drivers/hv/ring_buffer.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  *
   4  * Copyright (c) 2009, Microsoft Corporation.
   5  *
   6  * Authors:
   7  *   Haiyang Zhang <haiyangz@microsoft.com>
   8  *   Hank Janssen  <hjanssen@microsoft.com>
   9  *   K. Y. Srinivasan <kys@microsoft.com>
  10  */
  11 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
  12
  13 #include <linux/kernel.h>
  14 #include <linux/mm.h>
  15 #include <linux/hyperv.h>
  16 #include <linux/uio.h>
  17 #include <linux/vmalloc.h>
  18 #include <linux/slab.h>
  19 #include <linux/prefetch.h>
  20 #include <linux/io.h>
  21 #include <asm/mshyperv.h>
  22
  23 #include "hyperv_vmbus.h"
  24
  25 #define VMBUS_PKT_TRAILER       8
  26
  27 /*
  28  * When we write to the ring buffer, check if the host needs to
  29  * be signaled. Here is the details of this protocol:
  30  *
  31  *      1. The host guarantees that while it is draining the
  32  *         ring buffer, it will set the interrupt_mask to
  33  *         indicate it does not need to be interrupted when
  34  *         new data is placed.
  35  *
  36  *      2. The host guarantees that it will completely drain
  37  *         the ring buffer before exiting the read loop. Further,
  38  *         once the ring buffer is empty, it will clear the
  39  *         interrupt_mask and re-check to see if new data has
  40  *         arrived.
  41  *
  42  * KYS: Oct. 30, 2016:
  43  * It looks like Windows hosts have logic to deal with DOS attacks that
  44  * can be triggered if it receives interrupts when it is not expecting
  45  * the interrupt. The host expects interrupts only when the ring
  46  * transitions from empty to non-empty (or full to non full on the guest
  47  * to host ring).
  48  * So, base the signaling decision solely on the ring state until the
  49  * host logic is fixed.
  50  */
  51
  52 static void hv_signal_on_write(u32 old_write, struct vmbus_channel *channel)
  53 {
  54         struct hv_ring_buffer_info *rbi = &channel->outbound;
  55
  56         virt_mb();
  57         if (READ_ONCE(rbi->ring_buffer->interrupt_mask))
  58                 return;
  59
  60         /* check interrupt_mask before read_index */
  61         virt_rmb();
  62         /*
  63          * This is the only case we need to signal when the
  64          * ring transitions from being empty to non-empty.
  65          */
  66         if (old_write == READ_ONCE(rbi->ring_buffer->read_index)) {
  67                 ++channel->intr_out_empty;
  68                 vmbus_setevent(channel);
  69         }
  70 }
  71
  72 /* Get the next write location for the specified ring buffer. */
  73 static inline u32
  74 hv_get_next_write_location(struct hv_ring_buffer_info *ring_info)
  75 {
  76         u32 next = ring_info->ring_buffer->write_index;
  77
  78         return next;
  79 }
  80
  81 /* Set the next write location for the specified ring buffer. */
  82 static inline void
  83 hv_set_next_write_location(struct hv_ring_buffer_info *ring_info,
  84                      u32 next_write_location)
  85 {
  86         ring_info->ring_buffer->write_index = next_write_location;
  87 }
  88
  89 /* Get the size of the ring buffer. */
  90 static inline u32
  91 hv_get_ring_buffersize(const struct hv_ring_buffer_info *ring_info)
  92 {
  93         return ring_info->ring_datasize;
  94 }
  95
  96 /* Get the read and write indices as u64 of the specified ring buffer. */
  97 static inline u64
  98 hv_get_ring_bufferindices(struct hv_ring_buffer_info *ring_info)
  99 {
 100         return (u64)ring_info->ring_buffer->write_index << 32;
 101 }
 102
 103 /*
 104  * Helper routine to copy from source to ring buffer.
 105  * Assume there is enough room. Handles wrap-around in dest case only!!
