Drivers: hv: vmbus: Add comments on ring buffer signaling

Add comments describing intricacies of Hyper-V ring buffer
signaling code.  This information is not in Hyper-V public
documents, so include here to capture the knowledge for
future coders.

There are no code changes in this commit.

Signed-off-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hv/ring_buffer.c

@@ -431,7 +431,24 @@ static u32 hv_pkt_iter_bytes_read(const struct hv_ring_buffer_info *rbi,
 }
 
 /*
- * Update host ring buffer after iterating over packets.
+ * 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)
 {
@@ -447,22 +464,30 @@ void hv_pkt_iter_close(struct vmbus_channel *channel)
 	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.
-	 * Here is the reason for having this barrier:
-	 * If the reading of the pend_sz (in this function)
-	 * were to be reordered and read before we commit the new read
-	 * index (in the calling function)  we could
-	 * have a problem. If the host were to set the pending_sz after we
-	 * have sampled pending_sz and go to sleep before we commit the
+	 * 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;
@@ -476,14 +501,32 @@ void hv_pkt_iter_close(struct vmbus_channel *channel)
 	bytes_read = hv_pkt_iter_bytes_read(rbi, start_read_index);
 
 	/*
-	 * If there was space before we began iteration,
-	 * then host was not blocked.
+	 * 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;
 
-	/* If pending write will not fit, don't give false hope. */
+	/*
+	 * Similarly, if the new state is "not enough space", then
+	 * there's no need to signal.
+	 */
 	if (curr_write_sz <= pending_sz)
 		return;
 

include/linux/hyperv.h

@@ -89,18 +89,33 @@ struct hv_ring_buffer {
 	u32 interrupt_mask;
 
 	/*
-	 * Win8 uses some of the reserved bits to implement
-	 * interrupt driven flow management. On the send side
-	 * we can request that the receiver interrupt the sender
-	 * when the ring transitions from being full to being able
-	 * to handle a message of size "pending_send_sz".
+	 * WS2012/Win8 and later versions of Hyper-V implement interrupt
+	 * driven flow management. The feature bit feat_pending_send_sz
+	 * is set by the host on the host->guest ring buffer, and by the
+	 * guest on the guest->host ring buffer.
 	 *
-	 * Add necessary state for this enhancement.
+	 * The meaning of the feature bit is a bit complex in that it has
+	 * semantics that apply to both ring buffers.  If the guest sets
+	 * the feature bit in the guest->host ring buffer, the guest is
+	 * telling the host that:
+	 * 1) It will set the pending_send_sz field in the guest->host ring
+	 *    buffer when it is waiting for space to become available, and
+	 * 2) It will read the pending_send_sz field in the host->guest
+	 *    ring buffer and interrupt the host when it frees enough space
+	 *
+	 * Similarly, if the host sets the feature bit in the host->guest
+	 * ring buffer, the host is telling the guest that:
+	 * 1) It will set the pending_send_sz field in the host->guest ring
+	 *    buffer when it is waiting for space to become available, and
+	 * 2) It will read the pending_send_sz field in the guest->host
+	 *    ring buffer and interrupt the guest when it frees enough space
+	 *
+	 * If either the guest or host does not set the feature bit that it
+	 * owns, that guest or host must do polling if it encounters a full
+	 * ring buffer, and not signal the other end with an interrupt.
 	 */
 	u32 pending_send_sz;
-
 	u32 reserved1[12];
-
 	union {
 		struct {
 			u32 feat_pending_send_sz:1;