virtio_ring: don't update event idx on get_buf

In virtio_net, if we disable napi_tx, when we trigger a tx interrupt,
the vq->event_triggered will be set to true. It is then never reset
until we explicitly call virtqueue_enable_cb_delayed or
virtqueue_enable_cb_prepare.

If we disable the napi_tx, virtqueue_enable_cb* will only be called when
the tx ring is getting relatively empty.

Since event_triggered is true, VRING_AVAIL_F_NO_INTERRUPT or
VRING_PACKED_EVENT_FLAG_DISABLE will not be set. As a result we update
vring_used_event(&vq->split.vring) or vq->packed.vring.driver->off_wrap
every time we call virtqueue_get_buf_ctx. This causes more interrupts.

To summarize:
1) event_triggered was set to true in vring_interrupt()
2) after this nothing will happen in virtqueue_disable_cb() so
   VRING_AVAIL_F_NO_INTERRUPT is not set in avail_flags_shadow
3) virtqueue_get_buf_ctx_split() will still think the cb is enabled
   and then it will publish a new event index

To fix:
update VRING_AVAIL_F_NO_INTERRUPT or VRING_PACKED_EVENT_FLAG_DISABLE in
the vq when we call virtqueue_disable_cb even when event_triggered is
true.

Tested with iperf:
iperf3 tcp stream:
vm1 -----------------> vm2
vm2 just receives tcp data stream from vm1, and sends acks to vm1,
there are many tx interrupts in vm2.
with the patch applied there are just a few tx interrupts.

v2->v3:
-update the interrupt disable flag even with the event_triggered is set,
-instead of checking whether event_triggered is set in
-virtqueue_get_buf_ctx_{packed/split}, will cause the drivers  which have
-not called virtqueue_{enable/disable}_cb to miss notifications.

v3->v4:
-remove change for
-"if (vq->packed.event_flags_shadow != VRING_PACKED_EVENT_FLAG_DISABLE)"
-in virtqueue_disable_cb_packed

Fixes: 8d622d21 ("virtio: fix up virtio_disable_cb")
Signed-off-by: Albert Huang <huangjie.albert@bytedance.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20230329102300.61000-1-huangjie.albert@bytedance.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_ring.c

@@ -854,6 +854,14 @@ static void virtqueue_disable_cb_split(struct virtqueue *_vq)
 
 	if (!(vq->split.avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) {
 		vq->split.avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
+
+		/*
+		 * If device triggered an event already it won't trigger one again:
+		 * no need to disable.
+		 */
+		if (vq->event_triggered)
+			return;
+
 		if (vq->event)
 			/* TODO: this is a hack. Figure out a cleaner value to write. */
 			vring_used_event(&vq->split.vring) = 0x0;
@@ -1699,6 +1707,14 @@ static void virtqueue_disable_cb_packed(struct virtqueue *_vq)
 
 	if (vq->packed.event_flags_shadow != VRING_PACKED_EVENT_FLAG_DISABLE) {
 		vq->packed.event_flags_shadow = VRING_PACKED_EVENT_FLAG_DISABLE;
+
+		/*
+		 * If device triggered an event already it won't trigger one again:
+		 * no need to disable.
+		 */
+		if (vq->event_triggered)
+			return;
+
 		vq->packed.vring.driver->flags =
 			cpu_to_le16(vq->packed.event_flags_shadow);
 	}
@@ -2330,12 +2346,6 @@ void virtqueue_disable_cb(struct virtqueue *_vq)
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
 
-	/* If device triggered an event already it won't trigger one again:
-	 * no need to disable.
-	 */
-	if (vq->event_triggered)
-		return;
-
 	if (vq->packed_ring)
 		virtqueue_disable_cb_packed(_vq);
 	else