[PATCH v2 2/2] doc/howto: add code example to virtio-user exception path doc

[Bruce Richardson <bruce.richardson@intel.com>]

The HOWTO guide for using virtio-user as an exception path to the kernel
only provided an example of how testpmd may be used for that purpose.
However, a real application wanting to use virtio-user as exception path
would likely want to create such devices from code within the app
itself. Therefore, we update the doc with instructions and a code
snippet showing how this may be done.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>

---

 v2: fix http link to https
---
 .../howto/virtio_user_as_exceptional_path.rst | 55 +++++++++++++++++++
 1 file changed, 55 insertions(+)

diff --git a/doc/guides/howto/virtio_user_as_exceptional_path.rst b/doc/guides/howto/virtio_user_as_exceptional_path.rst
index 100376c32d..45d4ebd284 100644
--- a/doc/guides/howto/virtio_user_as_exceptional_path.rst
+++ b/doc/guides/howto/virtio_user_as_exceptional_path.rst
@@ -157,3 +157,58 @@ For example:
         /path/to/dpdk-testpmd --vdev=virtio_user0,path=/dev/vhost-net,queues=2,queue_size=1024 -- \
             -i --tx-offloads=0x002c --enable-lro --txq=2 --rxq=2 --txd=1024 --rxd=1024
 
+
+Creating Virtio-User Ports within an Application
+------------------------------------------------
+
+To use virtio-user ports within an application,
+it is not necessary to explicitly initialize those ports using EAL arguments at startup.
+Instead, one can use the generic EAL API
+`rte_eal_hotplug_add <https://doc.dpdk.org/api/rte__dev_8h.html#ad32e8eebf1f81ef9f290cb296b0c90bb>`_
+function to create a new instance at startup.
+For example, to create a basic virtio-user port, the following code could be used:
+
+.. code-block:: C
+
+   rte_eal_hotplug_add("vdev", "virtio_user0", "path=/dev/vhost-net");
+
+A fuller code example is shown below, where a virtio-user port, and hence kernel netdev,
+is created for each NIC port discovered by DPDK.
+Each virtio-user port is given the MAC address of its matching physical port
+(assuming app was run without vdev args on commandline, so all ports auto-discovered are HW ones).
+These new virtio-user netdevs will appear in the kernel port listings as ``virtio_user0``,
+``virtio_user1``, etc.,
+based on the names passed in as ``iface=`` via the ``portargs`` parameter.
+
+.. code-block:: C
+
+    nb_ports = rte_eth_dev_count_avail();
+
+    /* Create a vhost_user port for each physical port */
+    unsigned port_count = 0;
+    RTE_ETH_FOREACH_DEV(portid) {
+        char portname[32];
+        char portargs[256];
+        struct rte_ether_addr addr = {0};
+
+        /* don't create virtio_user ports for other virtio_user ports */
+        if (++port_count > nb_ports)
+            break;
+
+        /* get mac address of physical port to use as mac of virtio_user port */
+        rte_eth_macaddr_get(portid, &addr);
+
+        /* set the name and arguments */
+        snprintf(portname, sizeof(portname), "virtio_user%u", portid);
+        snprintf(portargs, sizeof(portargs),
+                "path=/dev/vhost-net,queues=1,queue_size=%u,iface=%s,mac=" RTE_ETHER_ADDR_PRT_FMT,
+                RX_RING_SIZE, portname, RTE_ETHER_ADDR_BYTES(&addr));
+
+        /* add the vdev for virtio_user */
+        if (rte_eal_hotplug_add("vdev", portname, portargs) < 0)
+            rte_exit(EXIT_FAILURE, "Cannot create paired port for port %u\n", portid);
+
+    }
+
+Once these virtio-user ports have been created in the loop, all ports, both physical and virtual,
+may be initialized and used as normal in the application.
-- 
2.34.1