ring_info->ring_buffer->read_index = next_read_location;
	ring_info->priv_read_index = next_read_location;
	}

	/* Get the size of the ring buffer. */
	static inline u32
	hv_get_ring_buffersize(const struct hv_ring_buffer_info *ring_info)
	{
	return ring_info->ring_datasize;
	}

	/* Get the read and write indices as u64 of the specified ring buffer. */
	static inline u64
	hv_get_ring_bufferindices(struct hv_ring_buffer_info *ring_info)
	{
	return (u64)ring_info->ring_buffer->write_index << 32;
	}

	/*
	* Helper routine to copy from source to ring buffer.
	* Assume there is enough room. Handles wrap-around in dest case only!!
	*/
	static u32 hv_copyto_ringbuffer(
	struct hv_ring_buffer_info *ring_info,
	u32 start_write_offset,
	const void *src,
	u32 srclen)
	{
	void *ring_buffer = hv_get_ring_buffer(ring_info);
	u32 ring_buffer_size = hv_get_ring_buffersize(ring_info);

	memcpy(ring_buffer + start_write_offset, src, srclen);

	start_write_offset += srclen;
	if (start_write_offset >= ring_buffer_size)
	start_write_offset -= ring_buffer_size;

	return start_write_offset;
	}

	/*
	*
	* hv_get_ringbuffer_availbytes()
	*
	* Get number of bytes available to read and to write to
	* for the specified ring buffer
	*/
	static void
	hv_get_ringbuffer_availbytes(const struct hv_ring_buffer_info *rbi,
	u32 read, u32 write)
	{
	u32 read_loc, write_loc, dsize;

	/* Capture the read/write indices before they changed */
	read_loc = READ_ONCE(rbi->ring_buffer->read_index);
	write_loc = READ_ONCE(rbi->ring_buffer->write_index);
	dsize = rbi->ring_datasize;

	*write = write_loc >= read_loc ? dsize - (write_loc - read_loc) :
	read_loc - write_loc;
	read = dsize - write;
	}

	/* Get various debug metrics for the specified ring buffer. */
	int hv_ringbuffer_get_debuginfo(struct hv_ring_buffer_info *ring_info,
	struct hv_ring_buffer_debug_info *debug_info)
	{
	u32 bytes_avail_towrite;
	u32 bytes_avail_toread;

	mutex_lock(&ring_info->ring_buffer_mutex);

	if (!ring_info->ring_buffer) {
	mutex_unlock(&ring_info->ring_buffer_mutex);
	return -EINVAL;
	}

	hv_get_ringbuffer_availbytes(ring_info,
	&bytes_avail_toread,
	&bytes_avail_towrite);
	debug_info->bytes_avail_toread = bytes_avail_toread;
	debug_info->bytes_avail_towrite = bytes_avail_towrite;
	debug_info->current_read_index = ring_info->ring_buffer->read_index;
	debug_info->current_write_index = ring_info->ring_buffer->write_index;
	debug_info->current_interrupt_mask
	= ring_info->ring_buffer->interrupt_mask;
	mutex_unlock(&ring_info->ring_buffer_mutex);

	return 0;
	}
	EXPORT_SYMBOL_GPL(hv_ringbuffer_get_debuginfo);

	/* Initialize a channel's ring buffer info mutex locks */
	void hv_ringbuffer_pre_init(struct vmbus_channel *channel)
	{
	mutex_init(&channel->inbound.ring_buffer_mutex);
	mutex_init(&channel->outbound.ring_buffer_mutex);
	}

	/* Initialize the ring buffer. */
	int hv_ringbuffer_init(struct hv_ring_buffer_info *ring_info,
	struct page *pages, u32 page_cnt)
	{
	int i;
	struct page **pages_wraparound;

	BUILD_BUG_ON((sizeof(struct hv_ring_buffer) != PAGE_SIZE));