 106  */
 107 static u32 hv_copyto_ringbuffer(
 108         struct hv_ring_buffer_info      *ring_info,
 109         u32                             start_write_offset,
 110         const void                      *src,
 111         u32                             srclen)
 112 {
 113         void *ring_buffer = hv_get_ring_buffer(ring_info);
 114         u32 ring_buffer_size = hv_get_ring_buffersize(ring_info);
 115
 116         memcpy(ring_buffer + start_write_offset, src, srclen);
 117
 118         start_write_offset += srclen;
 119         if (start_write_offset >= ring_buffer_size)
 120                 start_write_offset -= ring_buffer_size;
 121
 122         return start_write_offset;
 123 }
 124
 125 /*
 126  *
 127  * hv_get_ringbuffer_availbytes()
 128  *
 129  * Get number of bytes available to read and to write to
 130  * for the specified ring buffer
 131  */
 132 static void
 133 hv_get_ringbuffer_availbytes(const struct hv_ring_buffer_info *rbi,
 134                              u32 *read, u32 *write)
 135 {
 136         u32 read_loc, write_loc, dsize;
 137
 138         /* Capture the read/write indices before they changed */
 139         read_loc = READ_ONCE(rbi->ring_buffer->read_index);
 140         write_loc = READ_ONCE(rbi->ring_buffer->write_index);
 141         dsize = rbi->ring_datasize;
 142
 143         *write = write_loc >= read_loc ? dsize - (write_loc - read_loc) :
 144                 read_loc - write_loc;
 145         *read = dsize - *write;
 146 }
 147
 148 /* Get various debug metrics for the specified ring buffer. */
 149 int hv_ringbuffer_get_debuginfo(struct hv_ring_buffer_info *ring_info,
 150                                 struct hv_ring_buffer_debug_info *debug_info)
 151 {
 152         u32 bytes_avail_towrite;
 153         u32 bytes_avail_toread;
 154
 155         mutex_lock(&ring_info->ring_buffer_mutex);
 156
 157         if (!ring_info->ring_buffer) {
 158                 mutex_unlock(&ring_info->ring_buffer_mutex);
 159                 return -EINVAL;
 160         }
 161
 162         hv_get_ringbuffer_availbytes(ring_info,
 163                                      &bytes_avail_toread,
 164                                      &bytes_avail_towrite);
 165         debug_info->bytes_avail_toread = bytes_avail_toread;
 166         debug_info->bytes_avail_towrite = bytes_avail_towrite;
 167         debug_info->current_read_index = ring_info->ring_buffer->read_index;
 168         debug_info->current_write_index = ring_info->ring_buffer->write_index;
 169         debug_info->current_interrupt_mask
 170                 = ring_info->ring_buffer->interrupt_mask;
 171         mutex_unlock(&ring_info->ring_buffer_mutex);
 172
 173         return 0;
 174 }
 175 EXPORT_SYMBOL_GPL(hv_ringbuffer_get_debuginfo);
 176
 177 /* Initialize a channel's ring buffer info mutex locks */
 178 void hv_ringbuffer_pre_init(struct vmbus_channel *channel)
 179 {
 180         mutex_init(&channel->inbound.ring_buffer_mutex);
 181         mutex_init(&channel->outbound.ring_buffer_mutex);
 182 }
 183
 184 /* Initialize the ring buffer. */
 185 int hv_ringbuffer_init(struct hv_ring_buffer_info *ring_info,
 186                        struct page *pages, u32 page_cnt, u32 max_pkt_size)
 187 {
 188         struct page **pages_wraparound;
 189         unsigned long *pfns_wraparound;
 190         u64 pfn;
 191         int i;
 192
 193         BUILD_BUG_ON((sizeof(struct hv_ring_buffer) != PAGE_SIZE));
 194
 195         /*
 196          * First page holds struct hv_ring_buffer, do wraparound mapping for
 197          * the rest.
