summaryrefslogtreecommitdiff
path: root/drivers/hv/ring_buffer.c
blob: 3c9b02471760a33d340c1f50914ab305e5bec0eb (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
// 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 <linux/io.h>
#include <asm/mshyperv.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;
}

/* 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, u32 max_pkt_size)
{
	struct page **pages_wraparound;
	int i;

	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,
			pgprot_decrypted(PAGE_KERNEL));

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

	/*
	 * Ensure the header page is zero'ed since
	 * encryption status may have changed.
	 */
	memset(ring_info->ring_buffer, 0, HV_HYP_PAGE_SIZE);

	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;

	/* Initialize buffer that holds copies of incoming packets */
	if (max_pkt_size) {
		ring_info->pkt_buffer = kzalloc(max_pkt_size, GFP_KERNEL);
		if (!ring_info->pkt_buffer)
			return -ENOMEM;
		ring_info->pkt_buffer_size = max_pkt_size;
	}

	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);

	kfree(ring_info->pkt_buffer);
	ring_info->pkt_buffer = NULL;
	ring_info->pkt_buffer_size = 0;
}

/*
 * Check if the ring buffer spinlock is available to take or not; used on
 * atomic contexts, like panic path (see the Hyper-V framebuffer driver).
 */

bool hv_ringbuffer_spinlock_busy(struct vmbus_channel *channel)
{
	struct hv_ring_buffer_info *rinfo = &channel->outbound;

	return spin_is_locked(&rinfo->ring_lock);
}
EXPORT_SYMBOL_GPL(hv_ringbuffer_spinlock_busy);

/* Write to the ring buffer. */
int hv_ringbuffer_write(struct vmbus_channel *channel,
			const struct kvec *kv_list, u32 kv_count,
			u64 requestid, u64 *trans_id)
{
	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;
	struct vmpacket_descriptor *desc = kv_list[0].iov_base;
	u64 __trans_id, rqst_id = VMBUS_NO_RQSTOR;

	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);
	}

	/*
	 * Allocate the request ID after the data has been copied into the
	 * ring buffer.  Once this request ID is allocated, the completion
	 * path could find the data and free it.
	 */

	if (desc->flags == VMBUS_DATA_PACKET_FLAG_COMPLETION_REQUESTED) {
		if (channel->next_request_id_callback != NULL) {
			rqst_id = channel->next_request_id_callback(channel, requestid);
			if (rqst_id == VMBUS_RQST_ERROR) {
				spin_unlock_irqrestore(&outring_info->ring_lock, flags);
				return -EAGAIN;
			}
		}
	}
	desc = hv_get_ring_buffer(outring_info) + old_write;
	__trans_id = (rqst_id == VMBUS_NO_RQSTOR) ? requestid : rqst_id;
	/*
	 * Ensure the compiler doesn't generate code that reads the value of
	 * the transaction ID from the ring buffer, which is shared with the
	 * Hyper-V host and subject to being changed at any time.
	 */
	WRITE_ONCE(desc->trans_id, __trans_id);
	if (trans_id)
		*trans_id = __trans_id;

	/* 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) {
		if (rqst_id != VMBUS_NO_RQSTOR) {
			/* Reclaim request ID to avoid leak of IDs */
			if (channel->request_addr_callback != NULL)
				channel->request_addr_callback(channel, rqst_id);
		}
		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;

	/*
	 * The Hyper-V host writes the packet data, then uses
	 * store_release() to update the write_index.  Use load_acquire()
	 * here to prevent loads of the packet data from being re-ordered
	 * before the read of the write_index and potentially getting
	 * stale data.
	 */
	write_loc = virt_load_acquire(&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, *desc_copy;
	u32 bytes_avail, pkt_len, pkt_offset;

	hv_debug_delay_test(channel, MESSAGE_DELAY);

	bytes_avail = hv_pkt_iter_avail(rbi);
	if (bytes_avail < sizeof(struct vmpacket_descriptor))
		return NULL;
	bytes_avail = min(rbi->pkt_buffer_size, bytes_avail);

	desc = (struct vmpacket_descriptor *)(hv_get_ring_buffer(rbi) + rbi->priv_read_index);

	/*
	 * Ensure the compiler does not use references to incoming Hyper-V values (which
	 * could change at any moment) when reading local variables later in the code
	 */
	pkt_len = READ_ONCE(desc->len8) << 3;
	pkt_offset = READ_ONCE(desc->offset8) << 3;

	/*
	 * If pkt_len is invalid, set it to the smaller of hv_pkt_iter_avail() and
	 * rbi->pkt_buffer_size
	 */
	if (pkt_len < sizeof(struct vmpacket_descriptor) || pkt_len > bytes_avail)
		pkt_len = bytes_avail;

	/*
	 * If pkt_offset is invalid, arbitrarily set it to
	 * the size of vmpacket_descriptor
	 */
	if (pkt_offset < sizeof(struct vmpacket_descriptor) || pkt_offset > pkt_len)
		pkt_offset = sizeof(struct vmpacket_descriptor);

	/* Copy the Hyper-V packet out of the ring buffer */
	desc_copy = (struct vmpacket_descriptor *)rbi->pkt_buffer;
	memcpy(desc_copy, desc, pkt_len);

	/*
	 * Hyper-V could still change len8 and offset8 after the earlier read.
	 * Ensure that desc_copy has legal values for len8 and offset8 that
	 * are consistent with the copy we just made
	 */
	desc_copy->len8 = pkt_len >> 3;
	desc_copy->offset8 = pkt_offset >> 3;

	return desc_copy;
}
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;

	hv_debug_delay_test(channel, MESSAGE_DELAY);
	/* 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);