	/*
	* First page holds struct hv_ring_buffer, do wraparound mapping for
	* the rest.
	*/
	pages_wraparound = kcalloc(page_cnt * 2 - 1, sizeof(struct page *),
	GFP_KERNEL);
	if (!pages_wraparound)
	return -ENOMEM;

	pages_wraparound[0] = pages;
	for (i = 0; i < 2 * (page_cnt - 1); i++)
	pages_wraparound[i + 1] = &pages[i % (page_cnt - 1) + 1];

	ring_info->ring_buffer = (struct hv_ring_buffer *)
	vmap(pages_wraparound, page_cnt * 2 - 1, VM_MAP, PAGE_KERNEL);

	kfree(pages_wraparound);


	if (!ring_info->ring_buffer)
	return -ENOMEM;

	ring_info->ring_buffer->read_index =
	ring_info->ring_buffer->write_index = 0;

	/* Set the feature bit for enabling flow control. */
	ring_info->ring_buffer->feature_bits.value = 1;

	ring_info->ring_size = page_cnt << PAGE_SHIFT;
	ring_info->ring_size_div10_reciprocal =
	reciprocal_value(ring_info->ring_size / 10);
	ring_info->ring_datasize = ring_info->ring_size -
	sizeof(struct hv_ring_buffer);
	ring_info->priv_read_index = 0;

	spin_lock_init(&ring_info->ring_lock);

	return 0;
	}

	/* Cleanup the ring buffer. */
	void hv_ringbuffer_cleanup(struct hv_ring_buffer_info *ring_info)
	{
	mutex_lock(&ring_info->ring_buffer_mutex);
	vunmap(ring_info->ring_buffer);
	ring_info->ring_buffer = NULL;
	mutex_unlock(&ring_info->ring_buffer_mutex);
	}

	/* Write to the ring buffer. */
	int hv_ringbuffer_write(struct vmbus_channel *channel,
	const struct kvec *kv_list, u32 kv_count)
	{
	int i;
	u32 bytes_avail_towrite;
	u32 totalbytes_towrite = sizeof(u64);
	u32 next_write_location;
	u32 old_write;
	u64 prev_indices;
	unsigned long flags;
	struct hv_ring_buffer_info *outring_info = &channel->outbound;

	if (channel->rescind)
	return -ENODEV;

	for (i = 0; i < kv_count; i++)
	totalbytes_towrite += kv_list[i].iov_len;

	spin_lock_irqsave(&outring_info->ring_lock, flags);

	bytes_avail_towrite = hv_get_bytes_to_write(outring_info);

	/*
	* If there is only room for the packet, assume it is full.
	* Otherwise, the next time around, we think the ring buffer
	* is empty since the read index == write index.
	*/
	if (bytes_avail_towrite <= totalbytes_towrite) {
	++channel->out_full_total;

	if (!channel->out_full_flag) {
	++channel->out_full_first;
	channel->out_full_flag = true;
	}

	spin_unlock_irqrestore(&outring_info->ring_lock, flags);
	return -EAGAIN;
	}

	channel->out_full_flag = false;

	/* Write to the ring buffer */
	next_write_location = hv_get_next_write_location(outring_info);

	old_write = next_write_location;

	for (i = 0; i < kv_count; i++) {
	next_write_location = hv_copyto_ringbuffer(outring_info,
	next_write_location,
	kv_list[i].iov_base,
	kv_list[i].iov_len);
	}

	/* Set previous packet start */
	prev_indices = hv_get_ring_bufferindices(outring_info);

	next_write_location = hv_copyto_ringbuffer(outring_info,
	next_write_location,
	&prev_indices,
	sizeof(u64));

	/* Issue a full memory barrier before updating the write index */
	virt_mb();