 198          */
 199         if (hv_isolation_type_snp()) {
 200                 pfn = page_to_pfn(pages) +
 201                         PFN_DOWN(ms_hyperv.shared_gpa_boundary);
 202
 203                 pfns_wraparound = kcalloc(page_cnt * 2 - 1,
 204                         sizeof(unsigned long), GFP_KERNEL);
 205                 if (!pfns_wraparound)
 206                         return -ENOMEM;
 207
 208                 pfns_wraparound[0] = pfn;
 209                 for (i = 0; i < 2 * (page_cnt - 1); i++)
 210                         pfns_wraparound[i + 1] = pfn + i % (page_cnt - 1) + 1;
 211
 212                 ring_info->ring_buffer = (struct hv_ring_buffer *)
 213                         vmap_pfn(pfns_wraparound, page_cnt * 2 - 1,
 214                                  PAGE_KERNEL);
 215                 kfree(pfns_wraparound);
 216
 217                 if (!ring_info->ring_buffer)
 218                         return -ENOMEM;
 219
 220                 /* Zero ring buffer after setting memory host visibility. */
 221                 memset(ring_info->ring_buffer, 0x00, PAGE_SIZE * page_cnt);
 222         } else {
 223                 pages_wraparound = kcalloc(page_cnt * 2 - 1,
 224                                            sizeof(struct page *),
 225                                            GFP_KERNEL);
 226                 if (!pages_wraparound)
 227                         return -ENOMEM;
 228
 229                 pages_wraparound[0] = pages;
 230                 for (i = 0; i < 2 * (page_cnt - 1); i++)
 231                         pages_wraparound[i + 1] =
 232                                 &pages[i % (page_cnt - 1) + 1];
 233
 234                 ring_info->ring_buffer = (struct hv_ring_buffer *)
 235                         vmap(pages_wraparound, page_cnt * 2 - 1, VM_MAP,
 236                                 PAGE_KERNEL);
 237
 238                 kfree(pages_wraparound);
 239                 if (!ring_info->ring_buffer)
 240                         return -ENOMEM;
 241         }
 242
 243
 244         ring_info->ring_buffer->read_index =
 245                 ring_info->ring_buffer->write_index = 0;
 246
 247         /* Set the feature bit for enabling flow control. */
 248         ring_info->ring_buffer->feature_bits.value = 1;
 249
 250         ring_info->ring_size = page_cnt << PAGE_SHIFT;
 251         ring_info->ring_size_div10_reciprocal =
 252                 reciprocal_value(ring_info->ring_size / 10);
 253         ring_info->ring_datasize = ring_info->ring_size -
 254                 sizeof(struct hv_ring_buffer);
 255         ring_info->priv_read_index = 0;
 256
 257         /* Initialize buffer that holds copies of incoming packets */
 258         if (max_pkt_size) {
 259                 ring_info->pkt_buffer = kzalloc(max_pkt_size, GFP_KERNEL);
 260                 if (!ring_info->pkt_buffer)
 261                         return -ENOMEM;
 262                 ring_info->pkt_buffer_size = max_pkt_size;
 263         }
 264
 265         spin_lock_init(&ring_info->ring_lock);
 266
 267         return 0;
 268 }
 269
 270 /* Cleanup the ring buffer. */
 271 void hv_ringbuffer_cleanup(struct hv_ring_buffer_info *ring_info)
 272 {
 273         mutex_lock(&ring_info->ring_buffer_mutex);
 274         vunmap(ring_info->ring_buffer);
 275         ring_info->ring_buffer = NULL;
 276         mutex_unlock(&ring_info->ring_buffer_mutex);
 277
 278         kfree(ring_info->pkt_buffer);
 279         ring_info->pkt_buffer = NULL;
 280         ring_info->pkt_buffer_size = 0;
 281 }
 282
 283 /*
 284  * Check if the ring buffer spinlock is available to take or not; used on
 285  * atomic contexts, like panic path (see the Hyper-V framebuffer driver).
