diff --git a/include/xen/interface/io/sndif.h b/include/xen/interface/io/sndif.h
index d48175f1a3d2..131c3b469d4a 100644
--- a/include/xen/interface/io/sndif.h
+++ b/include/xen/interface/io/sndif.h
@@ -113,6 +113,8 @@
  *
  * /local/domain/1/device/vsnd/0/0/0/ring-ref = "386"
  * /local/domain/1/device/vsnd/0/0/0/event-channel = "15"
+ * /local/domain/1/device/vsnd/0/0/0/evt-ring-ref = "1386"
+ * /local/domain/1/device/vsnd/0/0/0/evt-event-channel = "215"
  *
  *------------------------------ Stream 1, capture ----------------------------
  *
@@ -122,6 +124,8 @@
  *
  * /local/domain/1/device/vsnd/0/0/1/ring-ref = "384"
  * /local/domain/1/device/vsnd/0/0/1/event-channel = "13"
+ * /local/domain/1/device/vsnd/0/0/1/evt-ring-ref = "1384"
+ * /local/domain/1/device/vsnd/0/0/1/evt-event-channel = "213"
  *
  *------------------------------- PCM device 1 --------------------------------
  *
@@ -135,6 +139,8 @@
  *
  * /local/domain/1/device/vsnd/0/1/0/ring-ref = "387"
  * /local/domain/1/device/vsnd/0/1/0/event-channel = "151"
+ * /local/domain/1/device/vsnd/0/1/0/evt-ring-ref = "1387"
+ * /local/domain/1/device/vsnd/0/1/0/evt-event-channel = "351"
  *
  *------------------------------- PCM device 2 --------------------------------
  *
@@ -147,6 +153,8 @@
  *
  * /local/domain/1/device/vsnd/0/2/0/ring-ref = "389"
  * /local/domain/1/device/vsnd/0/2/0/event-channel = "152"
+ * /local/domain/1/device/vsnd/0/2/0/evt-ring-ref = "1389"
+ * /local/domain/1/device/vsnd/0/2/0/evt-event-channel = "452"
  *
  ******************************************************************************
  *                            Backend XenBus Nodes
@@ -292,6 +300,23 @@
  *      The Xen grant reference granting permission for the backend to map
  *      a sole page in a single page sized ring buffer.
  *
+ *--------------------- Stream Event Transport Parameters ---------------------
+ *
+ * This communication path is used to deliver asynchronous events from backend
+ * to frontend, set up per stream.
+ *
+ * evt-event-channel
+ *      Values:         <uint32_t>
+ *
+ *      The identifier of the Xen event channel used to signal activity
+ *      in the ring buffer.
+ *
+ * evt-ring-ref
+ *      Values:         <uint32_t>
+ *
+ *      The Xen grant reference granting permission for the backend to map
+ *      a sole page in a single page sized ring buffer.
+ *
  ******************************************************************************
  *                               STATE DIAGRAMS
  ******************************************************************************
@@ -439,6 +464,19 @@
 #define XENSND_OP_GET_VOLUME		5
 #define XENSND_OP_MUTE			6
 #define XENSND_OP_UNMUTE		7
+#define XENSND_OP_TRIGGER		8
+
+#define XENSND_OP_TRIGGER_START		0
+#define XENSND_OP_TRIGGER_PAUSE		1
+#define XENSND_OP_TRIGGER_STOP		2
+#define XENSND_OP_TRIGGER_RESUME	3
+
+/*
+ ******************************************************************************
+ *                                 EVENT CODES
+ ******************************************************************************
+ */
+#define XENSND_EVT_CUR_POS		0
 
 /*
  ******************************************************************************
@@ -455,6 +493,8 @@
 #define XENSND_FIELD_VCARD_LONG_NAME	"long-name"
 #define XENSND_FIELD_RING_REF		"ring-ref"
 #define XENSND_FIELD_EVT_CHNL		"event-channel"
+#define XENSND_FIELD_EVT_RING_REF	"evt-ring-ref"
+#define XENSND_FIELD_EVT_EVT_CHNL	"evt-event-channel"
 #define XENSND_FIELD_DEVICE_NAME	"name"
 #define XENSND_FIELD_TYPE		"type"
 #define XENSND_FIELD_STREAM_UNIQUE_ID	"unique-id"
@@ -566,7 +606,9 @@
  * +----------------+----------------+----------------+----------------+
  * |                           gref_directory                          | 24
  * +----------------+----------------+----------------+----------------+
- * |                             reserved                              | 28
+ * |                             period_sz                             | 28
+ * +----------------+----------------+----------------+----------------+
+ * |                             reserved                              | 32
  * +----------------+----------------+----------------+----------------+
  * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
  * +----------------+----------------+----------------+----------------+
@@ -578,6 +620,14 @@
  * pcm_channels - uint8_t, number of channels of this stream,
  *   [channels-min; channels-max]
  * buffer_sz - uint32_t, buffer size to be allocated, octets
+ * period_sz - uint32_t, event period size, octets
+ *   This is the requested value of the period at which frontend would
+ *   like to receive XENSND_EVT_CUR_POS notifications from the backend when
+ *   stream position advances during playback/capture.
+ *   It shows how many octets are expected to be played/captured before
+ *   sending such an event.
+ *   If set to 0 no XENSND_EVT_CUR_POS events are sent by the backend.
+ *
  * gref_directory - grant_ref_t, a reference to the first shared page
  *   describing shared buffer references. At least one page exists. If shared
  *   buffer size  (buffer_sz) exceeds what can be addressed by this single page,
@@ -592,6 +642,7 @@ struct xensnd_open_req {
 	uint16_t reserved;
 	uint32_t buffer_sz;
 	grant_ref_t gref_directory;
+	uint32_t period_sz;
 };
 