	/* Now, update the write location */
	hv_set_next_write_location(outring_info, next_write_location);


	spin_unlock_irqrestore(&outring_info->ring_lock, flags);

	hv_signal_on_write(old_write, channel);

	if (channel->rescind)
	return -ENODEV;

	return 0;
	}

	int hv_ringbuffer_read(struct vmbus_channel *channel,
	void buffer, u32 buflen, u32 buffer_actual_len,
	u64 *requestid, bool raw)
	{
	struct vmpacket_descriptor *desc;
	u32 packetlen, offset;

	if (unlikely(buflen == 0))
	return -EINVAL;

	*buffer_actual_len = 0;
	*requestid = 0;

	/* Make sure there is something to read */
	desc = hv_pkt_iter_first(channel);
	if (desc == NULL) {
	/*
	* No error is set when there is even no header, drivers are
	* supposed to analyze buffer_actual_len.
	*/
	return 0;
	}

	offset = raw ? 0 : (desc->offset8 << 3);
	packetlen = (desc->len8 << 3) - offset;
	*buffer_actual_len = packetlen;
	*requestid = desc->trans_id;

	if (unlikely(packetlen > buflen))
	return -ENOBUFS;

	/* since ring is double mapped, only one copy is necessary */
	memcpy(buffer, (const char *)desc + offset, packetlen);

	/* Advance ring index to next packet descriptor */
	__hv_pkt_iter_next(channel, desc);

	/* Notify host of update */
	hv_pkt_iter_close(channel);

	return 0;
	}

	/*
	* Determine number of bytes available in ring buffer after
	* the current iterator (priv_read_index) location.
	*
	* This is similar to hv_get_bytes_to_read but with private
	* read index instead.
	*/
	static u32 hv_pkt_iter_avail(const struct hv_ring_buffer_info *rbi)
	{
	u32 priv_read_loc = rbi->priv_read_index;
	u32 write_loc = READ_ONCE(rbi->ring_buffer->write_index);

	if (write_loc >= priv_read_loc)
	return write_loc - priv_read_loc;
	else
	return (rbi->ring_datasize - priv_read_loc) + write_loc;
	}

	/*
	* Get first vmbus packet from ring buffer after read_index
	*
	* If ring buffer is empty, returns NULL and no other action needed.
	*/
	struct vmpacket_descriptor hv_pkt_iter_first(struct vmbus_channel channel)
	{
	struct hv_ring_buffer_info *rbi = &channel->inbound;
	struct vmpacket_descriptor *desc;

	if (hv_pkt_iter_avail(rbi) < sizeof(struct vmpacket_descriptor))
	return NULL;

	desc = hv_get_ring_buffer(rbi) + rbi->priv_read_index;
	if (desc)
	prefetch((char *)desc + (desc->len8 << 3));

	return desc;
	}
	EXPORT_SYMBOL_GPL(hv_pkt_iter_first);

	/*
	* Get next vmbus packet from ring buffer.
	*
	* Advances the current location (priv_read_index) and checks for more
	* data. If the end of the ring buffer is reached, then return NULL.
	*/
	struct vmpacket_descriptor *
	__hv_pkt_iter_next(struct vmbus_channel *channel,
	const struct vmpacket_descriptor *desc)
	{
	struct hv_ring_buffer_info *rbi = &channel->inbound;
	u32 packetlen = desc->len8 << 3;
	u32 dsize = rbi->ring_datasize;

	/* bump offset to next potential packet */
	rbi->priv_read_index += packetlen + VMBUS_PKT_TRAILER;
	if (rbi->priv_read_index >= dsize)
	rbi->priv_read_index -= dsize;

	/* more data? */
	return hv_pkt_iter_first(channel);
	}
	EXPORT_SYMBOL_GPL(__hv_pkt_iter_next);

	/* How many bytes were read in this iterator cycle */
	static u32 hv_pkt_iter_bytes_read(const struct hv_ring_buffer_info *rbi,
	u32 start_read_index)
	{
	if (rbi->priv_read_index >= start_read_index)
	return rbi->priv_read_index - start_read_index;
	else
	return rbi->ring_datasize - start_read_index +
	rbi->priv_read_index;
	}