 286  */
 287
 288 bool hv_ringbuffer_spinlock_busy(struct vmbus_channel *channel)
 289 {
 290         struct hv_ring_buffer_info *rinfo = &channel->outbound;
 291
 292         return spin_is_locked(&rinfo->ring_lock);
 293 }
 294 EXPORT_SYMBOL_GPL(hv_ringbuffer_spinlock_busy);
 295
 296 /* Write to the ring buffer. */
 297 int hv_ringbuffer_write(struct vmbus_channel *channel,
 298                         const struct kvec *kv_list, u32 kv_count,
 299                         u64 requestid, u64 *trans_id)
 300 {
 301         int i;
 302         u32 bytes_avail_towrite;
 303         u32 totalbytes_towrite = sizeof(u64);
 304         u32 next_write_location;
 305         u32 old_write;
 306         u64 prev_indices;
 307         unsigned long flags;
 308         struct hv_ring_buffer_info *outring_info = &channel->outbound;
 309         struct vmpacket_descriptor *desc = kv_list[0].iov_base;
 310         u64 __trans_id, rqst_id = VMBUS_NO_RQSTOR;
 311
 312         if (channel->rescind)
 313                 return -ENODEV;
 314
 315         for (i = 0; i < kv_count; i++)
 316                 totalbytes_towrite += kv_list[i].iov_len;
 317
 318         spin_lock_irqsave(&outring_info->ring_lock, flags);
 319
 320         bytes_avail_towrite = hv_get_bytes_to_write(outring_info);
 321
 322         /*
 323          * If there is only room for the packet, assume it is full.
 324          * Otherwise, the next time around, we think the ring buffer
 325          * is empty since the read index == write index.
 326          */
 327         if (bytes_avail_towrite <= totalbytes_towrite) {
 328                 ++channel->out_full_total;
 329
 330                 if (!channel->out_full_flag) {
 331                         ++channel->out_full_first;
 332                         channel->out_full_flag = true;
 333                 }
 334
 335                 spin_unlock_irqrestore(&outring_info->ring_lock, flags);
 336                 return -EAGAIN;
 337         }
 338
 339         channel->out_full_flag = false;
 340
 341         /* Write to the ring buffer */
 342         next_write_location = hv_get_next_write_location(outring_info);
 343
 344         old_write = next_write_location;
 345
 346         for (i = 0; i < kv_count; i++) {
 347                 next_write_location = hv_copyto_ringbuffer(outring_info,
 348                                                      next_write_location,
 349                                                      kv_list[i].iov_base,
 350                                                      kv_list[i].iov_len);
 351         }
 352
 353         /*
 354          * Allocate the request ID after the data has been copied into the
 355          * ring buffer.  Once this request ID is allocated, the completion
 356          * path could find the data and free it.
 357          */
 358
 359         if (desc->flags == VMBUS_DATA_PACKET_FLAG_COMPLETION_REQUESTED) {
 360                 if (channel->next_request_id_callback != NULL) {
 361                         rqst_id = channel->next_request_id_callback(channel, requestid);
 362                         if (rqst_id == VMBUS_RQST_ERROR) {
 363                                 spin_unlock_irqrestore(&outring_info->ring_lock, flags);
 364                                 return -EAGAIN;
 365                         }
 366                 }
 367         }
 368         desc = hv_get_ring_buffer(outring_info) + old_write;
 369         __trans_id = (rqst_id == VMBUS_NO_RQSTOR) ? requestid : rqst_id;
 370         /*
 371          * Ensure the compiler doesn't generate code that reads the value of
 372          * the transaction ID from the ring buffer, which is shared with the
 373          * Hyper-V host and subject to being changed at any time.
