[PATCH] doc: update async enqueue API usage

[xuan.ding@intel.com]

From: Xuan Ding <xuan.ding@intel.com>

This patch updates the correct usage for async enqueue APIs.
The rte_vhost_poll_enqueue_completed() needs to be
called in time to avoid packet loss due to full dma ring.

Signed-off-by: Xuan Ding <xuan.ding@intel.com>
---
 doc/guides/prog_guide/vhost_lib.rst | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/doc/guides/prog_guide/vhost_lib.rst b/doc/guides/prog_guide/vhost_lib.rst
index cd3f6caa9a..9cdd7da154 100644
--- a/doc/guides/prog_guide/vhost_lib.rst
+++ b/doc/guides/prog_guide/vhost_lib.rst
@@ -488,3 +488,12 @@ For PA mode, page by page mapping may exceed IOMMU's max capability,
 better to use 1G guest hugepage.
 
 For UIO driver, any VFIO related error message can be ignored.
+
+Async enqueue API usage
+-----------------------
+
+In async enqueue path, rte_vhost_poll_enqueue_completed() needs to be
+called in time. Otherwise, calling rte_vhost_submit_enqueue_burst()
+all the time will cause the DMA ring to be full, which will result
+in packet loss eventually.
+
-- 
2.17.1