doc: describe --builtin-net-driver option in vhost app

Very simple version of vhost-user driver in vhost sample will be used if
builtin-net-driver option is enabled. This driver is based on generic
vhost lib APIs. Unfortunately, the implementation is incompatible with
QEMU as protocol feature is not supported.

Signed-off-by: Marvin Liu <yong.liu@intel.com>
Acked-by: John McNamara <john.mcnamara@intel.com>

diff --git a/doc/guides/sample_app_ug/vhost.rst b/doc/guides/sample_app_ug/vhost.rst
index 9112e78..fd42cb3 100644 (file)
--- a/doc/guides/sample_app_ug/vhost.rst
+++ b/doc/guides/sample_app_ug/vhost.rst
@@ -158,6 +158,10 @@ when disabling VLAN strip. Such feature, which heavily depends on hardware,
 should be removed from this example to reduce confusion. Now, VLAN strip is
 enabled and cannot be disabled.
 
+**--builtin-net-driver**
+A very simple vhost-user net driver which demonstrates how to use the generic
+vhost APIs will be used when this option is given. It is disabled by default.
+
 Common Issues
 -------------
 
@@ -192,3 +196,8 @@ Common Issues
   according to the NIC's property. ::
 
       make EXTRA_CFLAGS="-DMAX_QUEUES=320"
+
+* Option "builtin-net-driver" is incompatible with QEMU
+
+  QEMU vhost net device start will fail if protocol feature is not negotiated.
+  DPDK virtio-user pmd can be the replacement of QEMU.