vhost: abstract vring call mechanism as it is transport-specific

vring notification mechanism is transport-specific.  At present, vhost
dataplane code in `lib/vhost/vhost.c` triggers guest notifications with
`eventfd_write()` system call. But this is an AF_UNIX specific
notification mechanism. This patch replaces `eventfd_write()` with the
existing generic `rte_vhost_vring_call()` function that is part of
DPDK's librte_vhost public API.

`rte_vhost_vring_call()` takes a vring_idx as an argument to associate
the `struct spdk_vhost_virtqueue` instance with the relevant `struct
vhost_virtqueue` instance. We introduce a new `vring_idx` field in
`struct spdk_vhost_virtqueue` to enable this association. This field is
initialized in `start_device()`. In addition, a stub for
`rte_vhost_vring_call()` is added in the vhost unit test file.

SPDK's internal `rte_vhost` copy will not be updated in order to support
the virtio-vhost-user transport. However, an `rte_vhost_vring_call()`
function is introduced in SPDK's `rte_vhost` in order to have a solid
API. This function is just a wrapper of `eventfd_write()`.

Change-Id: Ic93e25cd3f06e92f04766521bc850f1ee80b8ec8
Signed-off-by: Nikos Dragazis <ndragazis@arrikto.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/454373
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>

lib/vhost/rte_vhost/rte_vhost.h

@@ -472,4 +472,16 @@ int rte_vhost_set_vring_base(int vid, uint16_t queue_id,
 int rte_vhost_get_vring_base(int vid, uint16_t queue_id,
 		uint16_t *last_avail_idx, uint16_t *last_used_idx);
 
+/**
+ * Notify the guest that used descriptors have been added to the vring.
+ *
+ * @param vid
+ *  vhost device ID
+ * @param vring_idx
+ *  vring index
+ * @return
+ *  0 on success, -1 on failure
+ */
+int rte_vhost_vring_call(int vid, uint16_t vring_idx);
+
 #endif /* _RTE_VHOST_H_ */

lib/vhost/rte_vhost/vhost.c

@@ -503,3 +503,28 @@ rte_vhost_get_vring_base(int vid, uint16_t vring_idx,
 
 	return 0;
 }
+
+int
+rte_vhost_vring_call(int vid, uint16_t vring_idx)
+{
+	struct virtio_net *dev;
+	struct vhost_virtqueue *vq;
+
+	dev = get_device(vid);
+	if(!dev)
+		return -1;
+
+	if (vring_idx >= VHOST_MAX_VRING)
+		return -1;
+
+	vq = dev->virtqueue[vring_idx];
+	if (!vq)
+		return -1;
+
+	if (vq->callfd != -1) {
+		eventfd_write(vq->callfd, (eventfd_t)1);
+		return 0;
+	}
+
+	return -1;
+}

lib/vhost/vhost.c

@@ -278,8 +278,13 @@ spdk_vhost_vq_used_signal(struct spdk_vhost_session *vsession,
 		      "Queue %td - USED RING: sending IRQ: last used %"PRIu16"\n",
 		      virtqueue - vsession->virtqueue, virtqueue->last_used_idx);
 
-	eventfd_write(virtqueue->vring.callfd, (eventfd_t)1);
-	return 1;
+	if (rte_vhost_vring_call(vsession->vid, virtqueue->vring_idx) == 0) {
+		/* interrupt signalled */
+		return 1;
+	} else {
+		/* interrupt not signalled */
+		return 0;
+	}
 }
 
 
@@ -1118,9 +1123,11 @@ start_device(int vid)
 	for (i = 0; i < SPDK_VHOST_MAX_VQUEUES; i++) {
 		struct spdk_vhost_virtqueue *q = &vsession->virtqueue[i];
 
+		q->vring_idx = -1;
 		if (rte_vhost_get_vhost_vring(vid, i, &q->vring)) {
 			continue;
 		}
+		q->vring_idx = i;
 
 		if (q->vring.desc == NULL || q->vring.size == 0) {
 			continue;
@@ -1160,9 +1167,7 @@ start_device(int vid)
 	 * Tested on QEMU 2.10.91 and 2.11.50.
 	 */
 	for (i = 0; i < vsession->max_queues; i++) {
-		if (vsession->virtqueue[i].vring.callfd != -1) {
-			eventfd_write(vsession->virtqueue[i].vring.callfd, (eventfd_t)1);
-		}
+		rte_vhost_vring_call(vsession->vid, vsession->virtqueue[i].vring_idx);
 	}
 
 	spdk_vhost_session_set_coalescing(vdev, vsession, NULL);

lib/vhost/vhost_internal.h

@@ -113,6 +113,8 @@ struct spdk_vhost_virtqueue {
 	/* Next time when we need to send event */
 	uint64_t next_event_time;
 
+	/* Associated vhost_virtqueue in the virtio device's virtqueue list */
+	uint32_t vring_idx;
 } __attribute((aligned(SPDK_CACHE_LINE_SIZE)));
 
 struct spdk_vhost_session {

test/unit/lib/vhost/vhost.c/vhost_ut.c

@@ -55,6 +55,7 @@ DEFINE_STUB(spdk_event_allocate, struct spdk_event *,
 	    (uint32_t lcore, spdk_event_fn fn, void *arg1, void *arg2), NULL);
 DEFINE_STUB(spdk_mem_register, int, (void *vaddr, size_t len), 0);
 DEFINE_STUB(spdk_mem_unregister, int, (void *vaddr, size_t len), 0);
+DEFINE_STUB(rte_vhost_vring_call, int, (int vid, uint16_t vring_idx), 0);
 
 static struct spdk_cpuset *g_app_core_mask;
 struct spdk_cpuset *spdk_app_get_core_mask(void)