From 22098763a10d9c1340827fcf6edab66f153b27f0 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Sun, 13 Sep 2026 12:27:00 -0700 Subject: Merge tag 'trace-v7.3-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace Pull tracing fixes from Steven Rostedt: - Don't destroy user event fields when removal fails User event fields are destroyed before the event is removed from visibility. But that can fail leaving the still visible event with no fields. Move the destroying of the fields to after the event is successfully removed from visibility. - Initialize function graph state is fork before calling copy_exec_state() For non-CLONE_VM forks, copy_exec_state() allocates a new task_exec_state. If that allocation fails, ftrace_graph_exit_task() will free the tasks ret_stack pointer. Since that pointer is still using the parent's ret_stack, it mistakenly frees the parent's pointer too. Call ftrace_graph_init() on the task first which will NULL out the new tasks's ret_stack and if the copy fails, it will not free anything. - Remove FGRAPH_MAX_INDEX The macro FGRAPH_MAX_INDEX was added but never used. Remove it. - Save ent_size in function graph printing of nested functions The function graph tracer needs to look at the next event to see if the next event is the return of the current function entry. If it is, it prints a single line: ktime_get(); Otherwise it prints it like a nested function: tick_nohz_irq_exit() { ktime_get(); kcpustat_irq_exit(); } In order to look at the next event, it must save the current event so that it has the information to print from it. It saves the event in the iterator descriptor called "ent". What it doesn't save is the ent_size of the event which is now used to know if the function graph arguments are to be printed. The peek doesn't save the size so the size used happens to be that of the size of the last event that was seen. Save the entry event size in the iterator descriptor so that the correct size is used. - Fix several errors with freeing data in the histogram code The histogram code had a lot of leaked or or incorrect accounting when failures happen. Correct them. - Fix histogram regression of .percent and .graph modifiers Up until 6.3 histogram values could have "percent" or "graph" modifiers that changed how they were printed. But a change that added restricting histograms values from being strings, stack traces and other modifiers inadvertently prevented them from using the percent and graph modifiers, which were legal use cases for values. Put back the percent and graph modifiers. - Fix various typos in the comments - Set the trace_clock before initializing a histogram with clock argument The histogram API allows the user to specific which trace clock to use via a "clock=" string. The histogram is set up first before the clock is checked. If the passed in clock is not valid, it exits without fully fixing up the histogram leaving it on the list and a use-after-free can trigger. Update the clock argument first and if it fails then exit gracefully before the histogram trigger is placed on any lists. - Restore :mod: trailer after parsing in ftrace_set_clr_event The function ftrace_set_clr_event() modifies the parse string and needs to put it back to what was passed in. It searches for ":mod:" via a strsep() but fails to put back the first ':' in the string. Add back the ':' in the passed in string. - Take trace_array reference when opening a tracer options file The options files are dynamically created and some tracers add their own options. When a tracer adds their own list of options, the trace_array holding them has an array to hold the list of options for each tracer. This array increases in size via a krealloc(), and the new entry gets a newly allocated array to hold the options of the new tracer being added. The element in each entry of the tracer's option array holds a pointer back to the trace_array, a pointer to the tracer it is associated to, a pointer to the flags of the option. The issue is that these arrays are freed when the trace_array is freed when its instance it represents is removed from the instances directory. There's a race that an open of one of these options files can happen when the instance is being removed. Add a new helper function to be called by the open function of the options file to iterate all existing trace_arrays under a lock and find the one that has the given option element in one of it's tracer arrays. If found, then update the associated trace_array's reference counter to keep it from being freed. If not found, have the open call return -ENODEV. - Disable interrupts when acquiring the lock in rb_wake_up_waiters() The function rb_wake_up_waiters() assumes it will be called in interrupt context and does not disable irqs when taking cpu_buffer->reader_lock, which can be called in hard interrupt context. The issue is in PREEMPT_RT, this function is called in thread context leaving this lock open to a deadlock. Take the lock with interrupts disabled. - Use rcu_assign_pointer() for tmp_ops filter hash The tmp_ops used in update_ftrace_direct_mod() assigns its filter_hash field directly, but that field is annotated as __rcu and sparse complains. Assign it with rcu_assign_pointer() - Fix use-after-free in enable_trigger_private_data_free() The trace_event_call is accessed through the event_trigger_data's trace_event_file pointer to put the trace_event_call on freeing. The issue is that the trace_event_file data may have been freed already causing a use-after-free. Add a field to the event_trigger_data that points directly to the trace_event_call so that it can decrement its reference directly without needing to go through the trace_event_file. - Fix accounting of buffer data remote headers trace_buffer_desc_size() and trace_remote_alloc_buffer() undercount the number of pages is needed for the asked for size as it doesn't take into account the meta data on each page. Add a helper function to do the calculation properly and use that in these functions. - Catch nr_page_va overflow in ring_buffer_desc sizing The number of pages per remote ring buffer is capped by ring_buffer_desc::nr_page_va (32 bits). A buffer_size large enough to overflow that field would silently allocate a descriptor smaller than what was asked for. - Do not resize the subbuf order if any per_cpu buffer is disabled The mmapping of ring buffers disables resizing the subbuffers, but it is done per-cpu whereas the subbuf size change is done for all the per_cpu buffers under the buffer->mutex. It could change the size of some while the mapping is happening on others. Have the resize of the subbuf order check all the per_cpu buffers under the lock to see if any of them is disabled before starting and causing an inconsistency between buffers that are being mapped. * tag 'trace-v7.3-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace: (25 commits) ring-buffer: Check resize_disabled before publishing the new subbuf order tracing/remotes: Catch nr_page_va overflow in ring_buffer_desc sizing tracing/remotes: Account for ring buffer page header in size calculation tracing: Don't dereference trace_event_file in deferred trigger free ftrace: Use rcu_assign_pointer() for tmp_ops filter hash ring-buffer: Acquire the lock with irqsave in rb_wake_up_waiters() tracing: Take trace_array reference when opening a tracer options file tracing: Fix ring_buffer_read_page_size() kernel-doc tracing: Restore :mod: trailer after parsing in ftrace_set_clr_event() tracing: Fix memory corruption from a "STACKTRACE" histogram key tracing: Fix memory corruption from the histogram stacktrace modifier tracing: Undo the registration when enabling the histogram trigger fails tracing: Take the reference before publishing the named histogram trigger tracing: Set the trace clock before registering the histogram trigger tracing: Fix typo "preceeded" in comment tracing: Fix typo "availabe" in comment tracing: Let histogram values keep the percent and graph modifiers tracing: Keep the entry count when the histogram stats allocation fails tracing: Free histogram the field rejected for a bad modifier tracing: Free histogram the var ref when its initialization fails ... --- drivers/hv/ring_buffer.c | 654 +++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 654 insertions(+) create mode 100644 drivers/hv/ring_buffer.c (limited to 'drivers/hv/ring_buffer.c') diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c new file mode 100644 index 000000000..592a9601f --- /dev/null +++ b/drivers/hv/ring_buffer.c @@ -0,0 +1,654 @@ +// SPDX-License-Identifier: GPL-2.0-only +/* + * + * Copyright (c) 2009, Microsoft Corporation. + * + * Authors: + * Haiyang Zhang + * Hank Janssen + * K. Y. Srinivasan + */ +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt + +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include + +#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, + bool confidential) +{ + 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 = kzalloc_objs(struct page *, page_cnt * 2 - 1); + 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, + confidential ? PAGE_KERNEL : 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); -- cgit v1.3.1