 374          */
 375         WRITE_ONCE(desc->trans_id, __trans_id);
 376         if (trans_id)
 377                 *trans_id = __trans_id;
 378
 379         /* Set previous packet start */
 380         prev_indices = hv_get_ring_bufferindices(outring_info);
 381
 382         next_write_location = hv_copyto_ringbuffer(outring_info,
 383                                              next_write_location,
 384                                              &prev_indices,
 385                                              sizeof(u64));
 386
 387         /* Issue a full memory barrier before updating the write index */
 388         virt_mb();
 389
 390         /* Now, update the write location */
 391         hv_set_next_write_location(outring_info, next_write_location);
 392
 393
 394         spin_unlock_irqrestore(&outring_info->ring_lock, flags);
 395
 396         hv_signal_on_write(old_write, channel);
 397
 398         if (channel->rescind) {
 399                 if (rqst_id != VMBUS_NO_RQSTOR) {
 400                         /* Reclaim request ID to avoid leak of IDs */
 401                         if (channel->request_addr_callback != NULL)
 402                                 channel->request_addr_callback(channel, rqst_id);
 403                 }
 404                 return -ENODEV;
 405         }
 406
 407         return 0;
 408 }
 409
 410 int hv_ringbuffer_read(struct vmbus_channel *channel,
 411                        void *buffer, u32 buflen, u32 *buffer_actual_len,
 412                        u64 *requestid, bool raw)
 413 {
 414         struct vmpacket_descriptor *desc;
 415         u32 packetlen, offset;
 416
 417         if (unlikely(buflen == 0))
 418                 return -EINVAL;
 419
 420         *buffer_actual_len = 0;
 421         *requestid = 0;
 422
 423         /* Make sure there is something to read */
 424         desc = hv_pkt_iter_first(channel);
 425         if (desc == NULL) {
 426                 /*
 427                  * No error is set when there is even no header, drivers are
 428                  * supposed to analyze buffer_actual_len.
 429                  */
 430                 return 0;
 431         }
 432
 433         offset = raw ? 0 : (desc->offset8 << 3);
 434         packetlen = (desc->len8 << 3) - offset;
 435         *buffer_actual_len = packetlen;
 436         *requestid = desc->trans_id;
 437
 438         if (unlikely(packetlen > buflen))
 439                 return -ENOBUFS;
 440
 441         /* since ring is double mapped, only one copy is necessary */
 442         memcpy(buffer, (const char *)desc + offset, packetlen);
 443
 444         /* Advance ring index to next packet descriptor */
 445         __hv_pkt_iter_next(channel, desc);
 446
 447         /* Notify host of update */
 448         hv_pkt_iter_close(channel);
 449
 450         return 0;
 451 }
 452
 453 /*
 454  * Determine number of bytes available in ring buffer after
 455  * the current iterator (priv_read_index) location.
 456  *
 457  * This is similar to hv_get_bytes_to_read but with private
 458  * read index instead.
 459  */
 460 static u32 hv_pkt_iter_avail(const struct hv_ring_buffer_info *rbi)
 461 {
 462         u32 priv_read_loc = rbi->priv_read_index;
 463         u32 write_loc;
 464
 465         /*
 466          * The Hyper-V host writes the packet data, then uses
 467          * store_release() to update the write_index.  Use load_acquire()
 468          * here to prevent loads of the packet data from being re-ordered
 469          * before the read of the write_index and potentially getting
 470          * stale data.
 471          */
 472         write_loc = virt_load_acquire(&rbi->ring_buffer->write_index);
 473
 474         if (write_loc >= priv_read_loc)
 475                 return write_loc - priv_read_loc;
 476         else
 477                 return (rbi->ring_datasize - priv_read_loc) + write_loc;
 478 }
 479
 480 /*
 481  * Get first vmbus packet from ring buffer after read_index
 482  *
 483  * If ring buffer is empty, returns NULL and no other action needed.