	/*
	* Update host ring buffer after iterating over packets. If the host has
	* stopped queuing new entries because it found the ring buffer full, and
	* sufficient space is being freed up, signal the host. But be careful to
	* only signal the host when necessary, both for performance reasons and
	* because Hyper-V protects itself by throttling guests that signal
	* inappropriately.
	*
	* Determining when to signal is tricky. There are three key data inputs
	* that must be handled in this order to avoid race conditions:
	*
	* 1. Update the read_index
	* 2. Read the pending_send_sz
	* 3. Read the current write_index
	*
	* The interrupt_mask is not used to determine when to signal. The
	* interrupt_mask is used only on the guest->host ring buffer when
	* sending requests to the host. The host does not use it on the host->
	* guest ring buffer to indicate whether it should be signaled.
	*/
	void hv_pkt_iter_close(struct vmbus_channel *channel)
	{
	struct hv_ring_buffer_info *rbi = &channel->inbound;
	u32 curr_write_sz, pending_sz, bytes_read, start_read_index;

	/*
	* Make sure all reads are done before we update the read index since
	* the writer may start writing to the read area once the read index
	* is updated.
	*/
	virt_rmb();
	start_read_index = rbi->ring_buffer->read_index;
	rbi->ring_buffer->read_index = rbi->priv_read_index;

	/*
	* Older versions of Hyper-V (before WS2102 and Win8) do not
	* implement pending_send_sz and simply poll if the host->guest
	* ring buffer is full. No signaling is needed or expected.
	*/
	if (!rbi->ring_buffer->feature_bits.feat_pending_send_sz)
	return;

	/*
	* Issue a full memory barrier before making the signaling decision.
	* If reading pending_send_sz were to be reordered and happen
	* before we commit the new read_index, a race could occur. If the
	* host were to set the pending_send_sz after we have sampled
	* pending_send_sz, and the ring buffer blocks before we commit the
	* read index, we could miss sending the interrupt. Issue a full
	* memory barrier to address this.
	*/
	virt_mb();

	/*
	* If the pending_send_sz is zero, then the ring buffer is not
	* blocked and there is no need to signal. This is far by the
	* most common case, so exit quickly for best performance.
	*/
	pending_sz = READ_ONCE(rbi->ring_buffer->pending_send_sz);
	if (!pending_sz)
	return;

	/*
	* Ensure the read of write_index in hv_get_bytes_to_write()
	* happens after the read of pending_send_sz.
	*/
	virt_rmb();
	curr_write_sz = hv_get_bytes_to_write(rbi);
	bytes_read = hv_pkt_iter_bytes_read(rbi, start_read_index);

	/*
	* We want to signal the host only if we're transitioning
	* from a "not enough free space" state to a "enough free
	* space" state. For example, it's possible that this function
	* could run and free up enough space to signal the host, and then
	* run again and free up additional space before the host has a
	* chance to clear the pending_send_sz. The 2nd invocation would
	* be a null transition from "enough free space" to "enough free
	* space", which doesn't warrant a signal.
	*
	* Exactly filling the ring buffer is treated as "not enough
	* space". The ring buffer always must have at least one byte
	* empty so the empty and full conditions are distinguishable.
	* hv_get_bytes_to_write() doesn't fully tell the truth in
	* this regard.
	*
	* So first check if we were in the "enough free space" state
	* before we began the iteration. If so, the host was not
	* blocked, and there's no need to signal.
	*/
	if (curr_write_sz - bytes_read > pending_sz)
	return;

	/*
	* Similarly, if the new state is "not enough space", then
	* there's no need to signal.
	*/
	if (curr_write_sz <= pending_sz)
	return;

	++channel->intr_in_full;
	vmbus_setevent(channel);
	}
	EXPORT_SYMBOL_GPL(hv_pkt_iter_close);