Drivers: hv: ring_buffer: count on wrap around mappings in get_next_pkt_raw() (v2)

With wrap around mappings in place we can always provide drivers with
direct links to packets on the ring buffer, even when they wrap around.
Do the required updates to get_next_pkt_raw()/put_pkt_raw()

The first version of this commit was reverted (65a532f3d5) to deal with
cross-tree merge issues which are (hopefully) resolved now.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Tested-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/hyperv.h

@@ -1526,31 +1526,23 @@ static inline struct vmpacket_descriptor *
 get_next_pkt_raw(struct vmbus_channel *channel)
 {
 	struct hv_ring_buffer_info *ring_info = &channel->inbound;
-	u32 read_loc = ring_info->priv_read_index;
+	u32 priv_read_loc = ring_info->priv_read_index;
 	void *ring_buffer = hv_get_ring_buffer(ring_info);
-	struct vmpacket_descriptor *cur_desc;
-	u32 packetlen;
 	u32 dsize = ring_info->ring_datasize;
-	u32 delta = read_loc - ring_info->ring_buffer->read_index;
+	/*
+	 * delta is the difference between what is available to read and
+	 * what was already consumed in place. We commit read index after
+	 * the whole batch is processed.
+	 */
+	u32 delta = priv_read_loc >= ring_info->ring_buffer->read_index ?
+		priv_read_loc - ring_info->ring_buffer->read_index :
+		(dsize - ring_info->ring_buffer->read_index) + priv_read_loc;
 	u32 bytes_avail_toread = (hv_get_bytes_to_read(ring_info) - delta);
 
 	if (bytes_avail_toread < sizeof(struct vmpacket_descriptor))
 		return NULL;
 
-	if ((read_loc + sizeof(*cur_desc)) > dsize)
-		return NULL;
-
-	cur_desc = ring_buffer + read_loc;
-	packetlen = cur_desc->len8 << 3;
-
-	/*
-	 * If the packet under consideration is wrapping around,
-	 * return failure.
-	 */
-	if ((read_loc + packetlen + VMBUS_PKT_TRAILER) > (dsize - 1))
-		return NULL;
-
-	return cur_desc;
+	return ring_buffer + priv_read_loc;
 }
 
 /*
@@ -1562,16 +1554,14 @@ static inline void put_pkt_raw(struct vmbus_channel *channel,
 				struct vmpacket_descriptor *desc)
 {
 	struct hv_ring_buffer_info *ring_info = &channel->inbound;
-	u32 read_loc = ring_info->priv_read_index;
 	u32 packetlen = desc->len8 << 3;
 	u32 dsize = ring_info->ring_datasize;
 
-	if ((read_loc + packetlen + VMBUS_PKT_TRAILER) > dsize)
-		BUG();
 	/*
 	 * Include the packet trailer.
 	 */
 	ring_info->priv_read_index += packetlen + VMBUS_PKT_TRAILER;
+	ring_info->priv_read_index %= dsize;
 }
 
 /*