 484  */
 485 struct vmpacket_descriptor *hv_pkt_iter_first(struct vmbus_channel *channel)
 486 {
 487         struct hv_ring_buffer_info *rbi = &channel->inbound;
 488         struct vmpacket_descriptor *desc, *desc_copy;
 489         u32 bytes_avail, pkt_len, pkt_offset;
 490
 491         hv_debug_delay_test(channel, MESSAGE_DELAY);
 492
 493         bytes_avail = hv_pkt_iter_avail(rbi);
 494         if (bytes_avail < sizeof(struct vmpacket_descriptor))
 495                 return NULL;
 496         bytes_avail = min(rbi->pkt_buffer_size, bytes_avail);
 497
 498         desc = (struct vmpacket_descriptor *)(hv_get_ring_buffer(rbi) + rbi->priv_read_index);
 499
 500         /*
 501          * Ensure the compiler does not use references to incoming Hyper-V values (which
 502          * could change at any moment) when reading local variables later in the code
 503          */
 504         pkt_len = READ_ONCE(desc->len8) << 3;
 505         pkt_offset = READ_ONCE(desc->offset8) << 3;
 506
 507         /*
 508          * If pkt_len is invalid, set it to the smaller of hv_pkt_iter_avail() and
 509          * rbi->pkt_buffer_size
 510          */
 511         if (pkt_len < sizeof(struct vmpacket_descriptor) || pkt_len > bytes_avail)
 512                 pkt_len = bytes_avail;
 513
 514         /*
 515          * If pkt_offset is invalid, arbitrarily set it to
 516          * the size of vmpacket_descriptor
 517          */
 518         if (pkt_offset < sizeof(struct vmpacket_descriptor) || pkt_offset > pkt_len)
 519                 pkt_offset = sizeof(struct vmpacket_descriptor);
 520
 521         /* Copy the Hyper-V packet out of the ring buffer */
 522         desc_copy = (struct vmpacket_descriptor *)rbi->pkt_buffer;
 523         memcpy(desc_copy, desc, pkt_len);
 524
 525         /*
 526          * Hyper-V could still change len8 and offset8 after the earlier read.
 527          * Ensure that desc_copy has legal values for len8 and offset8 that
 528          * are consistent with the copy we just made
 529          */
 530         desc_copy->len8 = pkt_len >> 3;
 531         desc_copy->offset8 = pkt_offset >> 3;
 532
 533         return desc_copy;
 534 }
 535 EXPORT_SYMBOL_GPL(hv_pkt_iter_first);
 536
 537 /*
 538  * Get next vmbus packet from ring buffer.
 539  *
 540  * Advances the current location (priv_read_index) and checks for more
 541  * data. If the end of the ring buffer is reached, then return NULL.
 542  */
 543 struct vmpacket_descriptor *
 544 __hv_pkt_iter_next(struct vmbus_channel *channel,
 545                    const struct vmpacket_descriptor *desc)
 546 {
 547         struct hv_ring_buffer_info *rbi = &channel->inbound;
 548         u32 packetlen = desc->len8 << 3;
 549         u32 dsize = rbi->ring_datasize;
 550
 551         hv_debug_delay_test(channel, MESSAGE_DELAY);
 552         /* bump offset to next potential packet */
 553         rbi->priv_read_index += packetlen + VMBUS_PKT_TRAILER;
 554         if (rbi->priv_read_index >= dsize)
 555                 rbi->priv_read_index -= dsize;
 556
 557         /* more data? */
 558         return hv_pkt_iter_first(channel);
 559 }
 560 EXPORT_SYMBOL_GPL(__hv_pkt_iter_next);
 561
 562 /* How many bytes were read in this iterator cycle */
 563 static u32 hv_pkt_iter_bytes_read(const struct hv_ring_buffer_info *rbi,
 564                                         u32 start_read_index)
 565 {
 566         if (rbi->priv_read_index >= start_read_index)
 567                 return rbi->priv_read_index - start_read_index;
 568         else
 569                 return rbi->ring_datasize - start_read_index +
 570                         rbi->priv_read_index;
 571 }
 572
 573 /*
 574  * Update host ring buffer after iterating over packets. If the host has
 575  * stopped queuing new entries because it found the ring buffer full, and
 576  * sufficient space is being freed up, signal the host. But be careful to
 577  * only signal the host when necessary, both for performance reasons and
 578  * because Hyper-V protects itself by throttling guests that signal
 579  * inappropriately.