 /*
@@ -754,8 +805,32 @@ struct xensnd_rw_req {
  *
  * The 'struct xensnd_rw_req' is also used for XENSND_OP_SET_VOLUME,
  * XENSND_OP_GET_VOLUME, XENSND_OP_MUTE, XENSND_OP_UNMUTE.
+ *
+ * Request stream running state change - trigger PCM stream running state
+ * to start, stop, pause or resume:
+ *
+ *         0                1                 2               3        octet
+ * +----------------+----------------+----------------+----------------+
+ * |               id                |   _OP_TRIGGER  |    reserved    | 4
+ * +----------------+----------------+----------------+----------------+
+ * |                             reserved                              | 8
+ * +----------------+----------------+----------------+----------------+
+ * |      type      |                     reserved                     | 12
+ * +----------------+----------------+----------------+----------------+
+ * |                             reserved                              | 16
+ * +----------------+----------------+----------------+----------------+
+ * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
+ * +----------------+----------------+----------------+----------------+
+ * |                             reserved                              | 64
+ * +----------------+----------------+----------------+----------------+
+ *
+ * type - uint8_t, XENSND_OP_TRIGGER_XXX value
  */
 
+struct xensnd_trigger_req {
+	uint8_t type;
+};
+
 /*
  *---------------------------------- Responses --------------------------------
  *
@@ -778,8 +853,51 @@ struct xensnd_rw_req {
  * id - uint16_t, copied from the request
  * operation - uint8_t, XENSND_OP_* - copied from request
  * status - int32_t, response status, zero on success and -XEN_EXX on failure
+ *
+ *----------------------------------- Events ----------------------------------
+ *
+ * Events are sent via shared page allocated by the front and propagated by
+ *   evt-event-channel/evt-ring-ref XenStore entries
+ * All event packets have the same length (64 octets)
+ * All event packets have common header:
+ *         0                1                 2               3        octet
+ * +----------------+----------------+----------------+----------------+
+ * |               id                |      type      |   reserved     | 4
+ * +----------------+----------------+----------------+----------------+
+ * |                             reserved                              | 8
+ * +----------------+----------------+----------------+----------------+
+ *
+ * id - uint16_t, event id, may be used by front
+ * type - uint8_t, type of the event
+ *
+ *
+ * Current stream position - event from back to front when stream's
+ *   playback/capture position has advanced:
+ *         0                1                 2               3        octet
+ * +----------------+----------------+----------------+----------------+
+ * |               id                |   _EVT_CUR_POS |   reserved     | 4
+ * +----------------+----------------+----------------+----------------+
+ * |                             reserved                              | 8
+ * +----------------+----------------+----------------+----------------+
+ * |                         position low 32-bit                       | 12
+ * +----------------+----------------+----------------+----------------+
+ * |                         position high 32-bit                      | 16
+ * +----------------+----------------+----------------+----------------+
+ * |                             reserved                              | 20
+ * +----------------+----------------+----------------+----------------+
+ * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
+ * +----------------+----------------+----------------+----------------+
+ * |                             reserved                              | 64
+ * +----------------+----------------+----------------+----------------+
+ *
+ * position - current value of stream's playback/capture position, octets
+ *
  */
 
+struct xensnd_cur_pos_evt {
+	uint64_t position;
+};
+
 struct xensnd_req {
 	uint16_t id;
 	uint8_t operation;
@@ -787,6 +905,7 @@ struct xensnd_req {
 	union {
 		struct xensnd_open_req open;
 		struct xensnd_rw_req rw;
+		struct xensnd_trigger_req trigger;
 		uint8_t reserved[56];
 	} op;
 };
@@ -799,6 +918,47 @@ struct xensnd_resp {
 	uint8_t reserved1[56];
 };
 
+struct xensnd_evt {
+	uint16_t id;
+	uint8_t type;
+	uint8_t reserved[5];
+	union {
+		struct xensnd_cur_pos_evt cur_pos;
+		uint8_t reserved[56];
+	} op;
+};
+
 DEFINE_RING_TYPES(xen_sndif, struct xensnd_req, struct xensnd_resp);
 
+/*
+ ******************************************************************************
+ *                        Back to front events delivery
+ ******************************************************************************
+ * In order to deliver asynchronous events from back to front a shared page is
+ * allocated by front and its granted reference propagated to back via
+ * XenStore entries (evt-ring-ref/evt-event-channel).
+ * This page has a common header used by both front and back to synchronize
+ * access and control event's ring buffer, while back being a producer of the
+ * events and front being a consumer. The rest of the page after the header
+ * is used for event packets.
+ *
+ * Upon reception of an event(s) front may confirm its reception
+ * for either each event, group of events or none.
+ */
+
+struct xensnd_event_page {
+	uint32_t in_cons;
+	uint32_t in_prod;
+	uint8_t reserved[56];
+};
+
+#define XENSND_EVENT_PAGE_SIZE XEN_PAGE_SIZE
+#define XENSND_IN_RING_OFFS (sizeof(struct xensnd_event_page))
+#define XENSND_IN_RING_SIZE (XENSND_EVENT_PAGE_SIZE - XENSND_IN_RING_OFFS)
+#define XENSND_IN_RING_LEN (XENSND_IN_RING_SIZE / sizeof(struct xensnd_evt))
+#define XENSND_IN_RING(page) \
+	((struct xensnd_evt *)((char *)(page) + XENSND_IN_RING_OFFS))
+#define XENSND_IN_RING_REF(page, idx) \
+	(XENSND_IN_RING((page))[(idx) % XENSND_IN_RING_LEN])
+
 #endif /* __XEN_PUBLIC_IO_SNDIF_H__ */