 580  *
 581  * Determining when to signal is tricky. There are three key data inputs
 582  * that must be handled in this order to avoid race conditions:
 583  *
 584  * 1. Update the read_index
 585  * 2. Read the pending_send_sz
 586  * 3. Read the current write_index
 587  *
 588  * The interrupt_mask is not used to determine when to signal. The
 589  * interrupt_mask is used only on the guest->host ring buffer when
 590  * sending requests to the host. The host does not use it on the host->
 591  * guest ring buffer to indicate whether it should be signaled.
 592  */
 593 void hv_pkt_iter_close(struct vmbus_channel *channel)
 594 {
 595         struct hv_ring_buffer_info *rbi = &channel->inbound;
 596         u32 curr_write_sz, pending_sz, bytes_read, start_read_index;
 597
 598         /*
 599          * Make sure all reads are done before we update the read index since
 600          * the writer may start writing to the read area once the read index
 601          * is updated.
 602          */
 603         virt_rmb();
 604         start_read_index = rbi->ring_buffer->read_index;
 605         rbi->ring_buffer->read_index = rbi->priv_read_index;
 606
 607         /*
 608          * Older versions of Hyper-V (before WS2102 and Win8) do not
 609          * implement pending_send_sz and simply poll if the host->guest
 610          * ring buffer is full.  No signaling is needed or expected.
 611          */
 612         if (!rbi->ring_buffer->feature_bits.feat_pending_send_sz)
 613                 return;
 614
 615         /*
 616          * Issue a full memory barrier before making the signaling decision.
 617          * If reading pending_send_sz were to be reordered and happen
 618          * before we commit the new read_index, a race could occur.  If the
 619          * host were to set the pending_send_sz after we have sampled
 620          * pending_send_sz, and the ring buffer blocks before we commit the
 621          * read index, we could miss sending the interrupt. Issue a full
 622          * memory barrier to address this.
 623          */
 624         virt_mb();
 625
 626         /*
 627          * If the pending_send_sz is zero, then the ring buffer is not
 628          * blocked and there is no need to signal.  This is far by the
 629          * most common case, so exit quickly for best performance.
 630          */
 631         pending_sz = READ_ONCE(rbi->ring_buffer->pending_send_sz);
 632         if (!pending_sz)
 633                 return;
 634
 635         /*
 636          * Ensure the read of write_index in hv_get_bytes_to_write()
 637          * happens after the read of pending_send_sz.
 638          */
 639         virt_rmb();
 640         curr_write_sz = hv_get_bytes_to_write(rbi);
 641         bytes_read = hv_pkt_iter_bytes_read(rbi, start_read_index);
 642
 643         /*
 644          * We want to signal the host only if we're transitioning
 645          * from a "not enough free space" state to a "enough free
 646          * space" state.  For example, it's possible that this function
 647          * could run and free up enough space to signal the host, and then
 648          * run again and free up additional space before the host has a
 649          * chance to clear the pending_send_sz.  The 2nd invocation would
 650          * be a null transition from "enough free space" to "enough free
 651          * space", which doesn't warrant a signal.
 652          *
 653          * Exactly filling the ring buffer is treated as "not enough
 654          * space". The ring buffer always must have at least one byte
 655          * empty so the empty and full conditions are distinguishable.
 656          * hv_get_bytes_to_write() doesn't fully tell the truth in
 657          * this regard.
 658          *
 659          * So first check if we were in the "enough free space" state
 660          * before we began the iteration. If so, the host was not
 661          * blocked, and there's no need to signal.
 662          */
 663         if (curr_write_sz - bytes_read > pending_sz)
 664                 return;
 665
 666         /*
 667          * Similarly, if the new state is "not enough space", then
 668          * there's no need to signal.
 669          */
 670         if (curr_write_sz <= pending_sz)
 671                 return;
 672
 673         ++channel->intr_in_full;
 674         vmbus_setevent(channel);
 675 }
 676 EXPORT_SYMBOL_GPL(hv_pkt_iter_close);